Seasonal affective disorder (SAD) is a subtype of depression.
Research suggests that SAD arises from some people's sensitivity to seasonal changes in the amount of available daylight.
People sometimes react emotionally to changes in the amount of daylight available.
For example, with less sunlight in the winter, some individuals become depressed, sad and irritable. They want to sleep a lot and eat constantly. When spring arrives with more daylight hours, their mood enhances; they feel better and start getting their energy back.
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