How long do astronauts sleep?

Life on board runs on a strict timetable. The station's daily program reserves an 8.5-hour sleep window, from 9:30 p.m. to 6 a.m., protected every night under NASA rules. This window is no mere luxury: a study published in 2022 on six-month missions linked sleep deprivation in flight to degraded neurobehavioral functions and increased stress.

Crew members also have two and a half hours of physical exercise scheduled each day.

Studies conducted on crews show a persistent gap between the time allotted and the time actually slept. A study published in 2014 in The Lancet Neurology measured an average of 6.09 hours per night for crews on long-duration missions; the 2022 one reports 6.5 hours self-reported and a full 8.5-hour sleep window achieved in only 5.9% of nights. Several causes add up. The noise level on board is unfavorable to a good night's sleep, and astronauts generally wear earplugs and a sleep mask against noise and light; in orbit, they see 16 sunrises and sunsets every 24 hours. Workload also weighs in: the 2022 study found that astronauts go to bed later than planned and that a heavier workload goes hand in hand with less time spent in bed.

Sleep efficiency, on the other hand, remained stable, at an average of 91.8%: it's mainly the time spent in bed that shrinks.

Each astronaut has an individual sleeping compartment about the size of a phone booth, with a sleeping bag, pillow, lamp, air vent, laptop and storage. Those who sleep elsewhere strap their bag to the floor, the ceiling or the wall: without weight, sleeping on the floor or against a wall amounts to the same thing. The air vent is not a minor detail: exhaled carbon dioxide could form a bubble around the head, hence the requirement to sleep near a flow of air.