Depression is a very serious medical illness. Early diagnosis of depression may be missed because the onset of distinguishable symptoms often occurs gradually. Eventually however, the symptoms of depression may create significant changes in how a person functions, and medical care becomes necessary. Some of the more common symptoms of depression are: persistent sadness or irritability, memory loss and difficulty concentrating, changes in sleep habits, appetite, energy level, disinterest in activities that were once favorites, feelings of worthlessness, hopelessness, guilt, and emptiness. These symptoms, individually or combined, may produce recurring thoughts of death or suicide.