A constant sense of hopelessness and despair is a sign you may have major depression, also known as clinical depression. Video Transcript World Health Organization: "Depression."; Mayo Clinic: "Depression."; American Psychiatric Association: "What Is Depression?"; Anxiety and Depression Association of America: "Understand the Facts: Depression."; Getty; Comstock Images;i Stock/Getty Images Plus; Thinkstock; AudioJungle; Rike; Vanessa Clara Ann Vokey; Somos/Veer; KatarzynaBialasiewicz; funduck [MUSIC PLAYING] With major depression, it may be difficult to work, study, sleep, eat, and enjoy friends and activities. Some people have clinical depression only once in their life, while others have it several times in a lifetime. Major depression can sometimes occur from one generation to the next in families, but often it may affect people with no family history of the illness.

What Is Major or Clinical Depression? Most people feel sad or low at some point in their lives. But clinical depression is marked by a depressed mood most of the day, sometimes particularly in the morning, and a loss of interest in normal activities and relationships -- symptoms that are present every day for at least 2 weeks. In addition, according to the DSM-5 -- a manual used to diagnose mental health conditions -- you may have other symptoms with major depression. Those symptoms might include: Fatigue or loss of energy almost every day

Feelings of worthlessness or guilt almost every day

Impaired concentration, indecisiveness

Insomnia or hypersomnia (excessive sleeping) almost every day

Markedly diminished interest or pleasure in almost all activities nearly every day (called anhedonia, this symptom can be indicated by reports from significant others)

Restlessness or feeling slowed down

Recurring thoughts of death or suicide

Significant weight loss or gain (a change of more than 5% of body weight in a month)

Who Is at Risk for Major Depression? Major depression affects about 6.7% of the U.S. population over age 18, according to the National Institute of Mental Health. Overall, between 20% and 25% of adults may suffer an episode of major depression at some point during their lifetime. Major depression also affects older adults, teens, and children, but frequently goes undiagnosed and untreated in these populations.