Circadian Rhythms
Circadian rhythms are biological cycles that repeat approximately every 24 hours, regulating sleep-wake cycles and numerous other physiological processes throughout the body. The term derives from the Latin words “circa” (around) and “dies” (day). These internal rhythms are generated by the body’s own biological clock and are synchronized with external environmental cues, most notably the light-dark cycle.
The Body’s Master Clock
The suprachiasmatic nucleus (SCN), a small region within the brain’s hypothalamus, functions as the master clock that controls and coordinates circadian rhythms across the body. The SCN receives direct input from the eyes about ambient light levels, allowing it to synchronize internal timing with the external environment. This process, called entrainment, ensures that biological processes remain aligned with the 24-hour day-night cycle.
Regulated Processes and Effects
Beyond sleep-wake cycles, circadian rhythms regulate hormone production, body temperature, digestion, and cognitive function. Cortisol levels peak in the early morning to promote wakefulness, while melatonin increases in the evening to promote sleep. These rhythms influence alertness, appetite, metabolism, and immune function throughout the day. When circadian rhythms are disrupted—through shift work, jet lag, or irregular sleep schedules—physical and mental health can be negatively affected.