circadiaans

Circadiaans are the term used here to refer to circadian rhythms, endogenous approximately 24-hour cycles in physiology and behavior that persist in constant conditions. They regulate sleep-wake timing, hormone secretion, body temperature, metabolism, and cognitive performance. The word circadian comes from Latin circa diem, about a day.

Molecularly, circadian rhythms arise from transcription-translation feedback loops. In mammals, CLOCK and BMAL1 drive the expression of PERIOD and CRY genes; accumulation of PER and CRY proteins inhibits CLOCK-BMAL1, producing rhythmic gene expression with ~24-hour periodicity. The central clock is the suprachiasmatic nucleus (SCN) of the hypothalamus, which coordinates peripheral clocks in tissues through neural and hormonal signals. Light is the primary cue to reset the clock, perceived by melanopsin-containing retinal ganglion cells and transmitted via the retinohypothalamic tract.

Circadian systems are adaptive but flexible, aligning internal time with the external environment. Entrainment is achieved by light-dark cycles, meal timing, activity, and temperature changes. Individual differences in timing, called chronotypes, influence when people are most alert or sleepy. Circadian disruption can arise from jet lag, shift work, or irregular routines, and is a feature of several circadian rhythm sleep disorders, such as delayed or advanced sleep phase disorders and non-24-hour sleep-wake disorder.

The study of circadian biology, sometimes referred to in the plural as circadiaans, informs medicine, work scheduling, and strategies for improving sleep and overall health.