Patterns of behavioural and physiological activity in most organisms vary in a cyclical way. A circadian rhythm is a bodily rhythm that lasts about 24 hours. Other bodily rhythms can be infradian (lasting more than 24 hours) or ultradian (lasting less than 24 hours).

A lot of examples suggest that these rhythms are inbuilt (endogenous). For example, in countries with bad winters, the squirrel population hibernates. If these squirrels are kept in a laboratory, in a constant warm environment, with altering 12 hour periods of light and dark, they go through the hibernation routine at the appropriate time of the year, increasing food intake and body weight and decreasing body temperature. In addition, they awake as spring approaches in the world outside. However, even with these endogenous pacemakers, in the real world behaviour has to be adapted to external events exogenous factors) such as winter or night-time, so that leaf opening and hibernation happen at exactly the right time. These external events are called zeitgebers. Behaviour has to respond to zeitgebers to adapt to what is happening in the natural world.

Endogenous are sometimes called biological clocks. The main one in mammals (human beings), is the supra-chiasmatic nucleus (SCN). This is situated in the hypothalamus. The SCN has an inbuilt circadian firing pattern. This pattern continues even in cells of the SCN that have been removed from the brain and kept alive artificially. This proves that it is the SCN and not some other brain area generating them. The SCN causes sleep by stimulating the pineal gland to produce melatonin. Melatonin increases sleepiness by acting on the hypnogenic centres situated in various parts of the brain. Because the SCN generates a rhythm that lasts longer than 24 hours, it needs to be reset each day. The SCN is connected to the retina...