MODEL RELEASED: Depression. Sadness, loss of interest in life, pessimism and reduced emotional well-being are natural responses to some circumstances. However, if they are longer term and without any apparent cause, they may indicate psychiatric illness. Depressive illness involves changes including loss of appetite, sleep disorders, anxiety, tiredness, sometimes thoughts of suicide. It can be caused by viral infections and hormonal changes such as those involved in birth (many women suffer post-natal depression). Depression is treated using psychotherapy, antidepressant drugs or electroconvulsive therapy in severe cases.