Seasonal affective disorder may occur as a result of deficiencies in zeitgebers (such as light) during the winter months that stimulate the reward activation system, resulting in a depressed mood. Some studies have pointed to the hormone melatonin, which is regulated by circadian rhythms, as a possible mechanism.[19] Because circadian clocks synchronize human sleep-wake cycles to coincide with periods of the day during which reward potential is highest – that is, during the daytime[2] – and recent studies have determined that daily rhythms in reward activation in humans are modulated by circadian clocks as well,[11] external influences on those rhythms may influence an individual's mood.