Depression is a serious medical condition that affects how a person feels, thinks, and functions. Common symptoms include persistent sadness, loss of interest or pleasure, changes in sleep or appetite, low energy, difficulty concentrating, feelings of guilt or hopelessness, and, in some cases, thoughts of death or suicide.

Related mood disorders, such as bipolar disorder, involve cyclical changes in mood, energy, and functioning. Bipolar disorder includes periods of depression as well as episodes of elevated or irritable mood, which may involve racing thoughts, decreased need for sleep, increased energy, and impulsive behavior. Mood disorders can result from a combination of brain chemistry, genetics, personality, and environmental stressors, and benefit from evaluation and treatment by experienced professionals.