What Is a Circadian Rhythm? A Clock That Runs Without the Sun
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Put a person in a bunker with no clocks, no daylight and no schedule, and they do not drift into chaos. They keep a regular cycle of sleeping and waking, close to twenty-four hours but not exactly, which drifts slowly out of step with the outside world. That result, obtained repeatedly since the 1960s, establishes that the rhythm is generated internally and merely corrected by daylight rather than caused by it.
The clock and its parts
The master clock in mammals is the suprachiasmatic nucleus, a pair of small clusters of neurons in the hypothalamus containing a few tens of thousands of cells. Each of those cells contains a molecular oscillator: a set of genes whose proteins accumulate, then feed back to switch off their own production, then degrade, allowing the cycle to restart, with the whole loop taking roughly twenty-four hours. The mechanism was worked out in fruit flies and won the Nobel prize in 2017. Nearly every tissue in the body contains the same clock genes and runs its own local rhythm, so the liver, the heart and the skin each keep time, and the master clock's job is to keep them synchronised with each other and with the outside world. That distributed arrangement explains why circadian disruption affects metabolism, immunity and cardiovascular function rather than only sleepiness, since a great many physiological processes are scheduled rather than constant.
How it is set
The clock runs slightly long in most people, so it must be reset daily, and the dominant signal is light:
- •Specialised retinal cells containing melanopsin detect light independently of the rods and cones used for vision, which is why some blind people still entrain normally and others do not
- •These cells are most sensitive to short-wavelength blue light, which is why blue-enriched light at night has an outsized effect relative to its brightness
- •Light in the morning advances the clock, making a person sleep and wake earlier, while light in the evening delays it, and the size of the shift depends on timing more than on duration
- •Melatonin is released by the pineal gland when the clock signals darkness, and it is a signal of night rather than a sedative, which is why small doses timed correctly shift the clock while large doses mostly make people drowsy
- •Secondary cues include meal timing, exercise, temperature and social schedules, which entrain peripheral clocks and can pull them out of alignment with the master clock
- •Sleep itself is governed by two processes: this circadian signal and a separate homeostatic pressure that builds the longer a person is awake, and the interaction of the two explains why sleepiness is not simply proportional to hours awake
Chronotypes and the mismatch
Individuals differ substantially in the phase their clock settles at, a trait called chronotype, which is largely genetic, shifts with age, and is not a matter of discipline. Adolescents are biologically shifted later, with a delay that begins around puberty and reverses in the early twenties, which is well enough established that it underpins arguments for later school start times, and trials of delayed starts have reported more sleep and better attendance. Late chronotypes living on schedules set for early ones accumulate social jetlag, the difference between the sleep timing they choose on free days and the timing imposed on work days, which is associated in population studies with worse metabolic and mental health outcomes. Shift work is the extreme case, requiring wakefulness at the circadian low point and sleep at the peak, and long-term night shift work has been classified as a probable carcinogen, with associations reported for cardiovascular disease, metabolic syndrome and depression. Jet lag is the acute version, and it is worse travelling east because advancing a clock that already runs long is harder than delaying it.
What can be done about it
Interventions work best when they use the same signals the system already uses. Bright light in the morning and reduced light in the evening are the most effective and least invasive tools, and light boxes are established treatment for seasonal affective disorder and for delayed sleep phase. Correctly timed low-dose melatonin shifts the clock and is used for jet lag, shift work and some circadian sleep disorders, with the timing mattering far more than the dose. Consistency of wake time anchors the rhythm more effectively than consistency of bedtime. For shift workers, scheduling rotations forward rather than backward, controlling light exposure during and after shifts and protecting sleep opportunity are the practical measures. The evidence on evening screen use is more modest than the popular discussion suggests, with effects real but smaller than the effect of what people do on the device rather than the light it emits, and with blue-blocking filters showing limited benefit in trials.
The takeaway
A circadian rhythm is generated internally by a molecular feedback loop present in nearly every tissue, coordinated by a small cluster of neurons in the hypothalamus, and it runs slightly longer than twenty-four hours so it must be reset daily. Light detected by specialised retinal cells does the resetting, advancing the clock in the morning and delaying it in the evening, with melatonin acting as a signal of darkness rather than as a sedative. Chronotype is largely genetic, adolescents are biologically shifted later, and shift work forces a lasting mismatch.