BACKGROUND: The drop in blood pressure during the night sleeping hours is well documented. Less is known about the drop in blood pressure during daytime sleep. The objective of this survey is to describe the dropping during conventional and referred sleeping hours of night and afternoon sleep, and the relation of the dropping to individual characteristics.METHODS: One hundred and seven consecutive patients, who had undergone blood pressure monitoring between February 1993 and October 1994, were studied. Takeda automatic monitors programmed to provide readings at 0.5 h intervals during a period of 24 h were used. For each patient the following information was recorded: age, sex, height and weight, creatinine, and actual sleeping hours recorded on a diary. RESULTS: There was a clinically important decline both in diastolic (12 mmHg) and in systolic (15 mmHg) blood pressure during night sleep compared with awake time blood pressure. A greater decline was observed among untreated hypertensive individuals. No difference in the average blood pressure during the night sleep was observed between conventional sleeping hours and sleeping hours as recorded by the patient. Among patients with afternoon nap, a similar blood pressure was recorded during the sleeping hours (night and afternoon). No association was found between body mass index, estimated creatinine clearance and awake-asleep differences in blood pressure; only a weak correlation was found between age and blood pressure differences. CONCLUSION: Our findings confirm that a substantial decline of blood pressure occurs during night and afternoon sleep compared with that during awake time. We did not find a clinically relevant difference in blood pressure between conventional day-periods and awake and sleeping hours as reported by the patients.