The hypothalamus plays an important role in regulation of sleep/wakefulness states. While the ventrolateral preoptic nucleus (VLPO) plays a critical role in the initiation and maintenance of sleep, the lateral posterior part of the hypothalamus contains neuronal populations implicated in maintenance of arousal, including orexin-producing neurons (orexin neurons) in the lateral hypothalamic area (LHA) and histaminergic neurons in the tuberomammillary nucleus (TMN). During a search for neurons that make direct synaptic contact with histidine decarboxylase (HDC)-positive, histaminergic neurons (HDC neurons) in the TMN and orexin neurons in the LHA of male mice, we found that these arousal-related neurons are heavily innervated by GABAergic neurons in the preoptic area (POA) including the VLPO. We further electrophysiologically characterized GABAergic neurons in the VLPO (GABA neurons) that make direct synaptic contact with these hypothalamic arousal-related neurons. These neurons (GABA or GABA neurons) were both potently inhibited by noradrenaline (NA) and serotonin (5-HT), showing typical electrophysiological characteristics of sleep-promoting neurons in the VLPO. This work provides