Man Sleeping with Visible Brain
The sleep-wake cycle is an important circadian rhythm. When it is dark, the suprachiasmatic nucleus directs the pineal gland (located in another part of the brain) to secrete melatonin. This hormone peaks at night and ebbs during the day, and plays a key role in the sleep-wake cycle. Light disrupts melatonin production, and individuals who are exposed to too much light in the evenings, or who work the night shift, may experience problems sleeping as well as disruption in the menstrual cycle and other systems.