Scientists and researchers at Stanford University have identified a brain circuit which vital to the sleep-wake cycle. The identified circuit has a key role within our reward system; that's why every time we wake up, we're excited for the day and how we know not to look for food when we sleep. This new finding can lead to more discovery in psychiatry disorders, which are characterized by sleep-wake cycle disturbances. 

Source:  http://medicalxpress.com/news/2016-09-brain-circuit-sleep-wake-states-sleep-preparation.html

Photo Source: By BruceBlaus (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons