How space travel affects an astronaut’s sleep cycle

What happened

The sun rises, daylight changes, darkness arrives and morning comes again. A spacecraft orbits Earth roughly every 90 minutes, so astronauts experience about 16 sunrises and sunsets every 24 hours.

They cannot go to sleep every time the sun disappears – but their bodies still need to know when it is time to sleep, wake, eat, exercise and work. This makes spaceflight more than just an engineering challenge.

Sleep is thought to be controlled by two interacting biological systems . One is the circadian clock , which helps determine when the brain expects sleep and wakefulness.

But jet lag, shift work and spaceflight can pull them apart. An astronaut may be exhausted after many hours awake but still struggle to sleep because their circadian system is signalling it is daytime. Feeling tired and being biologically ready to sleep are not always the same thing.