It's been found that exposure to bright artificial light can substantially reduce the symptoms of depression by as much as 80 per cent in some patients. There is not so much to look at and therefore a greater opportunity for you to attend to your tasks.'' For the treatment of the depression associated with SAD, Professor Farmer and an increasing number of doctors are advising using light boxes. Investigators are looking at whether light or temperature, or even some other trigger, may be at work. Some suspect that a change in the environment, especially the arrival of long dark nights, affects personality - that when it gets cold and dark man becomes more introverted and more focused on the tasks in hand. "It is a quite surprising and counter-intuitive finding that requires a lot of thought,'' says Professor Anne Farmer of the Institute of Psychiatry in London, who specialises in treating affective disorders, including SAD. "One possible explanation is that we are less distracted by other things in winter. But depression and improved cognitive performances are strange bedfellows, so the hunt is now on to find out what could be happening in the brain to produce such a paradox. They also reported extreme fatigue and lack of energy, and an increased need for sleep. SAD, which affects between one and 25 per cent of people depending upon which study is looked at, is accepted as a condition where depression is linked to the arrival of winter months. A National Institute of Mental Health survey of 1,500 American SAD patients found that more than 90 per cent reported decreased activity in winter, as well as difficulties with work.

It is at odds with many assumptions about health and the winter. The reaction times of the volunteers were on average 11 milliseconds quicker in the winter tests. The problem that Dr Brennen and his colleagues now have is explaining what kind of body mechanism could possibly be at work to produce the unexpected phenomenon of superior thinking in winter. Clearly the belief that people get groggier and more forgetful in the winter months is unfounded.

The findings contradict some of the claims found in the literature on SAD.'' Even on the simplest of the tests - measuring reaction times to a circle being flashed on to a computer screen - the winter performances were better. We didn't look at depression, we were only interested in cognitive performance. We tested them in summer and in the winter, and we were sure that we would be able to pick up the winter deficit that is so often talked about,'' he says. "If you read a lot of the literature on SAD, the biological psychiatrists expect concentration to be worse in winter, that speed of thought and memory will be poor, and that people will feel sluggish But we found no trace of that. For Dr Tim Brennen, who led the research and who briefly escaped the winter blackness of Tromso University this week to present his findings at the London conference of the British Psychological Society, the results were a big surprise. "We tested 100 people on a battery of cognitive tests, including memory, attention, reaction time, memory recall and confusability.