The Physiology of Biological Time Human physiology is organized around a near-24-hour cycle known as the circadian rhythm. This internal timing system regulates sleep–wake patterns, hormone secretion, metabolism, immune function, tissue repair, and nervous system tone. Circadian rhythms are endogenous — generated within the body — but they are synchronised to the external environment, primarily through light. central clock in the brain (SCN) connected to peripheral organs T