Sirkadian

Sirkadian rhythms are endogenous, approximately 24-hour cycles that regulate a wide range of biological processes in organisms. In humans and other mammals, they coordinate sleep-wake cycles, hormone secretion, metabolism, body temperature, and cognitive performance. Although they persist in constant conditions, they are typically synchronized (entrained) to the environment by cues, known as zeitgebers, with light being the dominant signal.

The central clock in mammals is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN synchronizes peripheral clocks present in nearly all tissues. Molecularly, circadian timekeeping relies on transcription-translation feedback loops involving clock genes and proteins, including CLOCK, BMAL1, PER, and CRY, with auxiliary elements like REV-ERB and ROR stabilizing the cycle.

Entrainment occurs mainly through light detected by retinal cells, which send signals to the SCN via the retinohypothalamic tract. Melatonin production by the pineal gland increases at night, reinforcing circadian timing. Feeding times and physical activity can also reset peripheral clocks.

Disruption of sirkadian rhythms—such as through shift work, irregular sleep schedules, or rapid time-zone changes—has been linked to sleep disorders, metabolic dysfunction, mood disorders, and elevated risk for cardiovascular disease. Chronotype, the preference for morning or evening activity, varies among individuals and can be influenced by genetics, age, and environment.

Research methods include actigraphy and polysomnography to study sleep–wake patterns, and measurement of dim-light melatonin onset (DLMO) to assess circadian timing. Beyond humans, circadian clocks regulate plant physiology and animal behavior, illustrating evolutionary conservation of the mechanism.

Chronotherapy and pharmacology increasingly consider circadian timing to optimize treatment. The term circadian derives from Latin circa diem, meaning about a day; sirkadian is the spelling used in some languages.