Depression

Depression

National institute of Mental Health (NIMH) describes depression as a common but serious mood disorder.  The symptoms of depression impact how you feel, think and manage the activities of daily living.  NIMH further describes types of depressions persistent depressive disorder (lasting more than two years)and seasonal affective disorder (onset during winter months)

Bipolar disorder is also included in the NIMH list because someone with this disorder can experience extremes in mood that include periods of depression and irritability. 

For further reading:
https://www.nimh.nih.gov/health/statistics/major-depression.shtml