Mammals naturally want to hibernate in the winter, and many humans feel as if they would like to just curl up and sleep for the entire winter as well. Seasonal Affective Disorder (SAD) is an atypical form of depression related to changes in seasons. SAD is cyclic and seasonal with symptoms coming back year after year, tending to come and go at about the same time every year. The changes in mood are not necessarily related to obvious things that would make a certain season stressful (like regularly being unemployed during the winter).
Signs and symptoms usually occur only in the darkest of winter months — when days are very short and nights are very long – however some people who suffer from SAD report that their symptoms can begin as early as mid September and last until March in Canada.