Sleep structure and EEG power density in morning types and evening types during a simulated day and night shift.

The objective of this study was to examine circadian and homeostatic regulation of sleep in humans. In 8 morning types (M-types) and in 8 evening types (E-types), sleep was recorded during 3 successive nights and, after shifting sleep to the daytime, during 3 consecutive days. Night sleep was highly similar in the M-types and E-types. Day sleep clearly differed from night sleep in both types: Day sleep was shorter and had a longer first REMS episode. Furthermore, EEG power density recorded during non-REMS in the delta and theta frequency bands was higher during all day-sleep periods. Remarkably, the enhancements did not occur in non-REMS episode 1 but were delayed. This was interpreted as an inhibition of EEG power density at the beginning of sleep, possibly caused by the time course of body temperature and/or by the higher REMS propensity. Also, clear differences between the types became apparent: Only in the E-types, the non-REMS episodes shortened in response to the shift in bedtime, and probably related to this, the time course of EEG power density over consecutive non-REMS episodes became almost flat. It was concluded that the circadian system exerts not only an influence on sleep duration and REMS propensity, but also affects the time course of the non-REMS process.