Time perception in astronauts on board the International Space Station

Our mental representation of the world is a constant integration of sensory inputs, knowledge, and expectations. Previous research on astronauts aboard the International Space Station (ISS) has demonstrated alterations in spatial perception (size, distance, depth) in microgravity. Given the overlap in neural networks involved in spatial and temporal processing, this study hypothesized that time perception would also be affected by spaceflight. The neurovestibular system, crucial for spatial orientation and balance, is significantly impacted by weightlessness, leading to changes in vestibular responses, cognitive tasks, and even reported syndromes like "time compression syndrome" and "space fog." Einstein's theory of relativity, suggesting the intertwining of space and time, further supports this hypothesis. Prior to this study, subjective time perception during long-duration spaceflight had not been thoroughly investigated, highlighting the operational implications for time-critical tasks like docking and landings.

Several studies have explored the impact of weightlessness on spatial perception, revealing underestimation of distances during parabolic and orbital flights. These effects are attributed to adaptive changes in the neurovestibular system's processing of gravitational information. Astronauts have reported "time compression syndrome," perceiving time as compressed in orbit, and requiring more time for standard mental activity. Other reported phenomena include "space fog" affecting early mission cognitive performance and "entry motion sickness" impacting decision-making upon re-entry. These observations highlight the significant neurovestibular challenges associated with spaceflight and the potential implications for cognitive functions.

Ten healthy astronauts (9 male, 1 female; mean age 44.1 years) participating in ISS missions were tested. A control group of 15 healthy subjects (6 female, 9 male; mean age 43.2 years) underwent ground-based testing. Testing occurred before (L−), during (FD), and after (R+) the 6–8-month spaceflights. The "How Long is a Minute?" test involved subjects using a head-mounted display and trackball to judge a one-minute interval. A questionnaire assessed perception of longer durations (hours and days) since events like the start of the workday, lunch, vehicle docking, and spacewalks. Ground-based testing mirrored the in-flight conditions as closely as possible. Linear mixed models (LMM) and Mann-Whitney tests were used for statistical analysis. The ground-based study used similar time intervals between testing sessions as the pre-flight astronaut sessions.

Ground-based testing showed no significant differences between astronauts and controls in judging one-minute intervals. However, in-flight, astronauts perceived one minute as significantly shorter (59.6 ± 9.1 s, a 20% decrease compared to pre-flight 74.5 ± 20.2 s), though this perception remained consistent across in-flight sessions. Astronauts underestimated longer durations: time since the start of the workday (−14.2%), time since lunch (−19.2%), and time since the last test session when a gravity shift occurred (−26.0%). However, estimations of time elapsed in days since vehicle docking and spacewalks were remarkably accurate (+2.2% and +5.6% error, respectively). The study highlights the dissociation between time perception for shorter intervals (minutes) and longer intervals (hours). The accuracy in estimating minutes during spaceflight might be linked to the detailed, minute-by-minute schedules displayed at ISS workstations.

The results indicate that astronauts' time perception is altered in space, particularly for durations measured in hours. The overestimation of 1-minute intervals, despite underestimation of longer intervals, underscores the different mechanisms governing time perception of various durations. The high accuracy in estimating durations measured in days is likely due to the reliance on structured schedules and the memorability of significant events. While stress is a potential factor influencing time perception, the relatively consistent underestimation of time across flight sessions suggests that adaptation does not fully counteract the effects of microgravity. The results support the hypothesis of an overlapping perception of space and time, potentially linked to shared cortical networks like the right parietal cortex, as evidenced by prior research on astronauts' brain activity and structure changes after spaceflight.

This study provides compelling evidence of altered time perception in astronauts during long-duration spaceflight. The underestimation of durations of hours, in contrast to the relatively accurate estimation of durations of days, suggests a differential impact of microgravity on time perception depending on the time scale considered. Further research should explore the underlying neural mechanisms and the potential operational implications for mission planning and crew performance. Future studies could also investigate the role of individual differences in time perception and the potential for countermeasures to mitigate the effects of altered time perception on astronaut performance during space missions.

The relatively small sample size of astronauts limits the generalizability of the findings. While the control group helped to establish baseline time perception, differences in the daily routines and experiences between astronauts and the control group could influence the findings. The lack of access to astronauts’ schedules during training limited the ability to assess pre-flight time perception for longer intervals. Finally, the methodology focused on specific intervals and events and may not fully capture the complexity of subjective time perception during spaceflight.