Humans have long observed and studied natural rhythms, including the daily activity patterns of plants and animals. Biological rhythms that occur on the scale of a day are called circadian rhythms, and they are driven by an internal clock. It is advantageous to organisms to anticipate changes in the environment, such as the rising and setting of the sun, and an internal clock can be used to anticipate important environmental events. Nearly every organism studied, including humans, has a circadian clock. Experiments in which individuals were kept in isolation in caves or bunkers have shown that humans have circadian clocks.
Circadian rhythms affect human physiology and activity and thus influence many aspects of health.
The clock in the brain uses specific biochemical and neural pathways for passing its timing message along to other parts of the body, such as the liver or kidneys. Chemical reactions and physical properties are dependent on temperature. Circadian clocks have mechanisms to keep accurate time even when the temperature varies.