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A prospective analysis of consecutive ambulatory blood pressure monitorings over a 5 month period identified 50 subjects (35%) who took an afternoon nap during the monitoring. The average duration of daytime sleep, as reported by the patients, was 1.8 +/- 0.6 h compared with the reported 7 +/- 2 h for nighttime sleep. Ambulatory blood pressure values during daytime awake periods were significantly higher compared with daytime sleep and nighttime sleep. The blood pressure decline during daytime sleep and nighttime sleep was similar. The pattern of blood pressure changes during daytime sleep was comparable in normotensive (n = 16), untreated (n = 10), and treated hypertensives (n = 24), irrespective of age, gender, and the level of blood pressure. The marked decline in blood pressure during daytime sleep suggests that sleep itself, rather than an endogenous circadian rhythm, is responsible for the blood pressure dip observed during both daytime sleep and nighttime sleep. Ignoring actual sleeping time in people who sleep during the day may greatly distort the day-night ambulatory blood pressure difference, when the latter is calculated on the basis of arbitrarily defined "day" and "night" periods.
Barr et al. (Mon,) studied this question.