Endogenous circadian control of the human autonomic nervous system.

To determine if an endogenous circadian rhythmicity, independent from sleep/wake cycles, exists in autonomic nervous system (ANS) function, heart rate variability analysis of electrocardiogram R-R intervals was applied to data collected during a 27-day forced desynchrony protocol. Results during wakefulness indicate that the circadian pacemaker may control both the sympathetic and vagal limbs of the ANS. Vagal tone was maximal during the circadian phase corresponding to the usual sleep episode (although these measurements were made in the absence of sleep) with an acrophase at 4 AM to 5 AM. Sympathovagal balance was minimal between 9 AM and 1 PM. These endogenous circadian rhythms in ANS function may contribute to mortality from cardiovascular disease and nocturnal asthma.