Fig. 1.

Circadian timekeeping and the alignment between environmental/behavioural rhythms and central/peripheral clocks. (a) Timing signals from the environment (e.g. light, as indicated by the image of the sun) and from behaviours (e.g. food intake, as indicated by the plate with knife and fork, and physical activity, as indicated by the runner) affect the rhythms of the central clock (i.e. the SCN) and/or peripheral clocks (shown for the pineal gland [orange], liver [brown], adrenal gland [yellow], pancreas [green], stomach [red], muscle [blue], white adipose tissue [purple] and gastrointestinal tract [grey]). Rhythms in the clock oscillators are represented by the cosine waves. Through hormonal/humoral, neural and temperature pathways (shown by the dashed blue arrows), temporal signals are also transmitted between the central clock and peripheral clocks. Through this relay of timing signals, the body’s rhythms entrain to external environmental and behavioural rhythms while, internally, the central and peripheral clocks maintain synchrony. (b) Alignment and misalignment between central clock and environmental/behavioural rhythms. Timing of the central clock, environment light exposure and behaviours are shown as yellow bars across a 24 h period. In the ‘aligned’ condition, the central clock is aligned with light exposure, wake, feeding and physically active period. In the ‘misaligned’ condition, the light exposure, wake, feeding and physical activity are shifted so that they are not aligned with the central clock. This misalignment between the central clock and the environmental and behavioural rhythms is a type of circadian disruption. (c) Alignment and misalignment between central and peripheral clocks. Timing of the central and peripheral clock rhythms are shown schematically as cosine waves across a 24 h period. In the aligned condition, the rhythms of the central and peripheral clocks are aligned. In the misaligned condition, the rhythms are dampened or flat and some rhythms are shifted such that not all are aligned. This misalignment between central and peripheral clocks is another form of circadian disruption, also called ‘internal misalignment’ or ‘internal desynchrony’. Note that the phases of the cosine waves do not necessarily show the rhythmic expression of specific clock genes but illustrate the concept of alignment (when the timing of different clocks occur at an optimal phase relationship, i.e. the timing within each rhythm’s cycle are in alignment with one another) vs misalignment (when these relationships are abnormal). GI, gastrointestinal; WAT, white adipose tissue. This figure is available as part of a downloadable slideset.