Seasonal Affective Disorder (SAD) is a type of depression which has a seasonal pattern. The most common form of SAD is ... called 'winter depression' because symptoms are worst in the winter months. Symptoms tend to start from around September and are worse when the days are shortest (in December, January and February). By springtime, most people with SAD will improve and symptoms usually disappear. Seasonal affective disorder can sometimes affect people in the summer rather than the winter, but this is rare and has different symptoms.