Circadianer

Circadianer are endogenous, roughly 24-hour cycles that coordinate many aspects of physiology and behavior in organisms, including the sleep–wake cycle, hormone release, body temperature, and metabolism. These rhythms persist in constant conditions, indicating an internal clock, but they are normally synchronized, or entrained, to the environment, primarily by the light-dark cycle.

In mammals, the central clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus. At the molecular level, circadianer arise from transcription-translation feedback loops involving core clock genes such as CLOCK and BMAL1, which activate PER and CRY genes. The PER and CRY proteins accumulate, inhibit CLOCK:BMAL1 activity, and once they degrade, the cycle restarts. Peripheral clocks are present in tissues like the liver, fat, and muscle, and they are synchronized with the SCN through neural and hormonal signals and by feeding schedules.

Light is the dominant environmental cue, or zeitgeber, that resets the SCN. Retinal cells containing melanopsin detect light and convey signals to the hypothalamus, adjusting the clock. Melatonin, produced by the pineal gland in darkness, signals night to the body and helps regulate sleep timing and seasonal rhythms.

Variation among individuals leads to different chronotypes, such as morning larks or evening owls, reflecting differences in circadian timing and sensitivity to zeitgebers. Disruption of circadianer, as seen in shift work or rapid time-zone changes, can affect sleep, metabolism, mood, and overall health. Chronotherapy and carefully scheduled light exposure are used to mitigate misalignment and optimize functioning.