Dublin: On Earth, most people rarely have to think about the signals that keep their internal clocks aligned. The Sun rises, daylight changes, darkness arrives, and morning comes again.
Our bodies have evolved around this roughly 24-hour cycle, with light serving as 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, meaning astronauts experience about 16 sunrises and sunsets in 24 hours. They cannot go to sleep every time the Sun disappears, yet their bodies still need to know when it is time to sleep, wake, eat, exercise and work.
This makes spaceflight more than an engineering challenge. It is an extraordinary experiment in human biology, showing what happens when the environmental cues that normally regulate sleep disappear.
Sleep is thought to be governed by two interacting biological systems. The first is the circadian clock, which helps determine when the brain expects sleep and wakefulness. The second is sleep pressure, which builds gradually the longer we remain awake and dissipates during 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 the circadian system is signalling that it is daytime. Feeling tired and being biologically ready for sleep are not always the same thing.
This distinction is one of the key lessons spaceflight can offer about sleep.
Research in chronobiology and sleep science has shown that carefully structuring astronaut schedules around exercise, meals, tailored lighting and work can help offset some of the effects of sleep loss and circadian disruption. Artificial lighting can also be controlled to provide more appropriate environmental cues.
But there is a catch: giving astronauts time to sleep does not necessarily mean they will sleep.
Astronauts have traditionally been given around eight and a half hours of sleep opportunity on each day of spaceflight. However, research across different types of missions suggests they often obtain only about six to six and a half hours.
Repeated night after night, that gap can become 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, the ability to notice and respond reliably to important information.
In an environment where a missed signal could have serious consequences, sleep is not simply a matter of comfort. It is part of mission safety.
But this creates an unexpected engineering trade-off. Recent Nasa-supported testing found that pre-sleep lighting allowed greater melatonin production but impaired colour discrimination.
A lighting setting that benefits the circadian system may therefore be unsuitable when astronauts need to identify colour-coded controls, equipment or warning signals.
Caffeine can protect some aspects of performance when sleep-deprived astronauts need to remain alert. However, it has also been found to interfere with the sleep they need to perform well the next day.
In space, as on Earth, caffeine can temporarily solve one problem while quietly helping to create the next one.
Beyond Earth days
The next challenge is even more fundamental: what happens when humans stop living according to an Earth day altogether?
A Martian “sol” lasts approximately 24 hours, 40 minutes. Those extra 40 minutes may sound trivial, but for the human circadian system, they present a persistent scheduling problem.
In late September 2026, the four-person crew of Nasa’s latest year-long Chapea Mars simulation was scheduled to move from an Earth-based 24-hour schedule to the longer Martian day.
The crew is now expected to remain on Martian time until the mission, based at Johnson Space Center in Houston, Texas, ends on October 31.
Forty 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 — not simply travelling across time zones, but travelling between planetary days.
