Circadian Rhythm

A circadian rhythm is a biological cycle lasting close to 24 hours that is generated internally, not merely a passive response to the day-night cycle. The proof is that it persists in constant conditions — a person kept in an environment with no clocks, no daylight, and no external time cues will still sleep and wake, and their body temperature and hormone levels will still cycle, on a period close to but rarely exactly 24 hours. That "free-running period," typically a bit longer than 24 hours in humans, is what makes external synchronization necessary at all.

The master clock sits in the suprachiasmatic nucleus (SCN), a small structure in the hypothalamus that receives direct input from the retina and uses it to reset the body's rhythm daily, keeping the internal clock's slightly-off period locked to the actual 24-hour day. External time cues that perform this resetting are called zeitgebers ("time-givers") — light is the dominant one, but meal timing, temperature, and social schedule contribute too.

Jet lag is what happens when the zeitgebers change abruptly (crossing time zones) faster than the internal clock can resynchronize; the SCN can typically shift by only about an hour a day, so the mismatch between "what the sun and clock now say" and "what the SCN still believes" persists for days and produces the disorientation, poor sleep, and daytime fatigue jet lag is known for. Shift work imposes a chronic version of the same mismatch by fighting the light zeitgeber on a recurring schedule rather than a single transition.

Circadian timing governs far more than sleep: core body temperature, cortisol, and melatonin all cycle on the same clock, and drug metabolism and even the timing of certain medical events (heart attacks cluster in early morning) track it as well.