Depression is a clinical condition broadly characterised by pervasive sadness and by lack of enjoyment of every-day life activites. It is distinguished from normal sadness because of its duration (at least 10-14 days of continuous or almost continuous presence of these emotional difficulties) and by its negative impact on daily functioning, in areas such as work performances and personal relationships. In most cases, depression is also associated with feeling of anxiety and with disrupted sleep, appetite and libido. At its worst, depression is accompanied by the very distressing thoughts such as that life is not worth living, and even by acts of self-harm leading to attempted or successful suicide. In the Western world, depression is considered one of the leading causes of disability, with one in five people experiencing depression in the lifetime.