Winter blues is a more general term to describe the feeling of sadness during the winter season. It is not a medical diagnosis. Furthermore, many people experience the winter blues, so it is quite common. It is also a mild condition, as it typically clears up on its own in a short period of time. In contrast, seasonal affective disorder (SAD), also known as winter depression disorder or seasonal depression, is a clinical diagnosis that is usually tied to shorter days during fall and winter. It significantly affects daily functioning over a long period of time. SAD is a serious condition as it is a type of depression. People with SAD tend to have a gloomy outlook and feel worthless or hopeless.