The body's master clock is the suprachiasmatic nucleus, or SCN, a small region in the hypothalamus that orchestrates circadian rhythms throughout the body. Light is the SCN's most powerful input: bright light, especially the short-wavelength blue light emitted by phones, tablets, and LEDs, suppresses melatonin production by the pineal gland and shifts the internal clock. In darkness, melatonin rises, signaling that it is time to sleep.
Cortisol, the body's main stress hormone, follows an opposite pattern, rising in the early morning hours to promote wakefulness and peaking around the time of waking. Chronic stress, however, flattens this rhythm and disrupts sleep. Together, melatonin and cortisol help align the body's internal clock with the external light-dark cycle, a process reinforced by morning sunlight exposure of roughly 10 to 30 minutes within an hour of waking, which helps set the clock for the night ahead.
Not everyone follows the same schedule. Chronotypes describe individual preferences for sleep timing, with roughly 20 to 25 percent of people being night owls, 25 to 40 percent morning types, and the rest somewhere in between, and chronotype has a significant genetic component involving clock genes such as PER and CRY. When personal schedules conflict with the body's clock, problems arise. Social jet lag refers to the mismatch between weekday and weekend sleep times, while true jet lag results from rapid travel across time zones. Shift work disorder occurs when work schedules force wakefulness against the body's clock, and light exposure timing, melatonin, and gradual schedule shifts are common countermeasures. Adolescents naturally experience a phase delay that pushes their preferred sleep later, which is why early school start times are associated with widespread sleep deprivation and worse outcomes.