Every ninety minutes, the crew orbiting high above Earth watches another sunrise break over the planet, which means that in a single day they see sixteen dawns and sixteen dusks, and their bodies have no idea what time it is anymore. - Space Daily

Astronauts on the International Space Station live in an environment where the usual signals of day and night are constantly shifting. Orbiting roughly 400 kilometres above Earth at about 28,000 kilometres per hour, the station moves so fast that it completes a trip around the planet in about 90 minutes.

That pace means the crew sees about 16 sunrises and 16 sunsets during a single Earth day. Instead of one morning and one evening, the view outside the station windows cycles through dawn and dusk again and again, making the normal rhythm of a 24-hour day hard to recognize.

The result is a major challenge for the human body clock. Circadian rhythms are built around regular patterns of light and darkness, but in low Earth orbit those cues are broken into repeated, compressed cycles. For astronauts, that can turn timekeeping into a biological strain rather than a simple matter of checking the clock.

The station’s path around Earth is a reminder that spaceflight is not only a technical achievement but also a test of human biology. While the ISS offers a spectacular view of the planet, its nonstop sequence of orbital dawns and dusks creates an unusual and demanding environment for the people living aboard.