Depression is a common mental disorder that affects how you feel, think and behave. It causes a persistent feeling of sadness and loss of interest in activities that you normally enjoy. It can also lead to a variety of emotional and physical problems, such as poor concentration, low self-worth, hopelessness, disrupted sleep, changes in appetite or weight, and feeling very tired or low in energy. Depression is a serious condition that can affect your quality of life and increase the risk of **suicide**.