Circadian VIPergic neurons of the suprachiasmatic nuclei sculpt the sleep-wake cycle

Although the mammalian rest-activity cycle is controlled by a "master clock" in the suprachiasmatic nuclei (SCN) of the hypothalamus, it is unclear how firing of individual SCN neurons gates individual features of daily activity. Here, we demonstrate that a specific transcriptomically identified population of mouse VIP+ SCN neurons is active at the "wrong" time of the day -nighttime- when most SCN neurons are silent. Using chemogenetic and optogenetic strategies, we show that these neurons and their cellular clocks are necessary and sufficient to gate and time nighttime sleep, but have no effect upon daytime sleep. We propose mouse nighttime sleep, analogous to the human siesta, is a "hard-wired" property gated by specific neurons of the master clock to favor subsequent alertness prior to dawn (a circadian "wake maintenance zone"). Thus, the SCN is not simply a 24h metronome: specific populations sculpt critical features of the sleep-wake cycle. SCN neurons were recorded and collected from cFOS-GFP mice at night before and during the siesta. Mice were between 6 and 8 weeks old. We collected 20 SCN neurons.