Blood Pressure Control by Day Versus Night

Abstract

It has long been known that blood pressure and heart rate vary considerably throughout the day, and that they fall with sleep, at which time they both become less variable [1–4] (Fig. 1). Detailed analysis of changes with time have been made possible by intra-arterial monitoring, and these have suggested that the major fluctuation is due (in part at least) to an inherent diurnal rhythm. This has been supported by the fact that blood pressure tends to rise in the early morning before awakening [5]. However, without EEG monitoring the time of the change from sleep to wakefulness is difficult to determine accurately. Moreover, sleep induces a fall in blood pressure at whatever time of day it occurs. The classical studies required to differentiate between an inherent rhythm and one secondary to the sleep/activity cycle have not yet been carried out.