• Valérie Mongrain and Marie Dumont.
• Chronobiology Laboratory, Sacré-Coeur Hospital of Montréal, Québec, Canada.
• Sleep. 2007 Jun 1; 30 (6): 773-80.

Study ObjectivesTo evaluate the influence of chronotype on sleep stages and quantitative sleep EEG when sleep pressure is increased and sleep schedule remains constant.DesignA 5-day session comprising an adaptation night, a baseline night, two nights of sleep fragmentation, and a recovery night.SettingChronobiology laboratory.ParticipantsTwenty-four healthy subjects aged 19-34 years: 12 morning types and 12 evening types selected by questionnaire. Each group included 6 men and 6 women with a habitual sleep duration of 7 to 9 hours.InterventionsTwo nights of behavioral sleep fragmentation induced by forced 5-min awakenings every half-hour.Measurements And ResultsEach night of polysomnography recording lasted 8 hours and was based on each subject's preferred sleep schedule. On both nights of sleep fragmentation, stage 1 sleep increased, while both total sleep time and minutes of slow wave sleep decreased. No difference was observed in sleep architecture between morning types and evening types during sleep fragmentation nights or during recovery night. Spectral analysis of all-night NREM sleep EEG showed that during the recovery night, morning types had a larger fronto-central increase in low frequency activities and a larger centro-parietal decrease in 14-15 Hz activity than evening types. The largest group difference was for slow wave activity in the fronto-central area during the first part of the sleep episode.ConclusionsThese results add further support to a postulated difference in homeostatic sleep regulation between morning types and evening types, with morning types showing indications of a higher homeostatic response to sleep disruption.

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