Circadian Rhythm: How Your Body Clock Actually Works

Your body runs on an internal clock that is close to but not exactly twenty-four hours. Understanding how it gets reset each day explains most practical sleep advice.

The master clock

The suprachiasmatic nucleus, a small region of the hypothalamus, is the master circadian pacemaker. It receives direct input from specialised retinal cells containing melanopsin, which are most sensitive to blue-wavelength light and are distinct from the rods and cones used for vision. This input is how external light sets the internal clock each day. Peripheral clocks exist in nearly every tissue and are synchronised by the master clock, with feeding time as a secondary influence.

Light is the dominant signal

Morning light advances the clock, promoting earlier sleep and wake times. Evening light delays it. The intensity difference between indoor and outdoor light is far larger than it appears - a bright office is a fraction of overcast daylight, which is itself a fraction of direct sun. This is why getting outside in the morning is more effective than any indoor lighting adjustment, and why the practical advice is a walk rather than a lamp for most people.

Why regularity matters

The clock predicts and prepares. Core temperature, cortisol, melatonin and alertness all follow a predictable daily curve, and that preparation only works if timing is consistent. Irregular schedules force continuous readjustment, which is why shift workers show elevated risk across multiple health outcomes and why social jet lag - large weekend shifts in sleep timing - is associated with poorer outcomes even when total sleep hours are adequate.

A note on NeuronGold

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