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Time perception in astronauts on board the International Space Station

Time perception in astronauts on board the International Space Station www.nature.com/npjmgrav ARTICLE OPEN Time perception in astronauts on board the International Space Station 1 1 1 1 1✉ Deborah C. Navarro Morales , Olga Kuldavletova , Gaëlle Quarck , Pierre Denise and Gilles Clément We perceive the environment through an elaborate mental representation based on a constant integration of sensory inputs, knowledge, and expectations. Previous studies of astronauts on board the International Space Station have shown that the mental representation of space, such as the perception of object size, distance, and depth, is altered in orbit. Because the mental representations of space and time have some overlap in neural networks, we hypothesized that perception of time would also be affected by spaceflight. Ten astronauts were tested before, during, and after a 6–8-month spaceflight. Temporal tasks included judging when one minute had passed and how long it had been since the start of the workday, lunch, docking of a vehicle, and a spacewalk. Compared to pre-flight estimates, there is a relative overestimation for the 1-min interval during the flight and a relative underestimation of intervals of hours in duration. However, the astronauts quite accurately estimated the number of days since vehicle dockings and spacewalks. Prolonged isolation in confined areas, stress related to workload, and high-performance expectations are potential factors contributing to altered time perception of daily events. However, reduced vestibular stimulations and slower motions in weightlessness, as well as constant references to their timeline and work schedule could also account for the change in the estimation of time by the astronauts in space. npj Microgravity (2023) 9:6 ; https://doi.org/10.1038/s41526-023-00250-x INTRODUCTION decision making and alters their ability to control the vehicle and their movements . The neurovestibular challenges that occur To construct a mental representation of our world, we perceive when the crewmember returns to normal gravity include our environment by constantly processing sensory inputs from the 16 17 alterations in manual control , inability to egress the vehicle , visual, vestibular, and somatosensory systems. This representation 18 19 postural imbalance , and impaired locomotion . is also influenced by our expectations and our experience, which Einstein revolutionized physics a century ago with his theory are derived from our knowledge of the costs and consequences of that space and time are intertwined . Based on this theory, we acting in this environment. The central neurovestibular system hypothesized that the absence of gravitational reference alters the naturally takes gravity into account during spatial orientation, construction of the mental representation of both space and time. balance, and motor control. This system is also indispensable for Because astronauts underestimate distances and because the constructing our mental representation of the world. It continu- pace of their motion is slower in weightlessness, we hypothesized ously processes data from the visual, vestibular, and somatosen- that astronauts would also underestimate the relative time sory channels to update our spatial maps. Previous research between events. Prior to the present study, subjective perception suggests that distances are underestimated when subjects are in 1,2 3 of time during long-duration spaceflight had not been investi- weightlessness during parabolic or orbital flight. This distance gated. The results of this study have operational implications: underestimation is thought to be due to adaptive changes in the altered perception of time during spaceflight might impact processing of gravitational information by the neurovestibular 4,5 operations that require critical timing, such as docking operations system that alter the construction of spatial maps . and piloted landings. Exposure to weightlessness during spaceflight is known to elicit changes in vestibular responses, i.e., orientation illusions, errors in sensory localization, changes in vestibulo-ocular reflexes, and 6 METHODS space motion sickness . Cognitive tasks involving the neuroves- 7–9 10 Participants tibular system, such as mental rotation , perceived orientation and judgments of distance are also affected during spaceflight. 10 healthy crewmembers (9 male, 1 female; age M = 44.1, Some astronauts and cosmonauts have reported a ‘time SD = 4.6) who flew on the International Space Station (ISS) compression syndrome’ in orbit, whereby they perceive time as participated in this study. All crewmembers passed a United States compressed relative to the perceptions gained during training and Air Force Class III medical examination and had no known history simulation . In weightlessness, routine tasks require different of vestibular or oculomotor abnormalities. 15 healthy subjects (6 cognitive demands than they would on Earth. Astronauts in orbit females, 9 males; age M = 43.2, SD = 18.8) participated in a control also report that they require more time than normal to execute study in the laboratory. 12,13 standard mental activity . ‘Space fog’ is another reported The test procedures were approved by the European Space syndrome that affects cognitive performance during the first Agency Medical Board and the NASA Johnson Space Center weeks of a mission . After astronauts have adapted to weight- Institutional Review Board and were performed in accordance lessness and they re-enter the atmosphere, they also encounter a with the ethical standards laid down in the 1964 Declaration of condition called ‘entry motion sickness’, which slows the speed of Helsinki. All subjects gave a written informed consent before UNICAEN, INSERM, CHU Caen, Normandy University, COMETE, CYCERON, Esplanade de la Paix, 14032 Caen, France. email: gilles.clement@unicaen.fr Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; D.C. Navarro Morales et al. participating in the study. Informed consent was obtained from spatial orientation. Previous studies have demonstrated that the the subject for publication of identifying information/images in an perception of distance and the depth of objects are altered when 3,4 online open-access publication. free-floating in orbit . To investigate the relationship between these changes in spatial perception and changes in time perception, the psychophysics test was performed in the same Experimental protocol conditions as the previous spatial perception tests. In the flight study, the tests were performed before, during, and The second part of the experiment included a series of after 6–8-month spaceflights (M = 202, SD = 28 days). The questions to document the potential changes in the astronauts’ pre-flight test sessions occurred at launch minus (L-) 205 ± 51 days, perception of longer periods of time, i.e. hours and days, while in L-149 ± 55 days, and L-116 ± 45 days. In-flight test sessions were orbit. The subjects doffed the head-mounted display and used the conducted approximately every month: i.e., on flight day (FD) laptop keyboard to answer the following 5 questions: (a) How long FD17 ± 6 (M ± SD), FD46 ± 8, FD71 ± 6, FD99 ± 7, FD134 ± 8, and has it been since the last time you performed this test?; (b) How FD164 ± 7. After the astronauts returned to Earth, tests were long has it been since you started your work day?; (c) How long performed at return plus (R + ) 1 day, R + 5 ± 1 day, and has it been since you had your lunch?; (d) How long has it been R + 9 ± 1 day. since the last vehicle docked at the ISS?; and (e) How long has it A psychophysics test was administered, during which the been since the last extra-vehicular activity (EVA)? subjects were asked to judge when one minute had passed. In the flight study, the start of the workday was defined as the During this test, subjects wore a head-mounted display (Oculus termination of the morning daily planning conference (DPC), Rift, Oculus VR, Menlo Park, CA), and used a finger trackball which occurred about one hour after the crew awoke. The time of connected to a laptop to report their responses. Subjects were lunch was determined by the astronauts’ daily schedule. The dates wearing earphones for listening to the instructions and for of docking and EVA was determined from the Flight Program attenuating/masking noises from the spacecraft. Using the finger Integration Panel document provided by NASA Mission Integra- mouse, they pressed a ‘go’ button and waited one minute before tions and Operations Office. Because this information was not pressing on a ‘stop’ button. Only the ‘go’ and’stop’ buttons were available before and after the flight, the questionnaire was only displayed during the test. Subjects were not allowed to count the administered during the flight. seconds passing by. In the ground-based study, the subjects performed the ‘How On the ground, this test was performed in the seated upright Long is a Minute?’ test in the laboratory while sitting upright and position; on the ISS, astronauts were in the free-floating conditions using identical computer hardware and software as on board the (Fig. 1). During the free-floating conditions there are no ISS. The subjects were also asked the perceived number of days proprioceptive, tactile, or static vestibular cues that participate in since the last test session and how many minutes since they woke up and had breakfast (which they noted in a diary). The number of days between test sessions of the control subjects in the laboratory (M = 44.1, SD = 10.2) was similar to the number of days separating the 3 pre-flight sessions with the astronauts (M = 45.2, SD = 28.4), and so were the durations since wake up and lunch. Statistical analysis The errors between the perceived durations and the actual durations were calculated and time errors were computed in terms of percentage or days. First, linear mixed models (LMM) were used to compare the ground-based responses of the 2 subject groups (astronauts and controls) and to establish whether they differed, and whether the results of the 3 test sessions differed (dependent variable: time error; fixed effects: tests sessions; group: astronauts or controls; random effects: subjects). A second set of LMM was used to compare measurements from different sessions within the flight phases (pre, in, post) in astronauts (dependent variable: time error; fixed effect: test sessions; random effects: subjects). A third set of LMM was used to compare the time errors during the pre-, in-, and post-flight sessions (depended variable: time error; fixed effects: flight phase; random effects: subjects). Post-hoc pairwise compar- isons were then conducted using Bonferroni adjustment for multiple comparisons. Fixed effects estimates, confidence limits, and random effects standard deviations of these LMMs are provided in Supplementary Table 1. When preflight measurements were not available in the astronauts (perceived durations since beginning of workday and lunch), an independent Sample Mann–Whitney test was used to compare their inflight responses with those of the control subjects on the ground. When these responses were not available in the control subjects neither (perceived durations since last docking and last EVA) a one-sample t-test was used to determine if the time errors during the flight were different from zero. Statistical analysis was performed with JASP 0.16.1.0 and IBM SPSS Statistics Fig. 1 Astronaut performing the experiment while free-floating on board the ISS. Photo credit NASA. 27.0 software. npj Microgravity (2023) 6 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; D.C. Navarro Morales et al. Reporting summary (31.0 ± 10 d). The time interval between the last pre-flight session (L-116) and FD17 was much longer (133.8 ± 43 d). When Further information on research design is available in the Nature comparing the astronaut’s perceived duration between the test Research Reporting Summary linked to this article. sessions, the largest time errors occurred on FD17 and R + 1, i.e., shortly after the transition in gravity levels (Fig. 3). The mean time RESULTS error for these two sessions was −26.0% (SD = 24.3). The LMM indicated no significant differences between the How long is a minute? 2 test sessions in the control subjects and the astronauts On the ground, on average subjects indicated the duration of [F(1,23) = 0.023, p = 0.881], which suggests that there was no one minute to be 74.1 ± 19.5 s (mean ± SD) (Fig. 2). The LMM learning or adaptive effect due to the repetition of the test. No indicated no significant differences between the 3 test sessions significant differences were detected between the 2 groups of in the control subjects and the astronauts [F(2,46) = 0.58, subjects [F(1,23) = 0.005, p = 0.942]. p = 0.56], which suggests that there was no learning or adaptive We compared whether the perceived durations were different effect due to the repetition of the test. No significant differences within each flight phase. A LMM indicated that these differences were detected between the 2 groups of subjects [F(1,23) = 0.008, were only significant within the post-flight measurements p = 0.928]. [F(2,27) = 4.913, p = 0.009), and particularly between R + 1 and A LMM on the perceived durations of 1 min during the test R +4(p = 0.018) and between R + 1 and R +8(p = 0.027). We sessions performed within each flight phase indicated that there then pooled measurements within each flight phases and we were no significant differences within the sessions pre-flight conducted another LMM, which indicated no significant main [F(2,18) = 0.939, p = 0.409], in-flight [F(5,45) = 0.593, p = 0.705] and post-flight [F(2,18) = 1.212, p = 0.321]. When measurements effect of flight phases [F(2,98) = 2.390, p = 0.097]. within each flight phases were pooled, the LMM indicated a significant main effect of flight phases [F(2,108) = 15.050, Duration since start of workday and lunch p < 0.001]. Post-hoc tests indicated that this difference was On the ISS, a typical workday begins with the morning DPC significant between pre-flight and in-flight (p < 0.001) and between the astronauts and the mission control center. On between post-flight and in-flight (p = 0.002), but not significant average, this experiment took place 4.4 ± 1.0 h (mean ± SD) after between pre-flight and post-flight (p = 0.484). During the flight, the end of the DPC. The same interval was used when testing the the averaged perceived duration of one minute was 59.6 ± 9.1 s control subjects in the laboratory (4.5 ± 1.1 h). Two separated (mean ± SD), which corresponds to a 20.0% decrease from before LMMs indicated that there were no significant differences in the flight (74.5 ± 20.2 s). Interestingly, although the perceived duration perceived duration since the beginning of the workday between of one minute was less during flight relative to before flight, the the 3 pre-flight sessions in the control subjects [F(2,42) = 0.286, perception during flight was essentially accurate throughout the 6 p = 0.880], and between the 6 pre-flight sessions in the astronauts in-flight sessions. [F(5,54) = 0.469, p = 0.797). However, there were significant differences between the perceived durations of the astronauts Duration between test sessions in-flight and the control subjects on the ground (Mann–Whitney, The time interval between the test sessions was approximately the p = 0.001) (Fig. 4a). Overall, the astronauts underestimated the same before flight (34–56 days) and for sessions FD45 to R + 1 duration since the beginning of their workday by −14.2% (SD = 24.2%). Fig. 2 How long is a minute. Box and whisker plots of 10 astronauts’ perceived duration of one minute before (L−), during Fig. 3 How long since last test session. Box and whisker plots of (FD), and after (R+) spaceflight. Filled symbol represents the mean, the error in 10 astronauts’ perceived duration since the last test center line represents the median, bounds of box represent the first session before (L−), during (FD), and after (R+) spaceflight. Filled and third quartiles, and whiskers represent the minimum and symbol represents the mean, center line represents the median, maximum values in the set. The dotted line represents the average bounds of box represent the first and third quartiles, and whiskers of all pre-flight measurements in the astronaut group. The gray area represent the minimum and maximum values in the set. The dotted represents the mean ± standard deviation of measurements taken line represents the average of all pre-flight measurements in the from 15 control subjects in the laboratory during 3 test sessions astronaut group. The gray area represents the mean ± standard separated by approximately one month. L− Days before launch, FD deviation of measurements taken from 15 control subjects in the Flight days, R+ Days after return. *p < 0.05 (linear mixed model). laboratory. *p < 0.05 (linear mixed model). Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2023) 6 D.C. Navarro Morales et al. Fig. 5 Time error for durations in days. Box and whisker plots of the error in 10 astronauts’ perceived duration since the last vehicle docking to the ISS (a) and the last spacewalk (Extra-Vehicular Activity, EVA) (b). Filled symbol represents the mean, center line represents the median, bounds of box represent the first and third quartiles, and whiskers represent the minimum and maximum values in the set. *p < 0.05 (one-sample t-test). Fig. 4 Time error for durations in hours. Box and whisker plots of the error in perceived duration since the start of the work day (a) and since lunch (b) in 10 astronauts during spaceflight and these durations because we did not have access to the astronaut’s 15 subjects during 3 sessions (S1, S2, S3) in the laboratory. Filled schedule during their training. symbol represents the mean, center line represents the median, bounds of box represent the first and third quartiles, and whiskers Duration since last docking and extra-vehicular activity (EVA) represent the minimum and maximum values in the set. *p < 0.05 (Sample Mann–Whitney test). The docking of a vehicle to the ISS occurred an average of 26.0 ± 12.4 days (mean ± SD) before administering the ques- tionnaire. Astronauts were quite accurate in their estimations of this duration: their errors were less than one day (+2.2%) On average, the experiment occurred 2.7 ± 0.7 h (mean ± SD) (Fig. 5a). A LMM indicated that there were no significant after lunch in orbit and 2.5 ± 0.5 h after lunch in the laboratory. differences in the perceived duration since the last docking Two separated LMMs indicated that there were no significant between the 6 in-flight sessions [F(5,45) = 0.695, p = 0.630]. differences between the 3 pre-flight sessions in the control However, this time error was not significantly different from subjects [F(2,42) = 0.014, p = 0.986], and between the 6 in-flight zero (t-test, p = 0.797). sessions in the astronauts [F(5,45) = 0.591, p = 0.707]. However, Similarly, the EVAs occurred an average of 49.4 ± 15.9 days there were significant differences between the perceived (mean ± SD) before administering the questionnaire. The time durations of the astronauts in-flight and the control subjects error for the estimations of this duration was 2.8 ± 11.9 days on the ground (Mann–Whitney, p = 0.037) (Fig. 4a, b). Overall, (+5.6%) (Fig. 5b). A LMM indicated that there were no the astronauts underestimated the duration since their lunch by significant differences in the perceived duration since the last −19.2% (SD = 36.1%). It is possible, however, that the astronauts could have under- EVA between the 6 in-flight sessions [F(5,45) = 0.312, p = 0.903]. estimated the duration after the start of their workday and/or However, this time error was not significantly different from lunch before the flight. Unfortunately, we could not measure zero (t-test, p = 0.167). npj Microgravity (2023) 6 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA D.C. Navarro Morales et al. DISCUSSION operations such as vehicle docking and EVA are documented in mission elapsed time, i.e. the time elapsed since the launch or This study indicates that astronauts’ perceived the duration of one entry into the airlock. minute to be less during spaceflight (−20.0%) than before flight. The conditions during spaceflight, including weightlessness, In addition, they underestimated durations ranging from 2 h (since prolonged isolation in confined areas, stress related to workload, lunch) and 5 h (since start of their workday) by −36.1% and and high-performance expectations, are known to affect human −24.2%, respectively. They also underestimated the time elapsed 6,12 physiological and psychological responses . These conditions since the last test session when there was a change in gravity level could also alter the perceived temporal relationships between between these sessions (−26.5% on R + 1). However, they were events. During his 8-min EVA, Alexey Leonov clearly experienced essentially correct in estimating the durations in days elapsed an underestimation of time: ‘I was disappointed,’ he wrote in his since the last docking of a vehicle to the ISS (+2.2%) and since the post-flight report, ‘that the time assigned for working outside the last EVA (+5.6%). craft flew by very quickly. The entire period I remained in outer The method for assessing time perception of a 1-min time space seemed to be only 1 or 2 min.’ period is the method of production, i.e. indicating a 1-min interval Only 2 studies have been performed previously to assess time by pressing a button. Before the flight, a clock time of on average perception during spaceflight. The firstexperimenttook place 74.1 s was judged as 60 s on average by the subjects. In other after the historical one-orbit flight of Yuri Gagarin, when words, at a clock time of 60 s the subjects still thought it was Gherman Titov flew on the Vostok-2 for a full day (17 Earth perhaps 50 s, and waited longer before pressing the button. orbits). The objective was to assess his ability to evaluate time Similarly, the relative underproduction during the flight by the intervals. After starting a stopwatch, he began to count 20 s in his astronauts (59.6 s) as compared to before (74.5 s) refers to a head; when he estimated subjectively that 20 s had passed, he relative overestimation of duration. In other words, astronauts in stopped the stopwatch and looked at the actual elapsed time. space feel that more time (the 60 s) has gone by after 60 s (they The results were recorded in his onboard diary. The average time then pressed the button) than the astronauts on the ground who estimates during the 4 in-flight sessions were 20.3, 20.2, 20.1, and at 60 s felt a relatively shorter duration (say, 50 s) and waiting a 20.1 s. These estimates were not significantly different from little longer to press the button (on average at 74.5 s). This is a those measured during training, but they were biased by the fact classic dissociation in the interpretation of a time production and that he was counting in his head and he had continuous a time estimation task. In the latter the observer waits through the feedbacks on his performance . designated time and then verbally reports clock time (‘about 60 s). The second experiment on time perception during spaceflight The fact that there is a relative overestimation for the 1-min was performed on 4 astronauts during a 4-day Space Shuttle interval is not at odds with the relative underestimation of mission. The test was a classic time reproduction (non-counting) intervals in the hours range. There is ample evidence that different task. Subjects viewed a visual target traversing a display and, time intervals are governed by different mechanisms. The 1-min while it was obscured, estimated the time of its arrival at a interval could still be just within the working memory span (and a predetermined point by any means other than counting. The clock mechanism could apply) but the hour time range is way target moved at various speeds, so that the duration of the task 22,23 beyond and memory processes apply . ranged from 2 to 16 s. The 4 astronauts were tested the day Why are the astronauts actually more accurate in their estimate before their flight, each day during the flight, 3 h after landing, of minutes during spaceflight? One possible interpretation is 25 and again 3 days later . As the time duration of the task related to the way the astronauts’ work is organized on board the increased, the subjects tended to underestimate duration and ISS. By every ISS workstation, there is a daily minute-by-minute these errors in duration estimates increased each day as the flight schedule displayed on a computer (the Onboard Short-Term Plan progressed. Three hours after landing the duration estimates were Viewer, or OSTPV). Superimposed on this schedule is a vertical red 25 also significantly larger than on FD4 . bar that moves from left to right and symbolizes the passing of The results of the above experiment and the present study time. Each day, the astronauts perform different activities (e.g. suggest that the ability to estimate brief intervals of time experiments, equipment maintenance, taking photographs, etc.) deteriorates during a space mission and shortly after landing. scheduled at a given time for a given duration. Consequently, Similar effects were also observed in subjects exposed to crewmembers may have lunch at different time from one day to hypergravity in a centrifuge . A potential consequence of these another. The large variability in the astronauts’ time errors for effects is that crewmembers who need to make quick decisions and estimating the time elapsed since the beginning of the workday perform critical tasks during flight and re-entry may exhibit some and lunch is presumably due to the different type and schedule of delays in their responses, which could compromise safety. activities they perform each day. The red bar displayed on the Gibson pointed out that some measures of time are intrinsic, OSTPV indicates when crewmembers must start and end each i.e., they are physical phenomena ‘out there’ in the world. These activity. The speed of the red bar movement, and the comparison include the year (the Earth’s rotation around the sun which, due to between the perceived time for completing an activity and the the tilt of the Earth’s axis, yields a sequence of seasons), the month actual time when crewmember looks at the OSTPV presumably (the Moon’s rotation around the Earth, with its visible phases), and contribute to them becoming more accurate in their estimates of the day (the Earth’s rotation around its own axis, yielding dawn, minutes during spaceflight. Noon, dusk, and so on). These intrinsic measures of time contrast , an observer According to Einstein’s theory of relativity with the extrinsic measures of time, such as seconds, minutes, and traveling at high speed will experience time passing more slowly hours. Whereas the duration of the year, the month, and the day than an observer at rest. At the speed of the International Space are fixed by physical facts, the duration of the second is arbitrary; −11 Station (28,160 km/h), this difference is in the order to 4 ×10 s. it is a convention that works solely because many people have This effect is negligible in our results, which show changes in the agreed to it. The duration of the second can be changed. We order of minutes or days. could, if we wished, have only 10 h in the day, with minutes and A confounding factor for accurate time perception during seconds that were much longer. By contrast, we cannot change space operations is the fact that different time scales are used on the physical duration of the day. The day is an event, in Gibson’s the ISS, which can be confusing. The official time is Greenwich sense, and is perceivable as any physical event. The second is not Mean Time (GMT), but the astronauts and cosmonauts often use an event—it does not exist ‘out there’ to be perceived, but exists Houston time (GMT-5 h) or Moscow time (GMT + 2 h) to commu- only in the mind, as a social convention. In addition, circadian and nicate with mission control centers and their families. Also, critical circalunar rhythms follow 24-h and 30-day cycles, respectively. Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2023) 6 D.C. Navarro Morales et al. The results of our study indicate that astronauts are quite accurate experimental procedures as on Earth. Less stimulation of the when estimating intrinsic measures of time (days, months), but are vestibular system could lead to an underestimation of the duration inaccurate when estimating extrinsic measures of time (seconds, of time in weightlessness. In agreement with this hypothesis, 1,2 42 minutes, hours). However, the accuracy when estimating intrinsic perception of both distances and durations are altered during measures of time (days) is altered following transitions between transient exposure to weightlessness in parabolic flight. gravity levels (0 g to 1 g). Our previous studies indicated that astronauts’ subjective The results of psychophysical experiments on Earth indicate perception of their body motion and position and the size and 3,4 that subjects have distorted time perception when they are in distance of objects were altered during spaceflight . The present stressful situations . For example, during dangerous events such study shows that astronauts’ perception of durations ranging from as plane and car accidents, many people report an overestimation one minute to several hours (which are human conventions) is to time: fractions of seconds can be perceived as minutes . also altered during long-duration spaceflight, and strikingly so in During parachute jumps, subjects tend to overestimate the time about the same percentage as for perceived distances .No intervals and delay the opening of their parachute . By contrast, significant differences were seen across flight days, which indicate the results of our study indicate that subjects either under- that these alterations occurred within 2 weeks in orbit and that no estimate periods of time, or report accurate time perception. It is adaptation took place during long-duration spaceflight. This therefore unlikely that the results of this study are due to the observation raises operational concerns regarding the ability for stress of spaceflight. crewmembers to manually perform docking and landing man- Depending on the situation, time is perceived as either passing euvers after two weeks in orbit without any assistance. slowly or flying by. When we are bored, we feel that time passes These results also support the existence of an overlapping more slowly than when we are entertained. Impulsive people feel perception of time and space. It has been proposed that that time is excruciatingly slow when nothing is happening . representations of space and time both share the same metrics Time experience and time judgments are also altered in and cortical network, presumably located in the right parietal 29 43 depressive and manic patients . In general, institutionalized cortex (for stimuli shorter than one-second duration) . A potential individuals, such as individuals in homes for the elderly, whose neuronal basis for the interaction between these representations days are highly regulated and monotonous, experience time as comes from neurophysiological recordings in rodents hippocam- 30,31 passing slowly, i.e. they overestimate time durations . But the pus and enthorhinal cortex (which have connections with the same individuals may also experience time as speeding up when parietal cortex) showing that place and grid cells can simulta- 44,45 enjoyable and memorable events occur, such as a visit from neously code for space and time . In agreement with this 32,33 family members or social events . This effect is also likely to interpretation, recent brain imaging studies have shown that the happen during spaceflight. As indicated above, astronauts’ spontaneous or evoked activity in the right parietal cortex was 46–50 working days are filled with various activities in a timeline that modified in astronauts after spaceflight . changes from one day to another. During this busy schedule, time is perceived as going faster, and durations in minutes or DATA AVAILABILITY hours are therefore underestimated . Whereas, when unique Data will be made available on reasonable request to Dr. Gilles Clement events take place, such as vehicle docking and EVA, these days (gilles.clement@unicaen.fr). are memorized more accurately. Vicario et al. . showed that optokinetic stimulation influences time perception. Subjects overestimated time intervals after Received: 28 May 2022; Accepted: 10 January 2023; optokinetic stimulation compared with their estimations before optokinetic stimulation. 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Neuropsychologia 51, 197–210 (2013). Attribution 4.0 International License, which permits use, sharing, 37. Hitier, M., Besnard, S. & Smith, P. F. Vestibular pathways involved in cognition. adaptation, distribution and reproduction in any medium or format, as long as you give Front. Integr. Neurosci. 8, 59 (2014). appropriate credit to the original author(s) and the source, provide a link to the Creative 38. Jacob, R. G. & Furman, J. M. Psychiatric consequences of vestibular dysfunction. Commons license, and indicate if changes were made. The images or other third party Curr. Opin. Neurol. 14,41–46 (2001). material in this article are included in the article’s Creative Commons license, unless 39. Smith, P. F., Zheng, Y., Horii, A. & Darlington, C. L. Does vestibular damage cause indicated otherwise in a credit line to the material. If material is not included in the cognitive dysfunction in humans? J. Vestib. Res. 15,1–9 (2005). article’s Creative Commons license and your intended use is not permitted by statutory 40. Hanes, D. A. & McCollum, G. Cognitive-vestibular interactions: a review of patient regulation or exceeds the permitted use, you will need to obtain permission directly difficulties and possible mechanisms. J. Vestib. Res. 95, 343–348 (2006). from the copyright holder. To view a copy of this license, visit http:// 41. Clément, G., Fraysse, M. J. & Deguine, O. Mental representation of space in ves- creativecommons.org/licenses/by/4.0/. tibular patients with otolithic or rotatory vertigo. NeuroReport 20, 457–461 (2009). 42. Clément, G. Perception of time in microgravity and hypergravity during parabolic flight. 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Time perception in astronauts on board the International Space Station

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www.nature.com/npjmgrav ARTICLE OPEN Time perception in astronauts on board the International Space Station 1 1 1 1 1✉ Deborah C. Navarro Morales , Olga Kuldavletova , Gaëlle Quarck , Pierre Denise and Gilles Clément We perceive the environment through an elaborate mental representation based on a constant integration of sensory inputs, knowledge, and expectations. Previous studies of astronauts on board the International Space Station have shown that the mental representation of space, such as the perception of object size, distance, and depth, is altered in orbit. Because the mental representations of space and time have some overlap in neural networks, we hypothesized that perception of time would also be affected by spaceflight. Ten astronauts were tested before, during, and after a 6–8-month spaceflight. Temporal tasks included judging when one minute had passed and how long it had been since the start of the workday, lunch, docking of a vehicle, and a spacewalk. Compared to pre-flight estimates, there is a relative overestimation for the 1-min interval during the flight and a relative underestimation of intervals of hours in duration. However, the astronauts quite accurately estimated the number of days since vehicle dockings and spacewalks. Prolonged isolation in confined areas, stress related to workload, and high-performance expectations are potential factors contributing to altered time perception of daily events. However, reduced vestibular stimulations and slower motions in weightlessness, as well as constant references to their timeline and work schedule could also account for the change in the estimation of time by the astronauts in space. npj Microgravity (2023) 9:6 ; https://doi.org/10.1038/s41526-023-00250-x INTRODUCTION decision making and alters their ability to control the vehicle and their movements . The neurovestibular challenges that occur To construct a mental representation of our world, we perceive when the crewmember returns to normal gravity include our environment by constantly processing sensory inputs from the 16 17 alterations in manual control , inability to egress the vehicle , visual, vestibular, and somatosensory systems. This representation 18 19 postural imbalance , and impaired locomotion . is also influenced by our expectations and our experience, which Einstein revolutionized physics a century ago with his theory are derived from our knowledge of the costs and consequences of that space and time are intertwined . Based on this theory, we acting in this environment. The central neurovestibular system hypothesized that the absence of gravitational reference alters the naturally takes gravity into account during spatial orientation, construction of the mental representation of both space and time. balance, and motor control. This system is also indispensable for Because astronauts underestimate distances and because the constructing our mental representation of the world. It continu- pace of their motion is slower in weightlessness, we hypothesized ously processes data from the visual, vestibular, and somatosen- that astronauts would also underestimate the relative time sory channels to update our spatial maps. Previous research between events. Prior to the present study, subjective perception suggests that distances are underestimated when subjects are in 1,2 3 of time during long-duration spaceflight had not been investi- weightlessness during parabolic or orbital flight. This distance gated. The results of this study have operational implications: underestimation is thought to be due to adaptive changes in the altered perception of time during spaceflight might impact processing of gravitational information by the neurovestibular 4,5 operations that require critical timing, such as docking operations system that alter the construction of spatial maps . and piloted landings. Exposure to weightlessness during spaceflight is known to elicit changes in vestibular responses, i.e., orientation illusions, errors in sensory localization, changes in vestibulo-ocular reflexes, and 6 METHODS space motion sickness . Cognitive tasks involving the neuroves- 7–9 10 Participants tibular system, such as mental rotation , perceived orientation and judgments of distance are also affected during spaceflight. 10 healthy crewmembers (9 male, 1 female; age M = 44.1, Some astronauts and cosmonauts have reported a ‘time SD = 4.6) who flew on the International Space Station (ISS) compression syndrome’ in orbit, whereby they perceive time as participated in this study. All crewmembers passed a United States compressed relative to the perceptions gained during training and Air Force Class III medical examination and had no known history simulation . In weightlessness, routine tasks require different of vestibular or oculomotor abnormalities. 15 healthy subjects (6 cognitive demands than they would on Earth. Astronauts in orbit females, 9 males; age M = 43.2, SD = 18.8) participated in a control also report that they require more time than normal to execute study in the laboratory. 12,13 standard mental activity . ‘Space fog’ is another reported The test procedures were approved by the European Space syndrome that affects cognitive performance during the first Agency Medical Board and the NASA Johnson Space Center weeks of a mission . After astronauts have adapted to weight- Institutional Review Board and were performed in accordance lessness and they re-enter the atmosphere, they also encounter a with the ethical standards laid down in the 1964 Declaration of condition called ‘entry motion sickness’, which slows the speed of Helsinki. All subjects gave a written informed consent before UNICAEN, INSERM, CHU Caen, Normandy University, COMETE, CYCERON, Esplanade de la Paix, 14032 Caen, France. email: gilles.clement@unicaen.fr Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; D.C. Navarro Morales et al. participating in the study. Informed consent was obtained from spatial orientation. Previous studies have demonstrated that the the subject for publication of identifying information/images in an perception of distance and the depth of objects are altered when 3,4 online open-access publication. free-floating in orbit . To investigate the relationship between these changes in spatial perception and changes in time perception, the psychophysics test was performed in the same Experimental protocol conditions as the previous spatial perception tests. In the flight study, the tests were performed before, during, and The second part of the experiment included a series of after 6–8-month spaceflights (M = 202, SD = 28 days). The questions to document the potential changes in the astronauts’ pre-flight test sessions occurred at launch minus (L-) 205 ± 51 days, perception of longer periods of time, i.e. hours and days, while in L-149 ± 55 days, and L-116 ± 45 days. In-flight test sessions were orbit. The subjects doffed the head-mounted display and used the conducted approximately every month: i.e., on flight day (FD) laptop keyboard to answer the following 5 questions: (a) How long FD17 ± 6 (M ± SD), FD46 ± 8, FD71 ± 6, FD99 ± 7, FD134 ± 8, and has it been since the last time you performed this test?; (b) How FD164 ± 7. After the astronauts returned to Earth, tests were long has it been since you started your work day?; (c) How long performed at return plus (R + ) 1 day, R + 5 ± 1 day, and has it been since you had your lunch?; (d) How long has it been R + 9 ± 1 day. since the last vehicle docked at the ISS?; and (e) How long has it A psychophysics test was administered, during which the been since the last extra-vehicular activity (EVA)? subjects were asked to judge when one minute had passed. In the flight study, the start of the workday was defined as the During this test, subjects wore a head-mounted display (Oculus termination of the morning daily planning conference (DPC), Rift, Oculus VR, Menlo Park, CA), and used a finger trackball which occurred about one hour after the crew awoke. The time of connected to a laptop to report their responses. Subjects were lunch was determined by the astronauts’ daily schedule. The dates wearing earphones for listening to the instructions and for of docking and EVA was determined from the Flight Program attenuating/masking noises from the spacecraft. Using the finger Integration Panel document provided by NASA Mission Integra- mouse, they pressed a ‘go’ button and waited one minute before tions and Operations Office. Because this information was not pressing on a ‘stop’ button. Only the ‘go’ and’stop’ buttons were available before and after the flight, the questionnaire was only displayed during the test. Subjects were not allowed to count the administered during the flight. seconds passing by. In the ground-based study, the subjects performed the ‘How On the ground, this test was performed in the seated upright Long is a Minute?’ test in the laboratory while sitting upright and position; on the ISS, astronauts were in the free-floating conditions using identical computer hardware and software as on board the (Fig. 1). During the free-floating conditions there are no ISS. The subjects were also asked the perceived number of days proprioceptive, tactile, or static vestibular cues that participate in since the last test session and how many minutes since they woke up and had breakfast (which they noted in a diary). The number of days between test sessions of the control subjects in the laboratory (M = 44.1, SD = 10.2) was similar to the number of days separating the 3 pre-flight sessions with the astronauts (M = 45.2, SD = 28.4), and so were the durations since wake up and lunch. Statistical analysis The errors between the perceived durations and the actual durations were calculated and time errors were computed in terms of percentage or days. First, linear mixed models (LMM) were used to compare the ground-based responses of the 2 subject groups (astronauts and controls) and to establish whether they differed, and whether the results of the 3 test sessions differed (dependent variable: time error; fixed effects: tests sessions; group: astronauts or controls; random effects: subjects). A second set of LMM was used to compare measurements from different sessions within the flight phases (pre, in, post) in astronauts (dependent variable: time error; fixed effect: test sessions; random effects: subjects). A third set of LMM was used to compare the time errors during the pre-, in-, and post-flight sessions (depended variable: time error; fixed effects: flight phase; random effects: subjects). Post-hoc pairwise compar- isons were then conducted using Bonferroni adjustment for multiple comparisons. Fixed effects estimates, confidence limits, and random effects standard deviations of these LMMs are provided in Supplementary Table 1. When preflight measurements were not available in the astronauts (perceived durations since beginning of workday and lunch), an independent Sample Mann–Whitney test was used to compare their inflight responses with those of the control subjects on the ground. When these responses were not available in the control subjects neither (perceived durations since last docking and last EVA) a one-sample t-test was used to determine if the time errors during the flight were different from zero. Statistical analysis was performed with JASP 0.16.1.0 and IBM SPSS Statistics Fig. 1 Astronaut performing the experiment while free-floating on board the ISS. Photo credit NASA. 27.0 software. npj Microgravity (2023) 6 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; D.C. Navarro Morales et al. Reporting summary (31.0 ± 10 d). The time interval between the last pre-flight session (L-116) and FD17 was much longer (133.8 ± 43 d). When Further information on research design is available in the Nature comparing the astronaut’s perceived duration between the test Research Reporting Summary linked to this article. sessions, the largest time errors occurred on FD17 and R + 1, i.e., shortly after the transition in gravity levels (Fig. 3). The mean time RESULTS error for these two sessions was −26.0% (SD = 24.3). The LMM indicated no significant differences between the How long is a minute? 2 test sessions in the control subjects and the astronauts On the ground, on average subjects indicated the duration of [F(1,23) = 0.023, p = 0.881], which suggests that there was no one minute to be 74.1 ± 19.5 s (mean ± SD) (Fig. 2). The LMM learning or adaptive effect due to the repetition of the test. No indicated no significant differences between the 3 test sessions significant differences were detected between the 2 groups of in the control subjects and the astronauts [F(2,46) = 0.58, subjects [F(1,23) = 0.005, p = 0.942]. p = 0.56], which suggests that there was no learning or adaptive We compared whether the perceived durations were different effect due to the repetition of the test. No significant differences within each flight phase. A LMM indicated that these differences were detected between the 2 groups of subjects [F(1,23) = 0.008, were only significant within the post-flight measurements p = 0.928]. [F(2,27) = 4.913, p = 0.009), and particularly between R + 1 and A LMM on the perceived durations of 1 min during the test R +4(p = 0.018) and between R + 1 and R +8(p = 0.027). We sessions performed within each flight phase indicated that there then pooled measurements within each flight phases and we were no significant differences within the sessions pre-flight conducted another LMM, which indicated no significant main [F(2,18) = 0.939, p = 0.409], in-flight [F(5,45) = 0.593, p = 0.705] and post-flight [F(2,18) = 1.212, p = 0.321]. When measurements effect of flight phases [F(2,98) = 2.390, p = 0.097]. within each flight phases were pooled, the LMM indicated a significant main effect of flight phases [F(2,108) = 15.050, Duration since start of workday and lunch p < 0.001]. Post-hoc tests indicated that this difference was On the ISS, a typical workday begins with the morning DPC significant between pre-flight and in-flight (p < 0.001) and between the astronauts and the mission control center. On between post-flight and in-flight (p = 0.002), but not significant average, this experiment took place 4.4 ± 1.0 h (mean ± SD) after between pre-flight and post-flight (p = 0.484). During the flight, the end of the DPC. The same interval was used when testing the the averaged perceived duration of one minute was 59.6 ± 9.1 s control subjects in the laboratory (4.5 ± 1.1 h). Two separated (mean ± SD), which corresponds to a 20.0% decrease from before LMMs indicated that there were no significant differences in the flight (74.5 ± 20.2 s). Interestingly, although the perceived duration perceived duration since the beginning of the workday between of one minute was less during flight relative to before flight, the the 3 pre-flight sessions in the control subjects [F(2,42) = 0.286, perception during flight was essentially accurate throughout the 6 p = 0.880], and between the 6 pre-flight sessions in the astronauts in-flight sessions. [F(5,54) = 0.469, p = 0.797). However, there were significant differences between the perceived durations of the astronauts Duration between test sessions in-flight and the control subjects on the ground (Mann–Whitney, The time interval between the test sessions was approximately the p = 0.001) (Fig. 4a). Overall, the astronauts underestimated the same before flight (34–56 days) and for sessions FD45 to R + 1 duration since the beginning of their workday by −14.2% (SD = 24.2%). Fig. 2 How long is a minute. Box and whisker plots of 10 astronauts’ perceived duration of one minute before (L−), during Fig. 3 How long since last test session. Box and whisker plots of (FD), and after (R+) spaceflight. Filled symbol represents the mean, the error in 10 astronauts’ perceived duration since the last test center line represents the median, bounds of box represent the first session before (L−), during (FD), and after (R+) spaceflight. Filled and third quartiles, and whiskers represent the minimum and symbol represents the mean, center line represents the median, maximum values in the set. The dotted line represents the average bounds of box represent the first and third quartiles, and whiskers of all pre-flight measurements in the astronaut group. The gray area represent the minimum and maximum values in the set. The dotted represents the mean ± standard deviation of measurements taken line represents the average of all pre-flight measurements in the from 15 control subjects in the laboratory during 3 test sessions astronaut group. The gray area represents the mean ± standard separated by approximately one month. L− Days before launch, FD deviation of measurements taken from 15 control subjects in the Flight days, R+ Days after return. *p < 0.05 (linear mixed model). laboratory. *p < 0.05 (linear mixed model). Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2023) 6 D.C. Navarro Morales et al. Fig. 5 Time error for durations in days. Box and whisker plots of the error in 10 astronauts’ perceived duration since the last vehicle docking to the ISS (a) and the last spacewalk (Extra-Vehicular Activity, EVA) (b). Filled symbol represents the mean, center line represents the median, bounds of box represent the first and third quartiles, and whiskers represent the minimum and maximum values in the set. *p < 0.05 (one-sample t-test). Fig. 4 Time error for durations in hours. Box and whisker plots of the error in perceived duration since the start of the work day (a) and since lunch (b) in 10 astronauts during spaceflight and these durations because we did not have access to the astronaut’s 15 subjects during 3 sessions (S1, S2, S3) in the laboratory. Filled schedule during their training. symbol represents the mean, center line represents the median, bounds of box represent the first and third quartiles, and whiskers Duration since last docking and extra-vehicular activity (EVA) represent the minimum and maximum values in the set. *p < 0.05 (Sample Mann–Whitney test). The docking of a vehicle to the ISS occurred an average of 26.0 ± 12.4 days (mean ± SD) before administering the ques- tionnaire. Astronauts were quite accurate in their estimations of this duration: their errors were less than one day (+2.2%) On average, the experiment occurred 2.7 ± 0.7 h (mean ± SD) (Fig. 5a). A LMM indicated that there were no significant after lunch in orbit and 2.5 ± 0.5 h after lunch in the laboratory. differences in the perceived duration since the last docking Two separated LMMs indicated that there were no significant between the 6 in-flight sessions [F(5,45) = 0.695, p = 0.630]. differences between the 3 pre-flight sessions in the control However, this time error was not significantly different from subjects [F(2,42) = 0.014, p = 0.986], and between the 6 in-flight zero (t-test, p = 0.797). sessions in the astronauts [F(5,45) = 0.591, p = 0.707]. However, Similarly, the EVAs occurred an average of 49.4 ± 15.9 days there were significant differences between the perceived (mean ± SD) before administering the questionnaire. The time durations of the astronauts in-flight and the control subjects error for the estimations of this duration was 2.8 ± 11.9 days on the ground (Mann–Whitney, p = 0.037) (Fig. 4a, b). Overall, (+5.6%) (Fig. 5b). A LMM indicated that there were no the astronauts underestimated the duration since their lunch by significant differences in the perceived duration since the last −19.2% (SD = 36.1%). It is possible, however, that the astronauts could have under- EVA between the 6 in-flight sessions [F(5,45) = 0.312, p = 0.903]. estimated the duration after the start of their workday and/or However, this time error was not significantly different from lunch before the flight. Unfortunately, we could not measure zero (t-test, p = 0.167). npj Microgravity (2023) 6 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA D.C. Navarro Morales et al. DISCUSSION operations such as vehicle docking and EVA are documented in mission elapsed time, i.e. the time elapsed since the launch or This study indicates that astronauts’ perceived the duration of one entry into the airlock. minute to be less during spaceflight (−20.0%) than before flight. The conditions during spaceflight, including weightlessness, In addition, they underestimated durations ranging from 2 h (since prolonged isolation in confined areas, stress related to workload, lunch) and 5 h (since start of their workday) by −36.1% and and high-performance expectations, are known to affect human −24.2%, respectively. They also underestimated the time elapsed 6,12 physiological and psychological responses . These conditions since the last test session when there was a change in gravity level could also alter the perceived temporal relationships between between these sessions (−26.5% on R + 1). However, they were events. During his 8-min EVA, Alexey Leonov clearly experienced essentially correct in estimating the durations in days elapsed an underestimation of time: ‘I was disappointed,’ he wrote in his since the last docking of a vehicle to the ISS (+2.2%) and since the post-flight report, ‘that the time assigned for working outside the last EVA (+5.6%). craft flew by very quickly. The entire period I remained in outer The method for assessing time perception of a 1-min time space seemed to be only 1 or 2 min.’ period is the method of production, i.e. indicating a 1-min interval Only 2 studies have been performed previously to assess time by pressing a button. Before the flight, a clock time of on average perception during spaceflight. The firstexperimenttook place 74.1 s was judged as 60 s on average by the subjects. In other after the historical one-orbit flight of Yuri Gagarin, when words, at a clock time of 60 s the subjects still thought it was Gherman Titov flew on the Vostok-2 for a full day (17 Earth perhaps 50 s, and waited longer before pressing the button. orbits). The objective was to assess his ability to evaluate time Similarly, the relative underproduction during the flight by the intervals. After starting a stopwatch, he began to count 20 s in his astronauts (59.6 s) as compared to before (74.5 s) refers to a head; when he estimated subjectively that 20 s had passed, he relative overestimation of duration. In other words, astronauts in stopped the stopwatch and looked at the actual elapsed time. space feel that more time (the 60 s) has gone by after 60 s (they The results were recorded in his onboard diary. The average time then pressed the button) than the astronauts on the ground who estimates during the 4 in-flight sessions were 20.3, 20.2, 20.1, and at 60 s felt a relatively shorter duration (say, 50 s) and waiting a 20.1 s. These estimates were not significantly different from little longer to press the button (on average at 74.5 s). This is a those measured during training, but they were biased by the fact classic dissociation in the interpretation of a time production and that he was counting in his head and he had continuous a time estimation task. In the latter the observer waits through the feedbacks on his performance . designated time and then verbally reports clock time (‘about 60 s). The second experiment on time perception during spaceflight The fact that there is a relative overestimation for the 1-min was performed on 4 astronauts during a 4-day Space Shuttle interval is not at odds with the relative underestimation of mission. The test was a classic time reproduction (non-counting) intervals in the hours range. There is ample evidence that different task. Subjects viewed a visual target traversing a display and, time intervals are governed by different mechanisms. The 1-min while it was obscured, estimated the time of its arrival at a interval could still be just within the working memory span (and a predetermined point by any means other than counting. The clock mechanism could apply) but the hour time range is way target moved at various speeds, so that the duration of the task 22,23 beyond and memory processes apply . ranged from 2 to 16 s. The 4 astronauts were tested the day Why are the astronauts actually more accurate in their estimate before their flight, each day during the flight, 3 h after landing, of minutes during spaceflight? One possible interpretation is 25 and again 3 days later . As the time duration of the task related to the way the astronauts’ work is organized on board the increased, the subjects tended to underestimate duration and ISS. By every ISS workstation, there is a daily minute-by-minute these errors in duration estimates increased each day as the flight schedule displayed on a computer (the Onboard Short-Term Plan progressed. Three hours after landing the duration estimates were Viewer, or OSTPV). Superimposed on this schedule is a vertical red 25 also significantly larger than on FD4 . bar that moves from left to right and symbolizes the passing of The results of the above experiment and the present study time. Each day, the astronauts perform different activities (e.g. suggest that the ability to estimate brief intervals of time experiments, equipment maintenance, taking photographs, etc.) deteriorates during a space mission and shortly after landing. scheduled at a given time for a given duration. Consequently, Similar effects were also observed in subjects exposed to crewmembers may have lunch at different time from one day to hypergravity in a centrifuge . A potential consequence of these another. The large variability in the astronauts’ time errors for effects is that crewmembers who need to make quick decisions and estimating the time elapsed since the beginning of the workday perform critical tasks during flight and re-entry may exhibit some and lunch is presumably due to the different type and schedule of delays in their responses, which could compromise safety. activities they perform each day. The red bar displayed on the Gibson pointed out that some measures of time are intrinsic, OSTPV indicates when crewmembers must start and end each i.e., they are physical phenomena ‘out there’ in the world. These activity. The speed of the red bar movement, and the comparison include the year (the Earth’s rotation around the sun which, due to between the perceived time for completing an activity and the the tilt of the Earth’s axis, yields a sequence of seasons), the month actual time when crewmember looks at the OSTPV presumably (the Moon’s rotation around the Earth, with its visible phases), and contribute to them becoming more accurate in their estimates of the day (the Earth’s rotation around its own axis, yielding dawn, minutes during spaceflight. Noon, dusk, and so on). These intrinsic measures of time contrast , an observer According to Einstein’s theory of relativity with the extrinsic measures of time, such as seconds, minutes, and traveling at high speed will experience time passing more slowly hours. Whereas the duration of the year, the month, and the day than an observer at rest. At the speed of the International Space are fixed by physical facts, the duration of the second is arbitrary; −11 Station (28,160 km/h), this difference is in the order to 4 ×10 s. it is a convention that works solely because many people have This effect is negligible in our results, which show changes in the agreed to it. The duration of the second can be changed. We order of minutes or days. could, if we wished, have only 10 h in the day, with minutes and A confounding factor for accurate time perception during seconds that were much longer. By contrast, we cannot change space operations is the fact that different time scales are used on the physical duration of the day. The day is an event, in Gibson’s the ISS, which can be confusing. The official time is Greenwich sense, and is perceivable as any physical event. The second is not Mean Time (GMT), but the astronauts and cosmonauts often use an event—it does not exist ‘out there’ to be perceived, but exists Houston time (GMT-5 h) or Moscow time (GMT + 2 h) to commu- only in the mind, as a social convention. In addition, circadian and nicate with mission control centers and their families. Also, critical circalunar rhythms follow 24-h and 30-day cycles, respectively. Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2023) 6 D.C. Navarro Morales et al. The results of our study indicate that astronauts are quite accurate experimental procedures as on Earth. Less stimulation of the when estimating intrinsic measures of time (days, months), but are vestibular system could lead to an underestimation of the duration inaccurate when estimating extrinsic measures of time (seconds, of time in weightlessness. In agreement with this hypothesis, 1,2 42 minutes, hours). However, the accuracy when estimating intrinsic perception of both distances and durations are altered during measures of time (days) is altered following transitions between transient exposure to weightlessness in parabolic flight. gravity levels (0 g to 1 g). Our previous studies indicated that astronauts’ subjective The results of psychophysical experiments on Earth indicate perception of their body motion and position and the size and 3,4 that subjects have distorted time perception when they are in distance of objects were altered during spaceflight . The present stressful situations . For example, during dangerous events such study shows that astronauts’ perception of durations ranging from as plane and car accidents, many people report an overestimation one minute to several hours (which are human conventions) is to time: fractions of seconds can be perceived as minutes . also altered during long-duration spaceflight, and strikingly so in During parachute jumps, subjects tend to overestimate the time about the same percentage as for perceived distances .No intervals and delay the opening of their parachute . By contrast, significant differences were seen across flight days, which indicate the results of our study indicate that subjects either under- that these alterations occurred within 2 weeks in orbit and that no estimate periods of time, or report accurate time perception. It is adaptation took place during long-duration spaceflight. This therefore unlikely that the results of this study are due to the observation raises operational concerns regarding the ability for stress of spaceflight. crewmembers to manually perform docking and landing man- Depending on the situation, time is perceived as either passing euvers after two weeks in orbit without any assistance. slowly or flying by. When we are bored, we feel that time passes These results also support the existence of an overlapping more slowly than when we are entertained. Impulsive people feel perception of time and space. It has been proposed that that time is excruciatingly slow when nothing is happening . representations of space and time both share the same metrics Time experience and time judgments are also altered in and cortical network, presumably located in the right parietal 29 43 depressive and manic patients . In general, institutionalized cortex (for stimuli shorter than one-second duration) . A potential individuals, such as individuals in homes for the elderly, whose neuronal basis for the interaction between these representations days are highly regulated and monotonous, experience time as comes from neurophysiological recordings in rodents hippocam- 30,31 passing slowly, i.e. they overestimate time durations . But the pus and enthorhinal cortex (which have connections with the same individuals may also experience time as speeding up when parietal cortex) showing that place and grid cells can simulta- 44,45 enjoyable and memorable events occur, such as a visit from neously code for space and time . In agreement with this 32,33 family members or social events . This effect is also likely to interpretation, recent brain imaging studies have shown that the happen during spaceflight. As indicated above, astronauts’ spontaneous or evoked activity in the right parietal cortex was 46–50 working days are filled with various activities in a timeline that modified in astronauts after spaceflight . changes from one day to another. During this busy schedule, time is perceived as going faster, and durations in minutes or DATA AVAILABILITY hours are therefore underestimated . Whereas, when unique Data will be made available on reasonable request to Dr. Gilles Clement events take place, such as vehicle docking and EVA, these days (gilles.clement@unicaen.fr). are memorized more accurately. Vicario et al. . showed that optokinetic stimulation influences time perception. Subjects overestimated time intervals after Received: 28 May 2022; Accepted: 10 January 2023; optokinetic stimulation compared with their estimations before optokinetic stimulation. 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Neuropsychologia 51, 197–210 (2013). Attribution 4.0 International License, which permits use, sharing, 37. Hitier, M., Besnard, S. & Smith, P. F. Vestibular pathways involved in cognition. adaptation, distribution and reproduction in any medium or format, as long as you give Front. Integr. Neurosci. 8, 59 (2014). appropriate credit to the original author(s) and the source, provide a link to the Creative 38. Jacob, R. G. & Furman, J. M. Psychiatric consequences of vestibular dysfunction. Commons license, and indicate if changes were made. The images or other third party Curr. Opin. Neurol. 14,41–46 (2001). material in this article are included in the article’s Creative Commons license, unless 39. Smith, P. F., Zheng, Y., Horii, A. & Darlington, C. L. Does vestibular damage cause indicated otherwise in a credit line to the material. If material is not included in the cognitive dysfunction in humans? J. Vestib. Res. 15,1–9 (2005). article’s Creative Commons license and your intended use is not permitted by statutory 40. Hanes, D. A. & McCollum, G. Cognitive-vestibular interactions: a review of patient regulation or exceeds the permitted use, you will need to obtain permission directly difficulties and possible mechanisms. J. Vestib. Res. 95, 343–348 (2006). from the copyright holder. To view a copy of this license, visit http:// 41. Clément, G., Fraysse, M. J. & Deguine, O. Mental representation of space in ves- creativecommons.org/licenses/by/4.0/. tibular patients with otolithic or rotatory vertigo. NeuroReport 20, 457–461 (2009). 42. Clément, G. Perception of time in microgravity and hypergravity during parabolic flight. NeuroReport 29, 247–251 (2018). © The Author(s) 2023 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2023) 6

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