Nearly all organisms contain a circadian biological clock that is responsible for coordinating the temporal functions of many physiological systems. The circadian clock is synchronized to the earth's day/night rhythms via changes in the intensity of light throughout the cycle. In mammals, the eyes and specifically the intrinsically photosensitive retinal ganglion cells are essential for transmitting light information to the brain to influence the physiology of the organism. Several biochemical, hormonal, molecular, and behavioral functions are affected by the interaction of the circadian clock with the daily light/dark cycle. Furthermore, many studies have shown an association between circadian biology and mood regulation. Here, we present several behavioral methods in mice and humans for the measurement of the interaction between the endogenous biological clock and light. By incorporating circadian phenomena into mood studies, the link between the clock, light, and mood could be better understood. Further, modification of the light/dark environment should provide tools to control sleep, mood, and cognition via direct light input on behaviors.