Seasonal affective disorder, or SAD, also known as winter depression is an affective, or mood, disorder. Most SAD sufferers experience normal mental health throughout most of the year, but experience depressive symptoms in the winter or summer. SAD is rare, if existent at all, in the tropics, but is measurably present at latitudes north of 30°N, or south of 30°S.

Info Source, Links and more info:
http://en.wikipedia.org/wiki/Seasona...ctive_disorder