In REM sleep behavior disorder (RBD) it has been reported that tachycardia may not accompany the impressive movements associated with dream mentation. Up to now there are no data concerning the cardiac autonomic activity during wakefulness as well as during sleep out of nocturnal dream-enacting behaviors, in RBD. We evaluated 14 RBD patients. Our study shows that only 36% of patients had normal results in all traditional autonomic tests during wakefulness. Moreover, RBD patients had a reduced tonic and phasic heart rate variability during sleep. Autonomic evaluation during sleep may show impairment earlier than the traditional tests during wakefulness. No difference was found between idiopathic RBD patients and symptomatic ones.