Murray, G., Nicholas, C., Kleiman, J., Dwyer, R., Carrington, M., Allen, N. & Trinder, J. (2009). Nature's clocks and human mood: The circadian system modulates reward motivation. Emotion (online version),9(5), 705-716. Retrieved from https://doi.org/10.1037/a0017080
Existing literature on reward motivation pays scant attention to the fact that reward potential of the environment varies dramatically with the light/dark cycle. Evolution, by contrast, treats this fact very seriously: In all species, the circadian system is adapted to optimize the daily rhythm of environmental engagement. We used 3 standard protocols to demonstrate that human reward motivation, as measured in the dynamics of positive affect (PA), is modulated endogenously by the circadian clock. Under naturalistic conditions, 13.0% of PA variance was explained by a 24-hr sinusoid. In a constant routine protocol, 25.0% of PA variance was explained by the unmasked circadian rhythm in core body temperature (CBT). A forced desynchrony study showed PA to align with CBT in exhibiting circadian periodicity independent of a 28-hr sleep/wake cycle. It is concluded that the circadian system modulates reward activation, and implications for models of normal and abnormal mood are discussed.
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