Circadian rhythm is the internal "biological clock" with a period of about 24 hours, setting the daily fluctuations of sleep and wakefulness, body temperature, hormones and metabolism. The master pacemaker is the suprachiasmatic nucleus (SCN) in the hypothalamus: it checks against the light reaching the retina and synchronizes the "clocks" in virtually all tissues of the body.
How the biological clock works
The SCN is the "conductor" that, by the level of light, controls production of the hormone melatonin: in the evening it rises and drowsiness sets in, in the morning light suppresses it. At the same time, almost every organ has its own peripheral clock, which the SCN synchronizes with one another. It is light that is the strongest signal "resetting the hands" of these clocks.
Sleep, hormones and metabolism
The circadian rhythm governs not only sleep. It drives the fluctuations of cortisol (peaking in the morning), appetite and digestion, body temperature and insulin sensitivity — which is higher during the day and lower at night. So the timing of meals and light affects metabolism no less than their amount.
Rhythm disruption and health
A mismatch between the internal clock and real time (shift work, jet lag, bright screen light in the evening) in the short term causes drowsiness and reduced concentration. In the long term it raises the risk of obesity, diabetes, metabolic syndrome, mood disorders and cardiovascular diseases. Constant desynchronization also increases chronic stress.
Circadian rhythms and aging
With age the SCN signal weakens, sleep becomes more shallow and fragmented, and melatonin production declines. Chronic disruption of circadian rhythms is linked to an accelerated biological age. Conversely, a steady sleep schedule, daylight and a dark bedroom support healthspan and work toward longevity.
Calculate using this marker
Calculators where Circadian rhythm of blood pressure is used directly:
Frequently asked questions
How does the circadian rhythm differ from a sleep schedule?
The circadian rhythm is the body's internal biological clock (endogenous, about 24 hours). A sleep schedule is behavior. Health suffers when behavior diverges from the clock: for example, with night shifts there is harm even with enough sleep.
Is screen light harmful in the evening?
Yes. Bright, especially blue, light in the evening suppresses melatonin and shifts the clock "later," making it harder to fall asleep. Dim light in the evening and bright daylight in the morning help.