Circadian rhythm

Circadian rhythms are recurring physical, mental and behavioral patterns that follow an approximately 24-hour cycle. They occur across many forms of life, from microorganisms and plants to animals. In humans, rhythms are produced by biological clocks in tissues and organs, which operate through molecular processes involving genes and proteins. A central clock coordinates the timing of these peripheral clocks so that bodily systems are organized in relation to one another and to the external day-night cycle.

The principal human circadian pacemaker is the suprachiasmatic nucleus (SCN), a group of nerve cells in the hypothalamus. Information about light received by the eyes helps the SCN synchronize internal timing with the environment. The SCN also coordinates rhythms across body tissues and influences the timing of melatonin production. Circadian control is distributed: local clocks and other physiological processes also contribute, rather than every daily rhythm being generated by a single mechanism.

Light and darkness are the most influential cues for aligning human circadian timing. Food, physical activity, stress, social conditions and temperature can also affect rhythms. The timing of light exposure matters: for people on a typical nighttime sleep schedule, bright light in the evening tends to delay the clock, while bright morning light tends to advance it. Effects vary with timing and individual circumstances, so these responses are general patterns rather than exact predictions for every person.

The sleep-wake cycle is a prominent circadian rhythm, but sleep timing also depends on sleep pressure that builds during wakefulness. These interacting influences help explain why the circadian system can promote sleepiness near a usual bedtime and alertness before a usual waking time. Circadian timing also affects physiological patterns such as hormone release, body temperature, appetite and digestion. A mismatch between internal timing and a person's schedule or light-dark environment can affect sleep, though the effects and causes differ among individuals.

Circadian biology is studied by observing daily patterns in organisms and investigating the genes, proteins and neural pathways involved in timekeeping. Because light can reset circadian timing, controlled light exposure is also studied and used in some clinical contexts. Research reviews report that light can influence human circadian rhythms, sleep and mood, while emphasizing that effects depend on exposure and context. Circadian rhythms are a general biological framework, not a diagnosis by themselves.