From the International Space Station, dawn is not just a singular daily occurrence. The station orbits the Earth in approximately 90 minutes, allowing an astronaut to transition from night to day roughly 16 times within a 24-hour period.
The clear implication is that the human body must adjust to experiencing 16 miniature days. However, it does not. Astronauts adhere to a meticulously regulated 24-hour timetable, while the sunrises visible from the windows become more of a backdrop than a timepiece.
This is reporting and interpretation, not medical counsel.
The more accurate narrative is also the more intriguing one. The body never adjusts to the station’s 90-minute cycle as its day, but that doesn’t imply every astronaut experiences chronic jet lag. Researchers have observed both effective circadian synchronization and significant sleep deprivation in space. These issues are interconnected but not identical.
## Sixteen sunrises are genuine, yet they do not dictate the crew’s schedule
[NASA states that the ISS moves at around 17,500 miles per hour](https://www.nasa.gov/missions/station/spot-the-station-frequently-asked-questions/) and completes one orbit approximately every 90 minutes. This results in 16 sunrises and 16 sunsets within a single Earth day.
An astronaut who looks out at the right time can see a narrow streak of light spreading across the planet’s rounded edge. However, much of the workday is conducted inside modules, where they perform experiments, maintain equipment, exercise, and communicate with teams on Earth. European Space Agency astronaut Thomas Pesquet once remarked that crews often miss most of the sunrises because they are occupied indoors.
Consequently, the station does not require individuals to sleep every 45 minutes. According to [NASA’s operational guide for the ISS](https://www.nasa.gov/wp-content/uploads/2018/04/iss-operating_an_outpost-tagged.pdf), the program utilizes Greenwich Mean Time, also referred to as Coordinated Universal Time, as the standard time for the crew and ground control centers worldwide. The guide outlines a typical routine with wake-up at around 6 a.m. GMT and bedtime around 9:30 p.m.
That timetable is more consequential for the body than the view beyond the windows. Meals, work, exercise, social interaction, darkness during sleep, and scheduled artificial light all provide the circadian system with repeated signals regarding when the day should begin and conclude.
## The body continues to attempt to create a 24-hour cycle
Human circadian rhythms are produced internally and influenced by environmental cues, particularly light. It is not merely a passive reaction to every alteration in brightness. If it were, a thunderstorm or a day spent in a dark cinema would completely reset the system.
The quick orbital cycle is just too brief to become a typical human day. Instead, misalignment can occur: sleep is scheduled for one time, while the body’s internal signals encourage wakefulness at another.
However, the orbit alone does not guarantee misalignment. In a small 1998 study published in the *Journal of Biological Rhythms*, Thomas Monk and colleagues examined four male astronauts during a 17-day shuttle mission designed to minimize disruption to their schedules. Their [circadian rhythms remained in sync with the prescribed work-rest plan](https://pubmed.ncbi.nlm.nih.gov/9615283/) and resembled data collected on Earth.
That finding was based on only four individuals and an unusually stable mission schedule, thus it cannot encompass every crew or every mission. However, it does illustrate a significant limitation of the popular narrative: witnessing 16 sunrises does not compel the body to pursue them all.
## Synced clocks do not necessarily equate to sufficient sleep
The same four astronauts averaged 6.1 hours of sleep while in flight and demonstrated less deep, slow-wave sleep. Their circadian phase was fairly aligned, yet their sleep duration remained shorter. This distinction clarifies why the matter is more complex than a straightforward tale about sunrises disorienting the brain.
A much broader observational investigation led by Laura Barger and published in *The Lancet Neurology* in 2014 tracked 64 astronauts on shuttle missions and 21 on 13 ISS missions. The [ISS crew members averaged 6.09 hours of sleep in flight](https://pubmed.ncbi.nlm.nih.gov/25127232/), compared to 6.95 hours during the initial week after returning to Earth. Twelve of the 16 astronauts aboard the ISS who noted medication use reported taking sleep-enhancing drugs at some stage.
This was an observational study rather than an experimental one capable of establishing causation. It also collected data between 2006 and 2011, prior to the full implementation of the station’s current lighting systems. It indicates that short sleep was prevalent in that sample, not that the mere presence of 16 sunrises caused it or that all contemporary crew members experience sleep in the same manner.
Workload, alarms, noise, cabin temperature, airflow, carbon dioxide levels, excitement, stress, and sleeping in a weightless environment can