how does our brain control sleep-wake cycles?

Sleep is regulated by two body systems: sleep/wake homeostasis and the circadian biological clock. When we have been awake for a long period of time, sleep/wake homeostasis tells us that a need for sleep is accumulating and that it is time to sleep. The internal circadian biological clocks regulate the timing of periods of sleepiness and wakefulness throughout the day. This biological clock is controlled by a part of the brain called the Suprachiasmatic Nucleus (SCN), a group of cells in the hypothalamus that respond to light and dark signals. Disruptions to these systems can cause difficulties with sleep, like insomnia.
Learn more, visit the Sleep Foundation website.