The sleep or wakefulness status is thought to be determined by a complex balance between the sleep promoting and arousal centers of the brain. For example,wakefulness is promoted by the brain stem and hypothalamus. These systems are inhibited, at least in part, by the secretion of γ-aminobutyric acid
during SWS by neurons near the arousal centers.The ventrolateral
preoptic nucleus also inhibits the arousal systems during sleep. Products
of the immune system, such as cytokines, also play a part in the balance
between sleep and wakefulness.