An endogenous, self-sustaining oscillation of biological processes in living organisms that cycles approximately every 24 hours, regulating critical physiological and behavioral functions. This fundamental rhythm dictates the temporal organization of the endocrine system, controlling the pulsatile release of hormones, core body temperature, and the sleep-wake cycle. Maintaining a robust circadian rhythm is essential for systemic homeostasis and long-term health resilience.
Origin
The term combines the Latin circa (about) and dies (day), first formally used in the mid-20th century to describe the near-daily periodicity of biological phenomena. The concept gained scientific rigor with the discovery of the suprachiasmatic nucleus (SCN) in the hypothalamus as the master pacemaker and the subsequent identification of core clock genes. This field is foundational to modern endocrinology and chronomedicine.
Mechanism
The rhythm is generated at the molecular level by a complex transcriptional-translational feedback loop involving core clock genes, such as Period and Cryptochrome. This central clock in the SCN receives input primarily from retinal light exposure, acting as the primary synchronizer, or zeitgeber. The SCN then communicates the time of day to the rest of the body via neural and hormonal signals, coordinating the rhythmic function of peripheral organ systems and their respective endocrine output.
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