Parasympathetic nervous system activity dominates during non-rapid eye movement (REM) sleep, whereas the sympathetic nervous system is dominant during REM sleep. The balance between the sympathetic and parasympathetic nervous systems reduces the heart rate and blood pressure during non-REM sleep and increases these parameters during REM sleep and wakefulness. Heart rate variability analysis, a non-invasive method of autonomic nervous system analysis, may be useful in screening for obstructive sleep apnea and also to predict orthostatic hypotension secondary to autonomic disturbances in patients with REM sleep behavior disorder.