Phase relationships between sleep-wake cycle and underlying circadian rhythms in Morningness-Eveningness

Abstract Ashorter phase angle between habitual wake time and underlying circa-dian rhythms has been reported in evening types (E types) compared to morn-ing-types (M types). In this study, phase angles were compared between 12 E types and 12 M types to verify if this difference was observed when the sleep schedule was relatively free from external social constraints. Subjects were selected according to their Morningness-Eveningness Questionnaire score (MEQ score). There were 6 men and 6 women in each group (ages 19-34 years), and all had a habitual sleep duration between 7 and 9 h. Sleep schedule was recorded by actigraphy and averaged over 7 days. Circadian phase was esti-mated by the hour of temperature minimum (Tmin) in a 26-h recording and by the timing of the onset of melatonin secretion (dim-light melatonin onset [DLMO]) measured in saliva samples. Phase angles were defined as the interval between phase markers and averaged wake time. Results showed that, in the present experimental conditions, phase angles were very similar in the 2 groups of sub-jects. However, results confirmed the previously reported correlation betwee