<p>It is now well established that all human physiological processes exhibit rhythmicity. The menstrual cycle is probably the best-known, but physiological variables, such as body temperature, blood pressure, hormone secretion and renal excretion, have also been shown to have a regular circadian pattern over a period of around 24 hours (Minors and Waterhouse 1981) (Figs. 1-4). Definitions associated with chronobiology are given in Box 1.</p>