Your body clock follows a daily rhythm that affects sleepiness and alertness at different hours.

The longer version

The circadian system is built around a group of cells in the brain that keeps time on its own, running slightly longer than a day and being reset each morning by light reaching the retina. Light signals travel along a dedicated pathway and adjust the clock, while the clock in turn drives daily patterns in hormone release, body temperature, digestion and alertness.

Sleep pressure, which builds the longer a person is awake, interacts with that timing to determine how sleepy someone feels at a given hour. Because the internal day is not exactly twenty-four hours, the clock needs daily resetting to stay aligned. Travelling across time zones outruns that adjustment, which is why the body feels out of step for a while.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.