Circadian Rhythms

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 

The Central Clock

At the center of circadian regulation is the Suprachiasmatic Nucleus (SCN), a cluster of neurons in the hypothalamus. The SCN receives direct input from retinal photo-receptors and uses this information to align internal physiology with the day–night cycle.

The SCN does not produce sleep or wakefulness directly. Instead, it coordinates timing across multiple systems, ensuring that physiological processes occur in the appropriate sequence.

Peripheral Clocks and Organ Timing

In addition to the central clock, almost every organ contains its own circadian machinery. These peripheral clocks regulate local functions such as glucose metabolism in the liver, motility in the gut, contractile readiness in muscles, and inflammatory responses in immune cells.

Peripheral clocks respond strongly to behavioural cues — feeding time, physical activity, temperature, and posture. When these cues conflict with the central light-based clock, internal de-synchrony occurs.

Hormonal and Autonomic Rhythms

Key hormones follow circadian patterns. Cortisol rises before waking, promoting alertness and energy mobilisation. Melatonin rises after dusk, facilitating parasympathetic dominance and cellular repair. Core body temperature, insulin sensitivity, and blood pressure all fluctuate predictably across the day.

The autonomic nervous system mirrors these rhythms. Sympathetic tone predominates during the biological day; parasympathetic activity increases during the biological night.

Circadian rhythms therefore form the temporal framework within which autonomic balance occurs.

Circadian Disruption as Physiological Stress

When circadian timing is repeatedly ignored — through irregular sleep, late-night eating, excessive artificial light, or shift work — the body compensates by maintaining partial activation.

This compensation is experienced as fatigue, light sleep, metabolic inefficiency, heightened pain sensitivity, and reduced resilience. Stress, in this context, arises not from emotional strain but from temporal misalignment.