Depression is a mood disorder that causes persistent feelings of sadness, hopelessness, loss of interest, and decreased energy. A diagnosis of major depression means symptoms have been consistent nearly every day for at least two weeks. However, depression is more than just unhappiness. It can be severe enough to interfere with relationships, work, school, and other daily activities. Doctors also refer to this condition as major depressive disorder or clinical depression.

An estimated 1 in 6 people, or nearly 17 percent of Americans, will experience depression at some point in their lifetime, according to the American Psychiatric Association. About 7 percent of Americans have at least one major depressive episode per year. Although the symptoms can start at any age, depression is most likely to begin during a person's teens or 20s.

Types of depression

• Along with major depression, there are several other types of depression, which are characterized by their symptoms or causes: Postpartum depression involves feelings of extreme sadness, fatigue, and anxiety that start after a woman gives birth.
• Premenstrual dysphoric disorder (PMDD) is severe depression, irritability, and anxiety that occurs in the week or two before a woman's period.
• Seasonal effective disorder (SAD) is depression that occurs during the winter months and is relieved by the change of season.
• Bipolar disorder, formerly called manic depression, is characterized by alternating episodes of extremely low mood (depression) and exuberant highs (mania).
• Persistent depressive disorder, which is depressions lasting two years or more. It combines dystopia, which is a low-grade but persistent depression with chronic major depression. People with persistent depressive disorder generally lack energy, have low self-esteem, and often feel helpless.
• Psychotic depression includes features of both depression and psychosis, such as having false beliefs (delusions) or seeing and hearing things that are not there (hallucinations).

Symptoms

People who have depression will experience some or all of the following symptoms (almost every day for at least two weeks for major depression):

• Persistent sad or empty mood
• Feelings of hopelessness, helplessness, emptiness, worthlessness, or guilt
• Low energy, fatigue
• Irritability, restlessness, anxiety
• Slowed thinking, speaking, or movement
• Loss of interest in activities they once enjoyed
• Trouble concentrating, remembering, or making decisions
• Loss of appetite, or eating too much
• Weight gain or loss
• Trouble sleeping, or sleeping too much
• Headache, stomachache, and other aches and pains that do not have a clear physical cause
• Thoughts of death or suicide

Some people with depression will experience many of these symptoms. Others will have just a few. The severity of depression symptoms can range from mild to severe enough to affect a person's day-to-day life.