How sleep works
Circadian Rhythm Explained: Your Body Clock
Your circadian rhythm is a roughly 24-hour internal clock that times sleep, alertness, hormones and body temperature. Understanding it explains why light, regular routines and timing matter so much for sleep.
The master clock
A small region in the hypothalamus called the suprachiasmatic nucleus (SCN) acts as the master clock. It coordinates clocks in nearly every tissue, influencing when you feel sleepy and alert, body temperature, hormone release, digestion and more. Research into the genes that run these clocks earned the 2017 Nobel Prize in Physiology or Medicine.
Slightly longer than a day
Studies in which people lived without time cues found the human circadian period averages slightly longer than 24 hours (a 1999 study in Science measured an average of about 24 hours and 11 minutes). That means your clock needs to be reset a little each day, mainly by light, or it will drift later.
Light is the main signal
Light reaching the eyes is the strongest time cue. Light in the morning shifts the clock earlier, and light in the evening and night shifts it later. Other cues, called zeitgebers, include meal timing, physical activity and social schedules, but they are weaker.
Two forces behind sleepiness
Sleep is regulated by two systems working together. Sleep pressure builds the longer you are awake. The circadian system sends an alerting signal that rises through the day and drops in the late evening. You fall asleep most easily when sleep pressure is high and the alerting signal has dropped. The well-known post-lunch dip in alertness is partly circadian.
Melatonin
As evening falls, the SCN signals the pineal gland to release melatonin, which tells the body it is night. Bright light, especially in the blue part of the spectrum, suppresses melatonin. Melatonin levels stay elevated through the night and fall toward morning.
Chronotypes: larks and owls
People differ in their natural timing. Morning types (larks) feel best early; evening types (owls) feel best later. Chronotype is partly genetic and changes with age: children tend to be early, teens shift later, and older adults shift earlier again.
When the clock and life do not match
Shift work, jet lag and social jet lag all create a mismatch between your internal clock and your schedule. Circadian rhythm sleep-wake disorders include delayed and advanced sleep-wake phase disorders, shift work disorder and jet lag disorder. Chronic circadian disruption has been linked in research to metabolic, cardiovascular and mood problems. Consistent wake times, morning light and dark nights are the foundations of a healthy rhythm.
Frequently asked questions
Can I change from a night owl to a morning person?
You can shift your timing somewhat with consistent wake times, morning light and dim evenings, but your underlying tendency is partly genetic.
Why do I get sleepy after lunch?
There is a natural circadian dip in alertness in the early afternoon. A heavy meal and sleep loss can make it more noticeable.
What resets the body clock fastest?
Light exposure at the right time is the most powerful tool. Morning light moves the clock earlier and evening light moves it later.
Sources
This page is general information, not a diagnosis or medical advice. Read our medical disclaimer.