Space sleep: What space travel reveals about our body clocks

Ronan Lordan

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 used 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 an extraordinary experiment in human biology, revealing what happens when the environmental cues that normally regulate 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 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.

Research has shown that carefully guiding astronaut schedules around exercise, meals, tailored lighting and work can help offset some effects of sleep loss and circadian disruption. But giving astronauts time to sleep does not necessarily mean they will sleep. They have traditionally been given around eight and a half hours of sleep opportunity each day, but research suggests they often obtain only six to six and a half hours.

Repeated night after night, that gap becomes chronic sleep restriction. Research aboard the International Space Station has linked daily sleep patterns of six hours or less with measurable reductions in vigilant attention. In an environment where a missed signal could have serious consequences, sleep is part of mission safety.
NASA’s Artemis II lunar flyby mission offered scientists another window into sleep. Crew members wore wrist-mounted movement and sleep monitors before, during and after the mission, allowing researchers to examine changes in sleep and activity alongside cognition, behaviour and team performance.

NASA is also testing ways to prevent sleep disruption. A preliminary study involving 14 healthy adults found no difference in sleep inertia or vigilant attention between pink-noise and control conditions during the first 40 minutes after waking.

In spacecraft, light is treated as part of the biological life-support system. Blue-enriched white light can support alertness and help synchronise the circadian clock, while dimmer, blue-depleted light can prepare the brain for sleep. But recent NASA-supported testing found that pre-sleep lighting that allowed greater melatonin production impaired colour discrimination — creating an engineering trade-off when astronauts need to identify colour-coded controls or warning signals.

The next challenge is Mars. A Martian “sol” lasts approximately 24 hours, 40 minutes. That extra 40 minutes may sound trivial, but repeatedly delaying sleep and wake times means continually asking the circadian system to adjust.
Future Mars explorers may experience the most literal form of jet lag imaginable — travelling between planetary days.