What Happens to Human Time When the Clock Disappears?

What happens to our sleep and circadian rhythms when virtually every external indication of time is removed? A remarkable new study in Chronobiology International provides a rare glimpse.

Fifteen adults spent 40 days living in Lombrives Cave in France, without daylight, clocks or external communication. The researchers continuously monitored rest and activity and also measured core body temperature, cortisol and melatonin.

Without the usual environmental signals anchoring people to a 24-hour day, their behavioral “days” began to stretch dramatically. Among the 13 participants included in the analysis, average sleep–wake cycle duration increased from about 24 hours before entering the cave to more than 31 hours underground.

But perhaps the more interesting finding was what didn't disappear.

As their days became longer, participants also slept longer, maintaining a remarkably stable proportion of sleep to wakefulness. In other words, sleep homeostasis remained relatively organized even while their relationship with clock time drifted substantially.

Simply living together didn't keep everyone synchronized, either. Rather than settling into a common group rhythm, collective synchronization progressively weakened and temporary subgroups appeared.

For those interested in fatigue, shift work and irregular operations, the study is an unusually vivid demonstration of an important principle: sleep pressure, circadian timing, behavior and social schedules are related - but they are not the same thing.

Remove strong environmental time cues, and the human timing system can become surprisingly flexible. And when normal time cues returned? Twenty-four-hour organization rapidly re-emerged.

A fascinating natural experiment in what actually keeps humans synchronized to the day. 

Read the study: Mauvieux B, Gabel V, Raverot V, Clot C & Besnard S. (2026). Circadian free-running and temporal organization during 40 days of human group isolation in a cave without external time cues. Chronobiology International. DOI: 10.1080/07420528.2026.2661312.