Breathing slows down when you sleep and speeds up during physical activity.

The longer version

Breathing rate tracks the body's need to take in oxygen and remove carbon dioxide. Carbon dioxide is the more urgent signal: when its level rises, sensors in the brain stem and in the large arteries near the heart increase the drive to breathe.

During sleep, metabolism slows and muscle tone drops, so less ventilation is needed and the rate falls. During exertion, working muscles consume oxygen and produce carbon dioxide quickly, and the control system responds within seconds. Signals from moving joints and from the body's temperature also feed in, so the adjustment starts before chemistry alone would demand it. Depth of breathing changes alongside rate, and the two together determine how much air actually reaches the lungs.

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.