Seasonal Affective Disorder or SAD primarily affects people during the fall and winter season. It is a form of depression, with periods of being sad, angry or irritable and may leave you feeling more tired than usual.
The shorter days and the lack of sun light, especially in areas located far from the equator can disturb your internal clock and you may feel the need to sleep more than usual.
Presumably, food was scarce during most of human prehistory, and a tendency toward low energy during the winter months would have been adapted by reducing the need for calorie intake. Hibernation is an extreme example, but even species that do not hibernate often exhibit changes in behavior during the winter months.
In the fall cravings for carbohydrates to prepare for the season ahead are common and weight gain is often experienced during these times of low energy and with less time spent outdoors it is getting progressively worse as the winter season continues.
Seasonal mood variations are believed to be related to light. Treatment with a light box (Bright Light Therapy) has been proven to be effective and should be started in the early fall and continued throughout the long winter months. Once the days get longer towards the spring and summer the symptoms usually fade away gradually and healthy eating and outdoor activities will prevent the weight gain to become permanent.