by the Harvard Mental Health Letter

About the author: The Harvard Mental Health Letter is a monthly publication of Harvard College.

A sunny disposition, by definition, is a happy one, and a wintry atmosphere is cheerless. Many people become somewhat sluggish or gloomy as the days grow short; the annals of the Far North tell repeatedly of cabin fever in winter and elation under the midnight sun. For some the winter mood change is a clinical depression. It was first noticed long ago but had no accepted name until the early 1980s, when researchers at the National Institute of Mental Health in Washington began to call it seasonal affective disorder (SAD).

In its most common form, SAD is a major (serious) depression that recurs each year at the same time, starting in fall or winter and ending...