Circadian Rhythms Explained

A Clock That Runs Slightly Long

A cluster of cells in the hypothalamus, called the suprachiasmatic nucleus (SCN), just above where the optic nerves cross, keeps time for the whole body. It schedules alertness, body temperature, hormone release and the drive to sleep across the day.^1,2

Left to itself, the human clock runs a little longer than a day. Measured under tightly controlled lighting, its average period was 24.18 hours, far closer to 24 than the 25-hour figure from earlier studies, which turned out to have been skewed by light exposure.¹ Because it is not exactly 24 hours, it needs resetting every day.

How Light Sets the Clock

Light is the reset signal. Specialized light-sensing cells in the retina, separate from the rods and cones used for vision, report light levels directly to the master clock.² Light in the morning pulls the clock earlier; light in the evening pushes it later.^6,7

Evening screens are a measurable example. In a controlled study, reading on a light-emitting tablet before bed suppressed the evening rise of melatonin, delayed the body clock, made it take longer to fall asleep and left readers less alert the next morning, compared with reading a printed book.⁷ More on that in screens, blue light and sleep.

Two Forces Behind Sleep

Sleep pressure. The longer you are awake, the stronger the drive to sleep becomes. Sleep discharges it.³

The clock. The circadian signal for wakefulness rises through the day and holds sleep pressure off until evening, then falls away so sleep can start.³

Good sleep happens when the two line up: high pressure and a clock that has moved into its night. Trying to sleep when the clock says day is why daytime sleep after a night shift is short, and why a long-haul flight can leave you wide awake at 3 a.m.³

Early Birds and Night Owls

People differ in where their clock sits, a trait called chronotype. It also moves with age. In a survey of about 25,000 people, sleep timing grew steadily later through adolescence, peaked at about 19.5 years in women and 20.9 years in men, then drifted earlier for the rest of life.⁴

When work or school forces a different schedule from the one the clock prefers, sleep on free days shifts later to compensate. Researchers call the gap social jetlag, and in a large population study people with more of it were more likely to be overweight.⁸

When Rhythm and Schedule Collide

Persistent mismatch between the clock and the schedule can become a circadian rhythm sleep-wake disorder.⁵ See shift work sleep disorder and managing jet lag.

The Main Types

Delayed sleep-wake phase. Sleep and waking happen much later than the schedule demands, most often in adolescents and young adults.⁵

Advanced sleep-wake phase. The reverse: sleepiness in the early evening and waking well before dawn.⁵

Irregular rhythm. Sleep broken into scattered bouts across the 24 hours, with no main sleep period.⁵

Non-24-hour rhythm. A clock that never locks to the day and drifts later and later, most common in people who are totally blind, since light cannot reach the clock.^2,5

Shift work and jet lag. Mismatch imposed from outside by work hours or travel across time zones.⁵

The usual result is insomnia at the intended bedtime, sleepiness during the intended day, or both.⁵ Over the long term, misalignment carries health costs too: a meta-analysis of 34 studies linked shift work to a 23% higher risk of heart attack.⁹

Supporting a Steady Rhythm

Because light and timing drive the clock, the practical levers are daylight early in the day, dimmer and screen-free evenings, and sleep timing that stays similar on work days and free days.^6,7,8 Timed light and planned naps are also part of the AASM’s recommendations for night work.⁶ The complete sleep hygiene guide covers the day-to-day detail.

If sleep timing problems or daytime sleepiness continue despite a steady routine, an evaluation can look for a circadian or other sleep disorder.