circadien

Circadien, or circadian in English, refers to biological processes that display endogenous, entrainable oscillations with a period of about 24 hours. These rhythms arise from internal clocks but are synchronized to the environment, mainly through the light-dark cycle.

At the cellular level, circadian timing is generated by transcription-translation feedback loops of clock genes, including BMAL1, CLOCK, PER, and CRY. The proteins produced by these genes regulate their own expression in roughly daily cycles, creating an approximately 24-hour rhythm. In mammals, the master clock sits in the suprachiasmatic nucleus (SCN) of the hypothalamus, coordinating timing signals to peripheral clocks found in most tissues such as the liver, heart, and lungs. Peripheral clocks can be entrained by cues like feeding and temperature in addition to signals from the SCN.

Light is the principal environmental cue for circadian systems. Intrinsically photosensitive retinal ganglion cells detect light and transmit information to the SCN, helping to synchronize internal time with the external day-night cycle. Melatonin, produced by the pineal gland, and cortisol, released by the adrenal glands, exhibit characteristic daily rhythms that reflect circadian organization.

Functions and relevance include the regulation of sleep-wake cycles, hormone secretion, metabolism, body temperature, and cognitive performance. Disturbances of circadian timing can lead to jet lag, shift-work disorder, and advanced or delayed sleep phase disorders. Understanding circadian biology informs approaches such as chronotherapy, light therapy, and the optimization of work and living environments to support healthy rhythms. The term derives from Latin circa diem, roughly “about a day.”