Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behavior, feelings and physical well-being. It may include feelings of sadness, anxiety, emptiness, hopelessness, worthlessness, guilt, irritability, or restlessness.
Depressed people may lose interest in activities that once were pleasurable, or suffer cognitive impairments (e.g., difficulty concentrating, remembering details, making decisions). They may contemplate or attempt suicide. Their weight may change dramatically. Insomnia, excessive sleeping, change in sleep patterns (e.g., waking in the middle of the night or early in the morning and being unable to fall asleep again), fatigue, loss of energy, and aches, pains or digestive problems that are resistant to treatment may be present.
Depressed mood is a normal reaction to certain life events, a symptom of many medical conditions (e.g., Addison's disease, hypothyroidism), and a feature of certain psychiatric syndromes.