On Earth, most of us rarely have to think about the signals that keep our internal clocks aligned. The Sun rises, daylight changes, darkness arrives and morning comes again.
Our bodies have evolved alongside this roughly 24-hour cycle. Light is one of the strongest signals our bodies use to synchronise the circadian clock. But what happens when the Sun no longer provides a reliable schedule?
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. It is an extraordinary experiment in human biology – one that can reveal what happens when the environmental cues that normally regulate our sleep disappear.
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. The other is sleep pressure, which gradually builds the longer we remain awake and dissipates while we sleep. Usually, these systems work together. 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. This distinction is one of the most important lessons spaceflight can teach us about sleep.
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