Histaminergic neurons are localized in the tuberomammillary nucleus (TMN) of the posterior hypothalamus. These neurons have been postulated to have wake-promoting and arousal-maintaining functions. Previously, we found that the acute and specific activation of histaminergic neurons in the TMN slightly increased wakefulness in mice during the light periods. However, the precise neural mechanism of histaminergic neurons in the regulation of sleep-wake cycles have remained to be elucidated. In this study, we examined the wake-regulatory mechanism of histaminergic neurons by using chemogenetic inhibition methods. We virally expressed hM4Di specifically in the TMN-histaminergic neurons of histidine decarboxylase (Hdc)-Cre mice. Then clozapine-N-oxide (CNO) or saline (SA) were injected at ZT3 (light period) or ZT12 (dark period). We found that CNO injection at ZT12 significantly decreased wakefulness and increased NREM sleep due to prolonged duration of NREM sleep. We also found that the wake to wake transition was significantly decreased and NREM to NREM sleep transition was increased in the CNO group. On the other hand, CNO injection at ZT3 did not alter sleep-wake status. These results indicated that histaminergic neurons in the TMN are important for maintaining the arousal status in mice during the dark periods.