AUTONOMOUS CIRCADIAN RHYTHMS IN MEN AS INFLUENCED BY DIFFERENT LIGHT CONDITIONS

Until now, the circadian rhythm, autonomous, as confirmed subsequently by the results, has been tested with 75 human subjects in complete isolation from environment. 52 subjects have lived under constant illumination, also while sleeping; 20 subjects have switched on the light while getting up, and switched it off while going to bed (illumination by choice); with 3 subjects, the illumination has been changed between the two kinds mentioned before. With 38 subjects, the intensity of illumination has been varied during the experiment, in order to examine the influence of light intensity on the circadian period. The interpretation of all the results shows that, under illumination by choice, (a) the period is longer (significant with p<0.001), (b) the standard deviation around the mean value of period is greater (p<0.005), and (c) the tendency for internal desynchronization is greater (p=0.005) than under constant illumination. These results correspond with predictions derived from a hypothesis for circadian rhythms. Only under illumination by choice, the circadian period depends on the intensity of illumination, according to the following rule: the correlation between light intensity and period tends to be positive in individual experiments, and it is negative in group experiments (p=0.048); this result corresponds with theoretical considerations. Against that, under constant illumination the period depends frequently on light intensity but not according to a rule, neither in individual experiments nor in group experiments. Also, the tendency for internal desynchronization, is independent of the existing light intensity. In order to explain these results, the hypothesis is offered that also under objectively constant illumination, the subject is exposed subjectively to a light-dark cycle, because the eyes are open during activity time and closed during rest time. It follows that, under constant illumination, the same feed-back of the self-selected light-dark cycle on the circadian period is effective as under illumination by choice, but to a smaller amount. © 1969 Springer-Verlag.