On the ISS, astronauts see 16 sunrises every day as the station circles Earth, but their bodies refuse to follow the 90-minute cycle, forcing crews to stick to a familiar 24-hour routine despite living through multiple days and nights

If you watched Earth from the International Space Station, sunrise would quickly lose its usual meaning. The ISS circles Earth roughly once every 90 minutes, so astronauts can see about 16 sunrises and 16 sunsets during a 24-hour period, as per a report.

But they do not start a new day every time the Sun appears. Instead, astronauts follow a carefully managed 24-hour schedule while the station repeatedly moves between darkness and daylight outside the windows. Their bodies can experience changes in sleep and circadian timing in space, but the station's 90-minute orbit does not become their biological day.

Sixteen sunrises, but one daily schedule

According to NASA, the ISS travels at about 17,500 miles per hour and completes an orbit roughly every 90 minutes, as per a Science Blog report. That works out to about 16 sunrises and 16 sunsets over an Earth day.


An astronaut looking outside at the right moment can watch a thin band of sunlight spread across the curved edge of Earth. But most of those sunrises happen while the crew is busy working.

Astronauts spend much of their time carrying out experiments, maintaining equipment, exercising and communicating with teams on Earth. The station therefore does not operate on a 90-minute sleep cycle.

NASA's guide to operating the ISS describes Greenwich Mean Time, also known as Coordinated Universal Time, as the common clock used by the crew and control centers. A typical schedule has astronauts waking around 6 a.m. GMT and going to bed around 9:30 p.m, as per the Science Blog report.

The clock inside the station matters far more to the crew's routine than the changing view outside.

The body still works around a 24-hour rhythm

Human circadian timing is generated internally and adjusted by environmental signals, particularly light. That does not mean the body automatically resets whenever the surroundings become bright or dark.

The ISS's 90-minute orbital cycle does not become the astronauts' normal biological day. Instead, astronauts can experience a mismatch between the schedule they are following and signals coming from their environment.

Sleep might be scheduled at one time while the body's internal signals are encouraging wakefulness at another. But that does not happen in exactly the same way on every mission.

A small 1998 study published in the 'Journal of Biological Rhythms' followed four male astronauts during a 17-day shuttle mission designed to minimize schedule disruption, as per the Science Blog report. Their circadian rhythms remained aligned with the required work-rest schedule and looked broadly similar to measurements taken on Earth.

The study involved only four astronauts and a relatively stable mission schedule, so it cannot describe every astronaut or every flight. Still, it showed that seeing 16 sunrises does not force the body to follow all 16.

Staying aligned does not always mean getting enough sleep

Circadian alignment and sleep duration are not the same thing. In the 1998 study, the four astronauts averaged about 6.1 hours of sleep during the mission and experienced less deep, slow-wave sleep.

A larger observational study published in 'The Lancet Neurology' in 2014 looked at 64 astronauts on shuttle missions and 21 astronauts on 13 ISS missions.

The ISS astronauts averaged 6.09 hours of sleep during flight, compared with 6.95 hours during their first week back on Earth, as per the Science Blog report. Twelve of the 16 ISS astronauts who recorded medication use reported taking sleep-promoting drugs at some point. The study was observational, so it could not establish that the 16 daily sunrises caused the shorter sleep.

Other conditions can also interfere with sleep in space, including workload, alarms, noise, cabin temperature, airflow, carbon dioxide, excitement, stress and the experience of sleeping while floating.

Schedules can also change around dockings, spacewalks and other operations. NASA therefore considers inadequate sleep, circadian disruption and heavy workloads together when assessing fatigue risks, as per the Science Blog report.

Artificial light helps create a normal day

The constantly changing light outside the ISS is not a useful substitute for a normal day-night cycle.

The station instead uses adjustable solid-state lighting that can change in intensity and color spectrum throughout the day.

A 2016 review led by George Brainard described the lighting system as a way to support sleep, circadian entrainment and daytime alertness.

Blue-enriched white light can strengthen alerting signals during waking periods, while blue-depleted light before sleep reduces that signal. This gives astronauts a more consistent indoor light pattern than simply following the sunlight outside.

Lighting cannot solve every problem, however. It cannot shorten a demanding workday, remove noise or prevent an emergency from occurring at an inconvenient time.

What it can do is give the body's internal clock a clearer signal about when it should be awake and when it should prepare for sleep.