Managing human biological rhythms in space requires overriding the natural cues provided by the stars and sun. While International Space Station (ISS) crews experience 16 sunrises and sunsets every 24 hours, they rely on Coordinated Universal Time (UTC) and specialized LED lighting to maintain a stable 24-hour sleep-wake cycle essential for long-term health and mission success.
Why does the ISS experience 16 sunrises every day?
The rapid cycle of light and dark isn’t a design choice; it’s a requirement of orbital physics. To stay in orbit at an altitude of roughly 400 kilometres, the station must travel at approximately 28,000 kilometres an hour. This velocity allows the station to complete a full lap around the Earth roughly every ninety minutes.
While NASA reports sixteen orbits and sixteen sunrises per day, the math becomes even more precise when looking at different agency data. The European Space Agency (ESA) calculates the speed at 28,800 kilometres an hour, resulting in a ninety-two minute orbit. Using this more precise timing, the station actually completes about fifteen and a half orbits every twenty-four hours, though “sixteen” remains the standard public figure.
The number of sunrises isn’t always constant. During “high-beta-angle” periods—when the angle between the orbit and the sun exceeds 70 degrees—the station can go for days without entering Earth’s shadow. According to the Alpha Magnetic Spectrometer team, this can happen for about a week during the summer and winter.
How do astronauts stay synchronized with Earth’s time?
If a crew followed the sun, they would be living sixteen “days” every single day. To prevent total biological chaos, the station operates on a single, artificial clock: Coordinated Universal Time (UTC), also known as GMT. This provides a neutral midpoint between American and Russian control centres.
Maintaining this schedule requires significant engineering. Because the human body is evolutionarily tuned to the single rotation of Earth, the station must manufacture its own day and night. NASA has fitted the station with adjustable LED lighting designed to support the crew’s sleep cycle. These lights shift in both brightness and colour throughout the day to mimic the natural progression of light on the ground.
Sleep itself is a highly controlled activity. Crew members reside in individual compartments roughly the size of a telephone booth. These small spaces allow astronauts to shut out the constant visual of the passing Earth and the frequent sunrises, using standard-issue eye masks and earplugs to maintain their eight-and-a-half-hour rest periods.
In environments where natural light cues are absent or erratic, strict adherence to a “manufactured” schedule is the most effective way to prevent fatigue and hormone imbalances.
The future of biological timekeeping in deep space
As we look toward missions to the Moon and Mars, the challenges of timekeeping will only intensify. On the ISS, the crew can at least look out the cupola and see the Earth. On a long-duration transit to Mars, the visual connection to a rotating planet disappears entirely.
Future trends in space habitation suggest that “lighting as medicine” will become a primary focus. We will likely see even more sophisticated, biologically-responsive lighting systems that don’t just mimic a 24-hour cycle, but actively manage hormone release and body temperature. As we move away from Low Earth Orbit, the ability to create a stable, predictable environment will be the difference between a successful mission and a crew suffering from chronic sleep deprivation.
Frequently Asked Questions
How many sunrises do astronauts see on the ISS?
On average, astronauts experience sixteen sunrises and sixteen sunsets every twenty-four hours due to the station’s ninety-minute orbital period.
What time zone do astronauts use?
The crew uses Coordinated Universal Time (UTC), also known as GMT, to maintain a consistent 24-hour schedule regardless of their position over Earth.
Why don’t astronauts just sleep when it gets dark?
Because the station orbits Earth every 90 minutes, “darkness” only lasts for a short period. Following these frequent shifts would disrupt the human body’s natural circadian rhythm and hormonal health.
What do you think about the challenges of living in a world where the sun rises every 90 minutes? Let us know your thoughts in the comments below, or subscribe to our newsletter for more deep dives into the future of space exploration!
