Humans circadian rhythm are controlled by a master biological clock- a tiny clusters of neurons in the hypothalamus in the brain called the Suprachiasmatic nucleus (SCN). The SCN is the internal pacemaker that governs the timing of circadian rhythms, including the sleep-wake and mental alertness cycle.
Bright light, especially sunlight plays important role in regulating sleep-wake cycle. As the sun sets each day, the decrease in availability of light is detected by the Suprachiasmatic nucleus (SCN) through its connections with the visual system. In turn, the SCN triggers an increased in the production of hormone called melatonin ( a hormone produced by pineal gland).
Increased in blood level of melatonin make you feel sleepy and reduce activity levels. At night, blood levels of melatonin rises, peaking between 1:00 and 3:00 A.M. shortly before sunrise, the pineal gland all but stops producing melatonin, and you soon wake up. As the sun rises, exposure to sunlight and other bright light suppresses melatonin level, and they remain very low throughout the day.
