Depression is a mood disorder characterised by extreme sadness, inability to perform daily tasks, decreased energy and ability to think, loss of interest and decreased interest in once pleasurable activities. There may also be crying, feelings of hopelessness, loss of self-esteem, with recurrent thoughts of suicide and death. Depressive disorder also has an impact on sleep with insomnia or hypersomnia, as well as on appetite. It is not a state that can be controlled and over which the person can act without help. Depression has many causes including biological, psychological, environmental and social, it is a very serious illness that can lead to suicide. Depression should not be confused with sadness, a transient, non-pathological condition that requires no special treatment.