Since the 1960s, it has been established that the endogenous rhythm of the circadian clock is approximately 24.5 h in the absence of any time donor. The objective of our study is to determine if a group of people living out of time will present the same rhythm as previously observed and if this group will synchronize socially or not.
For 40 days, 14 people (7♀ and 7♂) lived isolated in a cave without any time information. Their sleep/wake cycles were recorded continuously using an actimeter (MotionWatch R, CamNTech®) 14 days outside the cave (Pre), in the cave (Per) and 14 days after leaving the cave (Post) . Body temperature was measured by an ingestible electronic capsule (eCelsius Performance, BodyCAP®).
During 40 days, an average of 29.25 ± 2.56 sleep/wake cycles is recorded with an average duration of 31.7 ± 8.0 h per cycle (no gender difference). The sleep time to wake time ratio remains similar for the 3 Per-Pre-Post conditions (35.9%). No social synchronization is observed within the group. The temperature cycles are first desynchronized from the sleep/wake cycles and then synchronize at the end of the protocol over a period of 30.25 h ± 6.14.
We show for the first time that people prefer to follow their own internal rhythm rather than synchronize with the group when they are isolated from all external constraints. The duration of the cycles increases without changing the wake/sleep ratio necessary for a good recovery.