The mammalian circadian system: From genes to behavior

Virtually all organisms studied to date, from bacteria to humans, possess an internal clock that has an intrinsic period of approximately 24 h. This endogenous pacemaker drives circadian rhythms ("ecirca". about, "edies" = a day) of myriad biological processes, from cell metabolism to behavioral state. Although the anatomical location and genetic components of the pacemaker differ among species, all circadian systems can be described by a three-component model that includes: (1) the pacemaker, a molecular clock that has a period of approximately 24 h and that continues to oscillate even in the prolonged absence of any timing cues, (2) an input pathway that conveys environmental time cues to the pacemaker, and (3) output pathways by which the pacemaker can influence biochemistry and behavior (Takahashi et al., 2001). This review focuses on mammalian circadian rhythms, and covers the anatomical and molecular basis for the master pacemaker, the neurochemical processes responsible for pacemaker entrainment, and the diverse systems involved in pacemaker output.