NASA astronaut Sunita Williams has the unique opportunity to witness 16 sunrises and sunsets every day aboard the International Space Station (ISS).
In 2013, Williams remarked, “Having worked hard to go to space, I was lucky to see 16 sunrises and 16 sunsets daily aboard a fast-moving space shuttle.”
How Do Multiple Sunrises and Sunsets Occur?
Every 90 minutes, the ISS completes a full orbit around the Earth at a speed of approximately 28,000 km/h, which means astronauts see a sunrise or sunset every 45 minutes. Astronauts aboard the ISS experience 16 day-night cycles in a single Earth day. The ISS moves at a much faster rhythm compared to life on Earth, where day and night typically last about 12 hours. Astronauts experience a transition between light and darkness every 45 minutes, creating a unique cycle that repeats 16 times each day.
How Do Astronauts Know When to Sleep?
Astronauts follow a schedule based on Coordinated Universal Time (UTC) because there are no regular day-night cycles in space. Their days are divided into roughly 5-minute periods for work, exercise, food, and rest, a schedule they stick to rigidly in order to maintain both mental and physical health in space.
On Earth, humans use the Sun’s rising and setting as cues for sleep and wake cycles. However, in space, astronauts lack these natural cues, as the Sun neither rises nor sets in the same way. As a result, the ISS’s constant motion creates the need for a fixed schedule, and astronauts follow UTC to maintain a balanced lifestyle alongside their demanding work schedules.
Astronauts also rely on precise atomic clocks to stay synchronized with Earth-based teams, which is especially crucial for navigation during deep-space missions.
Sunita Williams, a NASA astronaut who has been stationed on the International Space Station since June 2023, faced delays in returning to Earth due to issues with Boeing’s Starliner. She is now expected to return to Earth in February 2025 aboard SpaceX’s Crew Dragon.