Feelings of sadness, difficulty sleeping, suicidal thoughts, weight fluctuations and changes in appetite are symptoms of major depressive disorder, the American Psychiatric Association states. Depression often causes individuals to lose interest in their normal activities and struggle with feelings of restlessness, guilt and worthlessness.
Doctors diagnose a patient with depression when symptoms last for at least two weeks, according to the American Psychiatric Association. The disorder causes difficulty concentrating and making decisions, unexplained fatigue and sluggish movement and speech. Depending on the person, symptoms may be mild or so severe that they interfere with daily functions. Depression symptoms often overlap with other medical conditions and nonclinical forms of grief, so doctors try to help patients distinguish between temporary feelings of intense sadness and continuous, overwhelming feelings of hopelessness and self-loathing.
Patients are diagnosed with persistent depressive disorder when they exhibit symptoms for at least two years, but the severity may fluctuate, the National Institute of Mental Health explains. Symptoms vary by person, and while some individuals only experience emotional changes, such as irritability, others develop physical problems, including headaches, body aches, cramps and digestive discomfort. People coping with psychotic depression develop additional symptoms caused by a chronic form of mental instability, known as psychosis. For example, some individuals have hallucinations or suffer from delusional episodes in which their perceptions conflict with reality.