Seasonal Affective Disorder Many people go through short periods when they feel sad or unlike their usual selves. Sometimes, these mood changes begin and end when the seasons change and these mood changes are more serious and can affect how a person feels, thinks, and behaves. If you have noticed significant changes in your mood and behavior when the seasons change, you may be experiencing seasonal affective disorder. In most cases, SAD symptoms start in the late fall pr early winter and go away