Rapid eye movement (REM) sleep behavior disorder (RBD) is characterized by the loss of normal muscle atonia during REM sleep, leading to an increase of phasic muscle activity and complex motor behaviors during the night. There is some evidence that RBD patients have more of slow-wave sleep (SWS) than healthy elderly subjects. No study has looked at quantitative electroencephalogram analysis during non-REM sleep in either primary or secondary RBD. The aim of this study was to assess the increase of SWS and to analyze different electroencephalographic frequency ranges during non-REM sleep in 28 idiopathic RBD patients compared with 28 age- and sex-matched healthy volunteers. Idiopathic RBD patients spent more time in SWS (men: 1.4%; women: 5.9%) than control subjects (men: 0.4%; women: 0.6%; p = 0.004). Spectral analyses demonstrated that idiopathic RBD patients had increased all-night delta power in comparison with control subjects (p = 002). This study shows an increase of SWS and power in the delta band during non-REM sleep in idiopathic RBD patients compared with control subjects. Results are discussed about the possible nigrostriatal dopaminergic impairment in RBD patients and the association between RBD and neurodegenerative disorders.