Arousal states, such as sleep and wake, are highly correlated with specific rhythmic activity patterns in the neocortex. For example, during sleep, neocortical rhythms are synchronized and dominated by low-frequency oscillations. Recent evidence suggests that inhibitory neurons (INs) are key regulators of neocortical states.Here we investigate how neocortical long-range INs that co-express somatostatin (SST) and neuronal nitric oxide synthase (nNOS) (“SST/nNOS cells”), control patterns of synchronous neocortical activity and arousal states in mice. While the in-vivo function of SST/nNOS cells remains unknown, ex-vivo studies have revealed that they are transcriptionally unique, evolutionarily old, have dense and long-range arbors, and are likely active during sleep.We utilize an intersectional genetic strategy to target and characterize the morphology of these cells, and to monitor and manipulate their activity across arousal states. By reconstructing SST/nNOS arbor, we identify that their dense neurites are restricted and expand across the neocortex. Using 2-photon calcium imaging, we observe that they are active during periods of high cortical synchrony, specifically quiet wakefulness and sleep. By combining optogenetic manipulation with in-vivo silicon probe recordings, we show that activating SST/nNOS increases local cortical synchrony of excitatory neurons and low frequency oscillations, similar as a sleep-like cortical state. Going beyond local activation, we find that chemogenetic activation of SST/nNOS neurons across the whole neocortex can promote the animal to sleep. Overall, our data suggests that neocortical SST/nNOS cells are not only a good sensor of low arousal states but are important effectors of network synchronization and sleep.