Ten astronauts who spent more than three months aboard the International Space Station showed two linked changes while in orbit: their reaction time slowed and several timing tasks made short intervals feel shorter than they had on Earth. The pattern points to a practical problem for space psychology, because crews depend on steady attention and accurate timing while they move through a demanding environment.
Researchers reported the findings in a Frontiers in Physiology study published on March 17, 2023. Olga Kuldavletova, Deborah C. Navarro Morales, Gaelle Quarck, Pierre Denise and Gilles Clement tracked the astronauts before launch, during long-duration ISS missions and again after landing to see whether spaceflight changed how quickly they responded and how they judged intervals from 2 to 38 seconds.
For crews, those are not abstract lab scores. A slipping sense of elapsed time can affect pacing, decision speed and the mental effort needed to split attention between a primary task and a second stream of information. The paper ties those changes to psychology as much as physiology, because attention, stress, workload and adaptation to weightlessness all meet in the same few seconds.
Why seconds can feel different in orbit
Time perception is one of the quiet mental tools people use all day without noticing it. On Earth, the brain usually keeps short intervals steady enough for conversation, hand movements, reading and quick visual reactions. In orbit, the sensory inputs that help steady that timing are less familiar, especially when the body is floating and the inner ear no longer handles gravity in its usual way.
A later Frontiers review on altered gravity and time perception describes the same broad trend: astronauts and microgravity analog studies often show shorter subjective timing and slower responding. That review also notes that the effect can grow when a person has to do something else at the same time, which matches the idea that attention and timing draw on overlapping mental resources.
The ISS study leaned on that same logic. One reference in the paper points to a meta-analysis of cognitive load showing that duration judgments shift when the brain has to divide attention. A crew member reading digits while also keeping track of seconds is using working memory and focus at the same moment, so any drag on those systems can change how long a short interval seems to last.
How the ISS timing study worked
The experiment compared 10 astronauts with 15 healthy control participants on the ground. The astronaut group had nine men and one woman, with an average age of 44.1 years. The control group had six women and nine men, with an average age of 43.2 years. Before flight, the astronaut group completed three baseline sessions. During flight, they repeated the tests on mission days 17, 46, 71, 99, 134 and 164. After landing, they tested again on days 1, 5 and 9.
Each session included two kinds of duration judgment. In one task, the astronaut had to produce a target interval after hearing or seeing instructions. In the other, the astronaut had to reproduce the length of a visual signal. The targets ranged from 2 to 38 seconds. Some trials were single-task trials with counting aloud, while others were dual-task trials that added a concurrent digit-reading task.
The setup was kept as consistent as possible. Participants used a head-mounted display, noise-cancelling earphones and a finger trackball connected to a laptop. On Earth they were seated upright. On the station they floated freely, which removed the usual mix of body-pressure and gravity cues that help anchor orientation. The researchers also ran a simple visual reaction time test in which participants pressed a key as fast as possible after a blue square appeared on a screen.
The design matters because it let the team compare three stages of the same missions rather than taking a single snapshot. The procedures were approved by the European Space Agency Medical Board and by the NASA Johnson Space Center Institutional Review Board. Statistical models then compared preflight, inflight and postflight performance while accounting for repeated testing and differences between target durations.
What changed during the mission
The clearest result was slower responding in orbit. Compared with the mean of the three preflight sessions, astronaut reaction times were significantly higher at every inflight session that the paper reported, from day 17 through day 164. After landing, the scores moved back toward baseline and the postflight sessions did not differ significantly from the earlier ground average.
Timing judgments shifted at the same time. In the single-task production condition, the astronauts produced shorter intervals during all six inflight sessions than they had before launch, which means their subjective clock was running faster than their preflight baseline. In the dual-task reproduction condition, the strongest underestimation showed up later, with significant changes by flight day 71 and beyond. The pattern suggests that microgravity alone was part of the story, while divided attention increased the effect in the harder condition.
The authors proposed two mechanisms in the paper: changed vestibular inputs in weightlessness and strain on attention and working memory during the dual task. Both ideas fit the numbers. The astronauts were not just drifting around with a different body posture. Their brains were also handling a demanding environment, repeated work and the extra mental cost of keeping two tasks active at once.
Another nearby paper from the same research line, published in npj Microgravity, found that astronauts aboard the ISS also underestimated one minute during longer timing tasks. Put together, the two studies suggest the effect is not limited to one exact test. Short intervals of a few seconds and longer intervals of about a minute both appear vulnerable to change during long missions.
Why the findings matter for astronaut psychology
Psychology sits at the center of the result because the paper is really about how the brain keeps pace under pressure. The study does not claim that astronauts lose basic competence in orbit. It shows something subtler and more important: a small, repeated shift in attention and subjective timing can show up while highly trained people are still functioning well enough to complete their work.
One reason the team focused on the inner ear is that timekeeping and gravity sensing may be linked more closely than they seem. A study on vestibular stimulation found that stimulating the balance system can contract subjective time. That makes the ISS result easier to understand. When weightlessness changes the stream of signals coming from the vestibular system, the brain may also change the pace it uses to judge brief durations.
The paper also connects the findings to the long-standing idea of space fog, a crew term for lapses in concentration, slower thinking and mental overload. Earlier work cited by the authors has linked long missions to broader cognitive strain and another Scientific Reports study found persistent deterioration in visuospatial performance after long-duration spaceflight. The new timing data do not prove one single cause, but they fit a wider picture in which life in orbit can nudge several mental systems at once.
Limits remain clear. The sample was small, the authors did not directly measure every source of stress or fatigue and the paper calls for future studies to collect better subjective reports of what astronauts describe as fogginess. Even with those limits, the result is valuable. A mission does not need dramatic confusion to face risk. A modest slowdown, paired with a slightly faster inner clock, is enough to tell scientists that the psychology of spaceflight is still changing deep into a mission.
