Orbiting daylight: how the ISS keeps a human day with engineered light
With ~90-minute orbits yielding 16 sunrises and sunsets each day, the ISS keeps a 24-hour rhythm not by the Sun but by UTC scheduling, window management, and a smart lighting system that shifts from day (neutral/blue-enriched) to pre-sleep (warm) and alertness (cool). Astronauts still average about six hours of sleep, using sleep shifts when needed; future missions will rely even more on engineered daylight since there may be no Earth sunrise to cue the body.
