These adaptations in behavior and activity result from alterations in their brain chemistry that is induced by the changing seasons. The impact of these changes on body chemistry can be easily observed in many mammals by changes in the thickness and coloring of their fur. The change in day length, as registered by the biological clock in the brain, is the signal that prompts these alterations in body chemistry. Humans are also affected by the seasons. Seasonal changes in brain chemistry that occur in mammals affect centers in the brain that can influence mood in people.
Many people feel less energetic and gain weight in the winter.
People often find it harder to get up in the morning, have more difficulty becoming motivated, and prefer to avoid social contact. Because of the eye hazard that may result from the high intensity of light used in conventional Bright Light therapy, eye safety experts have recommended that potential Bright Light therapy users be screened by an eye-care professional or ophthalmologist before treatment.