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Cardiac autonomic activity during sleep is only very slightly influenced by the emotional state of the patient and, unlike some of the traditional tests of autonomic function, may be studied in all patients. In an attempt to evaluate autonomic function in patients with different neuropsychiatric disorders, two different methods of quantifying the changes in sympathetic and parasympathetic cardiac control during sleep were used: (1) the ratios of consecutive R-wave (R-R) intervals before and after spontaneous body movements; (2) spectral analysis of R-R intervals. It was found that more than one third of patients with presenile Alzheimer's disease had defective cardiac sympathetic control. Untreated parkinsonian patients showed predominantly defective parasympathetic, and to a lesser extent sympathetic, function during sleep. In these patients, as well as in patients with multiple sclerosis, autonomic evaluation during sleep led to earlier detection of impairment than the traditional tests during wakefulness. Narcoleptic patients and patients with panic disorder showed normal autonomic function during sleep, but had altered control levels during the wakeful period before sleep. The findings in these narcoleptic patients were probably related to the impairment of their sleep-wake cycle. The sympathetic overactivity found in patients with panic disorder was probably a result of cognitive activity, as the nocturnal data excluded an intrinsic defect in autonomic regulation.