Table 4.

Subterranean studies reported from 1974 to 1994.

Study	Subjects	Number of days in isolation	Main findings	Neurophysiological measures
1. Chouvet et al., 1974	3 adult males (Mairetet, Chabert and Engelender), separately isolated	2 males in 1968 (Chabert and Engelender): 150 1 male in 1966 (Mairetet) 174	Subjects developed a circabidian/bicircadian rhythm (34 h of wakefulness and 14 h of sleep); the duration of sleep stages 3 and 4 was correlated with the duration of the previous waking period, providing evidence of homeostatic regulation mechanisms in sleep regulation	EEG, EMG, EOG
2. Siffre, 1987 (Proceedings of a Colloquium on “Space & Sea”)	Review: 7 adults from previous studies, separately isolated	60–174	5 subjects developed a circabidian/bicircadian rhythm; REM sleep duration is directly proportional to the duration of sleep (same subjects as in Chouvet et al., 1974); REM and SWS increased at the cost of lighter sleep stages (1 and 2) when the sleep wake cycle desynchronizes from circadian to circabidian	EEG, EMG, EOG
3. Sanchez da la Pena et al., 1989 (Proceedings. Second Annual IEEE Symposium on Computer-based Medical Systems)	1 adult female (Follini)	97	Subject maintained circadian systolic, diastolic, and mean arterial pressure rhythms that were slightly but significantly greater than 24 h. A circaseptan rhythm for heart rate was observed	Heart rate monitor only
4. Sonnenfeld et al., 1992	1 adult female	131	Sleep-wake cycle began to deviate from 24 h after 30 days of isolation, and thereafter ranged from 25 to 42 h in cyclical patterns	None reported
5. Hillman et al., 1994a,b (New Trends in Experimental and Clinical Psychiatry)	1 adult female (Le Guen)	103	Subject maintained a circadian sleep-wake rhythm but it varied slightly throughout the period of isolation, to a period somewhat longer than 24 h. Other physiological measurements such as urinary water excretion rate and caffeine metabolism developed circasemiseptan (half-weekly) rhythms (the authors attributed these rhythms to exposure to cosmic rays)	None reported

Note that “circabidian” refers to 2-day (~48 h) rhythms, whereas “bicircadian” refers to twice-daily (~12 h) cycles.