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

< 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.