Interdependence between locomotor activity and duration of wakefulness in humans during isolation.

Locomotor activity was recorded in 14 subjects who lived singly in an isolation unit for 16 to 88 days. Their free-running circadian rhythms had a mean period of 25.9 h, with individual means in the duration of wakefulness (alpha) ranging from 12.1 to 22.9 h. Intraindividually, the hourly means of activity were negatively correlated with alpha to such a degree that the total amount of activity per 'day' remained constant irrespective of large variations in alpha.