Figure 1.

The circadian clock system. The mammalian circadian system can be stratified into three main components: inputs from the external environment to clocks (photic and non‐photic time cues), clock mechanisms (at the cellular and systemic level), and output signalling pathways that modulate physiology and behaviour. The three components are interconnected (arrows in both directions): the effects of input on the clock is time dependent, determined by the clock itself; peripheral cellular clocks and the main pacemaker, in the suprachiasmatic nucleus, are synchronized; activity influences body temperature; and sleep–wake cycles determine feeding–fasting cycles. In this way, external inputs, individual cellular clocks and subnetworks, and outputs are integrated into a coherent functional system/network