Circadian Clocks and Rhythms

We have an internal “clock” that regulates sleep-wake cycles over roughly 24 hours (circadian). It is most directly set by light exposure, especially sunlight. Sunrise triggers a rise in cortisol, promoting wakefulness. 12–16 hours later, the pineal gland releases melatonin, promoting sleepiness. Avoiding light at night prevents disrupting this rhythm. Circadian rhythms also impact hormones, body temperature, hunger, etc. They are linked with health, mood, and longevity.