Circadian variation of cardiac autonomic regulation during 24-h bed rest.

We examined the effect of circadian variation on cardiac autonomic regulation in 11 young and middle-aged, healthy men during 24-h bed rest. Cardiac parasympathetic regulation expressed significant circadian variation: sensitivity of baroreceptor reflex, standard deviation of R-R intervals and the power of high frequency component of R-R interval variability (HFP) increased during the evening (2000-2100 h), were highest during the night (0200-0300 h), and gradually decreased again towards afternoon (1400-1500 h). Cardiac sympathetic regulation, the power of medium frequency component of R-R interval variability (MFP), did not show any significant circadian variation. The autonomic response to orthostatic stress changed after the 24-h bed rest; the sympathetic dominance in response to assuming upright position was before bed rest principally attributable to increased sympathetic activity (MFP increase), whereas after bed rest this was due to withdrawal of parasympathetic activity (HFP decrease). We conclude that the effect of circadian variation must be taken into account, while assessing cardiac autonomic regulation in patients with acute cardiovascular disorders.