The circadian rhythms of human subjects without timepieces or indication of the alternation of day and night.

1. Seven solitary subjects, and two groups of four, spent from 5 to 13 days in an isolation unit without knowledge of time. Three solitary subjects and one group of four adopted fairly regular activity habits with a period of 25-27 h; one subject adopted a period of 30 h, and one of 27 h initially, decreasing to 24-25 h after a few days. One group of four awoke roughly every 24 h, after a sleep which was alternately about 8 h, or about 4 h and believed by the subjects to be an afternoon siesta. Two solitary subjects alternated sleeps of about 8 or 16 h, separated by 24 h of activity.2. Deep temperature in all subjects oscillated with a period of 24-26 h, which was thus commonly distinct from their activity habits.3. Urinary potassium followed a rhythm whose period, though usually close to, was sometimes distinct from, that of temperature. A secondary period corresponding to that of activity was also sometimes present.4. Urinary sodium and chloride usually gave evidence of two periodic components, one corresponding to activity and the other to the rhythm of either temperature or of urinary potassium.5. Urinary creatinine and phosphate usually followed the subject's routine of activity.6. Plasma samples were collected on a few occasions and analysed for phosphate and 11-hydroxycorticosteroids. Changes in plasma phosphate were usually, but not always, associated with similar changes in urinary phosphate, and changes in plasma corticosteroids were often, but not always, associated with similar changes in urinary potassium shortly afterwards.7. Observations are recorded on a subject alone in a cave for 127 days. His activity habits, though wildly variable, gave evidence of a period of 25.1 h and his urinary electrolyte excretion indicated a shorter period, of 24.6 h. During the following 3 days, when he remained in the cave but was visited frequently, his plasma corticosteroids and urinary potassium oscillated with a period of 16 h.8. The possible mechanisms controlling these rhythms are discussed.