Abstract
Biological rhythms have been observed in practically all groups of laboratory mammals and at every level of physiological and behavioural organization. Biological rhythms are classified according to their period as ultradian (<24 h), circadian (~24 h), infradian(>24 h), and seasonal or circannual rhythms (~1 year). This review outlines what is known about the neurobiology of biological rhythms in mammals and describes the hierarchical order in which ultradian, circadian and infradian rhythms are related to each other. The article does not attempt to catalogue every physiological variable showing rhythmical fluctuations in laboratory mammals. Rather, it focuses on the basic concepts of circadian rhythms and recent advances made in our understanding of the physiology of the internal clock controlling circadian and other biological rhythms.