Interneurons control brain arousal states The underlying circuit mechanisms coordinating brain arousal and motor activity are poorly understood. Liu et al. found that glutamic acid decarboxylase 2 (GAD2)–expressing, but not parvalbumin-expressing, interneurons in a part of the brain known as the substantia nigra promote sleep (see the Perspective by Wisden and Franks). Parvalbuminergic neurons fire at higher rates in states of high motor activity, and their activation increases movement termination consistent with the function of the substantia nigra in suppressing unwanted movements during action selection. By contrast, GAD2 neurons are preferentially active in states of low motor activity. In addition to motor suppression, their activation powerfully enhances the transition from quiet wakefulness to sleep, which differ mainly in the arousal level rather than motor behavior. GAD2 interneurons thus provide general suppression of both motor activity and brain arousal to promote states of quiescence. Science , this issue p. 440 ; see also p. 366