Fig. 2.

A simplified depiction of mammalian sleep modulation. The suprachiasmatic nucleus (SCN) takes in stimuli from an array of inputs, most notably photic input via the retinohypothalamic tract (RHT). The SCN acts as the central circadian pacemaker, entraining other brain regions and a large array of peripheral tissues, including the liver and gastrointestinal tract, via, both, neural connections and humoral factors. There are direct projections from the SCN to the lateral hypothalamus (LH), which contains orexin neurons, and to the dorsomedial hypothalamus (DMH). The DMH projects broadly to sleep and arousal centers. Mutual inhibition exists between the arousal-promoting locus coeruleus (LC) and the sleep-promoting ventrolateral preoptic area (VLPO) which facilitates the flip/flop between wakefulness and sleep states