ABSENCE OF SEASONAL-VARIATION IN THE PHASE OF THE ENDOGENOUS CIRCADIAN-RHYTHM IN HUMANS

Humans may be subject to seasonal variations, as evidenced by the exis tence of seasonal affective disorder (SAD) and midwinter insomnia. How ever, some recent studies have shown that the seasonal variation in th e phase of the circadian rhythm is relatively weak in healthy humans. In the present study, evidence is found that there is no seasonal vari ation in the phase of the endogenous circadian rhythm at all. Body tem perature, cortisol excretion, and subjective alertness of six subjects recorded under constant routine conditions showed no systematic seaso nal variation in circadian phases. This finding indicates that seconda ry zeitgebers blocked or counterbalanced the seasonal variation in the entrainment effect of the natural photoperiod. The human being may li ve in an environment in which the photoperiod has lost its status of p rimary zeitgeber.