The Physiological Clock

Abstract

It has long been known to physiologists and biologists that many functions in animals and plants vary in a cyclic way with periods of approximately 24 hours. Such rhythmic activities are now usually referred to as ‘circadian’ (circa = about; dies = day). Rhythmic movements of leaves and petals are frequent in the plant kingdom. Linné, the Swedish botanist, referred to them as ‘sleep movements’ since they were subordinate to the cycles of day and night. Linné actually constructed a ‘flower-clock’ by planting a number of plants, the petals of which had different times for opening and closing. Early observations provided evidence of corresponding rhythms of certain functions in animals such as the emergence of insects from their pupae or the daily fluctuations in body temperature. In humans the existence of circadian rhythms was demonstrated in studies of glycogen synthesis in the liver, urinary secretion, body temperature and pulse frequency (Fig. 20.1).