Cirkadianska

Cirkadianska is an adjective used to describe phenomena related to circadian rhythms, the roughly 24-hour cycles that regulate many biological processes in living organisms, including humans. The term comes from Latin circa diem, meaning “about a day.” In biology, circadian rhythms arise from an endogenous clock that persists without external cues but is normally synchronized by the environment, primarily through light–dark cycles.

In mammals, the central clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN coordinates timing across the body by synchronizing peripheral clocks found in tissues such as the liver, heart, and pancreas. These clocks operate through transcription-translation feedback loops involving clock genes, including CLOCK and BMAL1 (ARNTL) that promote the expression of PER and CRY genes. The protein products eventually inhibit their own transcription, creating sustained ~24-hour rhythms.

Light information reaches the SCN via the retinohypothalamic tract, with intrinsically photosensitive retinal ganglion cells playing a key role. The pineal gland modulates melatonin production to signal night, while cortisol rhythms help promote wakefulness in the morning.

Disruptions to circadian timing are associated with sleep disorders, metabolic and cardiovascular risks, mood disturbances, and impaired cognitive function. Factors such as shift work, jet lag, and irregular eating or exposure to artificial light can desynchronize internal clocks. Chronobiology studies how timing affects health and behavior and explores interventions—like timed light exposure, meals, and drug administration—to optimize outcomes.