Circadian free-running and temporal organization during 40 days of human group isolation in a cave without external time cues.

Fifteen adults lived for 40 days in Lombrives Cave (France) without daylight, clocks, or external communication, providing a unique model of human temporal organization under complete environmental time isolation. After exclusions, 13 participants (7 women, 6 men) were analyzed. Rest-activity rhythms were continuously recorded by wrist actigraphy, core body temperature was measured using ingestible sensors, and salivary cortisol and melatonin were sampled at self-defined awakening and bedtime. During isolation, participants exhibited marked free-running behavior, completing 24-31 cycles (mean: 29.3 ± 2.6), with cycle duration increasing from 24.1 ± 2.3 h pre-isolation to 31.6 ± 8.0 h in-cave (p < 0.001). Sleep duration increased proportionally, preserving a stable sleep-to-wake ratio (~36.5%), indicating maintained homeostatic regulation despite temporal drift. At the group level, collective organization progressively deteriorated, with reduced co-activity and absence of stable synchronization. Instead, transient and weakly defined subgroups emerged. Core body temperature rhythms remained oscillatory but showed highly variable periods (19.5-86.4 h), with early internal desynchronization followed by partial re-alignment. Cortisol and melatonin maintained stable state-dependent contrasts across cycles. Upon re-exposure to external time cues, 24-h organization rapidly recovered. These findings demonstrate substantial plasticity of human temporal organization, characterized by extended free-running rhythms, preserved homeostasis, and absence of stable social synchronization.