Quite large numbers of the population experience periods of depression which, while not lasting long enough to be generally regarded as a depressive illness by the usual criteria, do bring significant problems. These individuals suffer from mood changes lasting a few days only. At its most extreme such a pattern of mood disturbance can lead to serious impulsive attempts at self-harm, which of course may be fatal, but it can also cause difficulties in employment or relationships etc. This type of depression is currently subject of research aiming to determine its relationship to other forms of depression and its response to treatment, as well as its usual course.