Figure 4.

Representation of circadian clock divisions. A circadian clock can be depicted as having input pathways, a central oscillator (or pacemaker), and output pathways. The central oscillator produces the endogenous biological rhythm and can be synchronized with the environment via input pathways through cues such as light or temperature. Output pathways convey the clock’s rhythms to downstream targets and drive overtly rhythmic activities. Some circadian systems consist of more elaborate pathways (shown as dashed lines) that include multiple, interlocking oscillators and positive or negative feedback from clock-controlled activities to oscillator and/or input components. (Figure based on data in Gardner et al. 2006.)