Seasonal depression is also known as seasonal affective disorder, or SAD. It is a form of depression that is associated with the change in seasons and occurs at the same time every year. Most people who suffer from seasonal affective disorder being to experience symptoms in the fall and continue experiencing them throughout the winter. However, there are some people who get seasonal affective disorder in the spring or summer. About six percent of the population in the United States is affected by seasonal affective disorder. In addition, another 14 percent of adults in the United States suffer from a form of seasonal affective disorder that is milder.