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Abstract
The purpose of this study was to describe and compare the circadian rhythm of body temperature and cortisol, as well as self-reported clock times of sleep onset and offset on weekdays and weekends in 19 healthy adult “larks” (morning chronotypes) and “owls” (evening chronotypes), defined by the Horne and Östberg questionnaire. Day-active subjects entered the General Clinical Research Center, where blood was sampled every 2h over 38h for later analysis for cortisol concentration by enzyme immunoassay. Rectal body temperature was measured continuously. Lights were turned off at 22:30 for sleep and turned on at 06:00, when subjects were awakened. The acrophases (peak times) of the cortisol and temperature rhythms occurred 55 minutes (P ≤.05) and 68 minutes (P <.01), respectively, earlier in the morningness group. The amplitude of the cortisol rhythm was lower in the eveningness than in the morningness group (P = n.s.). Subject groups differed on all indices of habitual and preferred timing of sleep and work weekdays and weekends (P =. 05–.001). (Chronobiology International, 18(2), 249–261, 2001)