circadian

Circadian refers to biological processes that follow an approximately 24-hour cycle, driven by endogenous clocks and synchronized by environmental cues. The term combines Latin circa, around, and dies, day. Circadian rhythms are found across many organisms, including mammals, plants, fungi, and bacteria, and regulate a wide range of physiological and behavioral functions.

In mammals, a central clock resides in the suprachiasmatic nucleus (SCN) of the brain, coordinating peripheral clocks present in nearly all tissues. At the molecular level, circadian timing arises from transcription-translation feedback loops involving core clock genes such as CLOCK and BMAL1, which activate transcription of PER and CRY genes. The PER and CRY proteins accumulate, inhibit CLOCK-BMAL1 activity, and produce roughly 24-hour cycles. Light is the primary zeitgeber, or time cue, resetting the SCN through signals from retinal photoreceptors, including melanopsin-containing cells, which project to the SCN via the retinohypothalamic tract.

In addition to light, other cues such as feeding times, temperature, and social activities can entrain clocks, particularly peripheral tissues. Melatonin, produced by the pineal gland in the dark, conveys night information and helps regulate sleep. Circadian organization influences metabolism, hormone secretion, body temperature, and cognitive performance, among other processes.

Disruption of circadian timing—through shift work, jet lag, or irregular sleep patterns—has been associated with sleep disorders and may contribute to metabolic, cardiovascular, and mental health risks. Research in chronobiology explores mechanisms, health implications, and therapeutic approaches, including light therapy, timed medication (chronotherapy), and sleep hygiene strategies to align internal clocks with external environments.