In humans, positive affect is an indicator of activation of the reward system, a neurobiological adaptation guiding motivation and behavior in the context of incentive cues. Disturbances of positive affect and related reward function are defining features of mood disorders and other serious psychopathology, so there are calls for a deeper understanding of normal and abnormal operation of positive affect/reward. One potentially useful avenue of research seeks to understand internal and external modulators of reward function, and a key hypothesis in this domain posits that positive affect/reward is modulated by endogenous biological rhythms. The reward potential of the environment varies with the light–dark cycle, and the fitness of an organism is enhanced by its being primed for environmental engagement when the likelihood of rewards is high (daytime for diurnal species). In all species, the endogenous circadian system is adapted for this purpose. It has therefore been hypothesized that the human reward system not only is reactive to external cues but also is modulated by timing information from the circadian system. Consistent with this prediction, a range of evidence suggests that positive affect is partly controlled by the endogenous circadian system, and there is emerging evidence for a circular relationship between features of sleep and daytime positive mood states. This chapter critically reviews evidence for circadian and sleep modulation of positive affect and situates these findings in a broader understanding of positive affect as an indicator of reward motivation.
Dopaminergic reward system: a short integrative review