title: Absence of seasonal variation in the phase of the endogenous circadian rhythm in humans.
abstract: Humans may be subject to seasonal variations, as evidenced by the existence of seasonal affective disorder (SAD) and midwinter insomnia. However, some recent studies have shown that the seasonal variation in the phase of the circadian rhythm is relatively weak in healthy humans. In the present study, evidence is found that there is no seasonal variation in the phase of the endogenous circadian rhythm at all. Body temperature, cortisol excretion, and subjective alertness of six subjects recorded under constant routine conditions showed no systematic seasonal variation in circadian phases. This finding indicates that secondary zeitgebers blocked or counterbalanced the seasonal variation in the entrainment effect of the natural photoperiod. The human being may live in an environment in which the photoperiod has lost its status of primary zeitgeber.
authors.affiliation: Unit for Experimental Psychiatry, University of Pennsylvania. VDongen@Mail.Med.UPenn.edu
authors.forename: H P
authors.initials: HP
authors.lastname: Van Dongen
authors.forename: G A
authors.initials: GA
authors.lastname: Kerkhof
authors.forename: J H
authors.initials: JH
authors.lastname: Souverijn
mesh_terms: D000293:Adolescent; D001683:Biological Clocks; D001831:Body Temperature; D002940:Circadian Rhythm / Q000502:physiology*; D003624:Darkness; D006801:Humans; D006854:Hydrocortisone / Q000378:metabolism* / Q000652:urine; D008027:Light; D008297:Male; D009426:Netherlands; D017440:Photoperiod; D012016:Reference Values; D016574:Seasonal Affective Disorder / Q000209:etiology; D012621:Seasons; D007319:Sleep Initiation and Maintenance Disorders / Q000209:etiology
chemical_list: D006854:Hydrocortisone