Glossary /
Circadian Rhythm
The roughly 24-hour internal clock that drives sleep, hormones, body temperature, and metabolism.
What it is
Every cell in your body keeps time. The master clock sits in a tiny region of the hypothalamus called the suprachiasmatic nucleus, and it synchronises a network of peripheral clocks in your liver, gut, fat, and muscle. Together they decide when cortisol rises in the morning, when melatonin climbs at night, when insulin sensitivity peaks, and when core body temperature dips for sleep.
Why it matters
The clock is not a metaphor — it is a measurable biochemical oscillator, and it expects daylight in the morning and darkness at night. When you eat at midnight, scroll a bright screen in bed, or work rotating shifts, the peripheral clocks fall out of phase with the central one. The downstream cost shows up as poor sleep, blood-sugar dysregulation, mood disorders, and elevated cardiovascular risk. Most of what you can do for your sleep happens during the day: light, food timing, and movement.