What type of mood disorder is characterized by prolonged periods of sadness?

The mood disorder characterized by prolonged periods of sadness is depression. This condition is often marked by a persistent feeling of sadness or a lack of interest in previously enjoyed activities, which can last for weeks or even months. Depression can also manifest with various symptoms such as changes in appetite, sleep disturbances, fatigue, and difficulty concentrating. It significantly affects daily functioning and quality of life.

In contrast, bipolar disorder involves alternating episodes of depression and mania, thus incorporating both high and low moods rather than solely prolonged sadness. Schizophrenia is primarily a mental disorder affecting thought processes, perceptions, and behaviors, rather than mood specifically. Mania refers to an elevated or irritable mood that can occur in bipolar disorder, but by itself, is an opposite state to the prolonged sadness seen in depressive episodes.