When a person experiences recurring depression and fatigue at the same time of the year he may be suffering from Seasonal Affective Disorder (or SAD). These kind of symptoms usually appear in the months of winter when the daylight hours become shorter. But they seem to disappear once spring returns and days become longer again. The person returns to his usual mood and energy level.
According to medical experts, two hormones known as melatonin and serotonin regulate a persons sleep-wake cycles, energy and mood. Sunlight seems to play a key role in the production of these two hormones. Lack of sunlight in winter, increases the level of melatonin which can cause a person to feel sleepy and lethargic.
The reverse effect occurs in summer or spring when longer hours of daylight produces serotonin which gets the person back to his normal mood.