53 healthly persons were monitored for skin temperature by a Hardy dermal radiometer every 60 sec. Readings continued until a spontaneous cycle appeared. 32 subjects were examined awake and 21 while asleep. A spontaneous rhythm with a mean of 4.25 min was found for the awake subjects, and one of 8.33 for the asleep subjects. To confirm these findings longer recordings were made by a thermocouple electronic thermometer in a further 23 subjects. Observations were continued until the awake subject slept. The results were analyzed by autocorrelation. In 13 records the sleeping rhythm was significantly longer than the waking. Another thermocouple probe in 14 subjects was used to measure axillary temperature. In 7 subjects a shorter cycle was seen while awake and a longer one while they slept. The properties of a perceptual clock are reviewed. It is suggested that the relative awareness of the subject be added to them.