CIRCADIAN RHYTHMS IN MEN AS INFLUENCED BY ARTIFICIAL LIGHT-DARK CYCLES OF VARIOUS PERIODS

Circadian rhythms of 10 subjects were studied under the influence of an artificial light-dark cycle including twilight-transitions. The Zeitgeber was changed to periods either longer or shorter than 24 hours. Three out of seven subjects became synchronized completely to 26 2/3 hours; two subjects were synchronized with their activity-rhythm but not with the rhythm of rectal temperature (internal desynchronization), and two were completely desynchronized. One out of five subjects followed a Zeitgeber-period of 22 2/3 hours, but only with the activity rhythm, while all others were completely desynchronized. These experiments demonstrate that an artificial light-dark cycle synchronizes circadian rhythms in men only to periods which are close to 24 hours. For the activity rhythm, this range of entrainment is larger than for the temperature rhythm. By other means than by the demonstration of free-running rhythms after the exclusion of Zeitgebers, these results establish the endogenous character of human circadian rhythms. The finding that the rhythms of activity and of rectal temperature can vary independently, suggests that the two rhythms have to be considered as separate oscillators. Both, the external phase-relationship between Zeitgeber and activity rhythm, as well as the internal phase-relationship between the rhythms of activity and of temperature, are functions of the Zeitgeber-period. With an increase of the Zeitgeber-period by one hour, the activity rhythm is advanced relative to the Zeitgeber by 17°, and the temperature rhythm is advanced relative to the activity rhythm by 21°. The results suggest either different types of action of the Zeitgeber on activity and on temperature, or different types of oscillations underlying the two rhythms. © 1969 Springer-Verlag.