ABSTRACT
In Chapter 7, we discussed how light synchronizes (or entrains) the circadian clock through a mostly nonparametric mechanism and how it otherwise affects the expression of circadian rhythms. Although light is the zeitgeber that has been most thoroughly studied, it is certainly not the only one. As diagramed in Figure 8.1, stimuli that have been shown to entrain circadian rhythms include not only light but also several so-called nonphotic stimuli such as ambient temperature, food availability, physical activity (exercise), and social contact. At least in mammals, none of these stimuli seems to have as strong an inuence on the circadian system as light has. Nonetheless, all of these stimuli can signicantly affect circadian rhythms, particularly when a light-dark (LD) cycle is not simultaneously present.
