Recall that seasonal affective disorder, or SAD, is a form of mood disorder in which people become depressed during the winter months, when there are fewest hours of daylight. Their moods then brighten in summer months, when there is more daylight each day. It turns out that people with SAD who are exposed to bright lights for a few hours each day during the winter months often experience complete relief form their depression within a couple of days.

SAD or Seasonal Affective Disorder is a kind of depression usually influenced by seasons. It is a subtype of major depressive disorder.
These could be of two types – winter onset and summer onset. Winter onset is the most common one among the two. It begins in late fall and ends with the advent of summer. On the other hand summer onset begins in late spring or early summer and wards off by the beginning of winter.