Difference in sleep regulation between morning and evening circadian types as indexed by antero-posterior analyses of the sleep EEG.

Circadian types classify individuals according to their preferred timing for activity and sleep, morning and evening types showing, respectively, early or late preferences. This characteristic has been associated with corresponding differences in circadian sleep propensity. In this study, quantitative analysis of the sleep EEG in antero-posterior derivations was used to test the hypothesis that morning and evening types differ not only in the circadian aspect of sleep regulation but also in the homeostatic aspect. Morning types and evening types (six men and six women per group, aged 19-34 years) were selected using the Morningness-Eveningness Questionnaire. They were studied by polysomnography according to their preferred sleep schedule. Spectral activity in four midline derivations (Fz, Cz, Pz, Oz) was calculated separately in nonrapid eye movement (NREM) sleep and in rapid eye movement (REM) sleep. In NREM sleep, morning types showed a steeper decrease of slow-wave activity (SWA; 1-5 Hz) per sleep cycle in the fronto-central derivations and a steeper increase in 13-14 Hz activity in the parieto-occipital derivations than did evening types. Nonlinear regression analysis revealed that the exponential decay rate of relative values of SWA in NREM sleep was faster in morning than evening types, in the frontal derivation. In REM sleep, morning types showed a steeper decrease of high sigma (14-16 Hz) and beta (16-24 Hz) activities across the night in centro-parietal derivations than did evening types. These results show for the first time a clear difference between morning types and evening types in homeostatic sleep regulation.