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After benzodiazepine (BDZ) administration, a decrease in cardiac vagal tone has been described during wakefulness, but data on cardiac autonomic function during sleep are lacking. Melatonin (MLT), reported to have hypnotic properties, caused an increase in vagal tone in animals. The aim of this study was to evaluate heart rate (HR) variability during sleep after a single bedtime dose of triazolam (TRI, 0.125 mg) and MLT (100 mg) in six healthy young subjects. We evaluated tonic (vagal activity) HR modifications in relation to sleep as well as phasic (sympathetic activity) HR modifications in relation to spontaneous body movements during rapid-eye-movement (REM) and non-REM (NREM) sleep. No significant change in sympathetic activity was observed after TRI and MLT in comparison with placebo, whereas TRI caused a significant decrease in vagal tone during sleep. Our nocturnal study seems to confirm previous diurnal findings about a decrease in vagal tone by BDZs.