zirkadian

Zirkadian rhythms, also known as circadian rhythms, are endogenous, entrainable 24-hour cycles that regulate numerous physiological and behavioral processes in living beings. The term derives from Latin circa diem, around a day. In mammals, a master clock resides in the suprachiasmatic nucleus of the hypothalamus, while peripheral clocks exist in nearly all tissues. Cellular timekeeping relies on transcription-translation feedback loops involving clock genes such as CLOCK, BMAL1, PER1-3 and CRY1-2. Light, detected by intrinsically photosensitive retinal ganglion cells with melanopsin, is the dominant environmental cue that synchronizes the SCN.

Outputs of the clock regulate the sleep-wake cycle, hormone secretion (notably melatonin and cortisol), body temperature, and metabolism. Circadian biology extends beyond animals to plants, fungi, and many microorganisms, enabling adaptation to daily light cycles.

Disruptions, such as irregular sleep schedules, jet lag, or shift work, are associated with adverse health outcomes including metabolic, cardiovascular, and mood disorders. Mitigating strategies include regular light exposure, consistent sleep timing, timed meals, and chronotherapy approaches for drug delivery. The field of zirkadian biology, or chronobiology, encompasses research across medicine, agriculture, and occupational health.