Biologists have known that variations in the amount of sunlight a person receives and her or his circadian clock play a role in the disorder. They have also proposed that the neurotransmitters serotonin and melatonin may be involved. However, they have not yet identified the underlying neurobiological mechanisms responsible. Biologists have now localized the seasonal light cycle effects that drive seasonal affective disorder to a small region of the brain called the dorsal raphe nucleus.