[Diurnal sleep].

The distribution of sleeping and waking over 24 h shows, apart from a high circadian propensity to sleep during nighttime, additional preferred phase position for sleep during daytime. This was the result of studies in monotonous experimental conditions. In addition to an increase in sleep propensity at about 2.00 p.m. further preferred phase positions for sleep could be found at 10.00 a.m. and 6.00 p.m., they however were less robust. Taken together the results indicate an ultradian 4-h periodicity for the propensity to sleep. The REM-sleep also demonstrates a characteristic daily variation with a maximum in the early morning and a minimum in the evening. On the other hand only delta-sleep shows interdependence to the proceeding waking time.