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.