When the clocks go back in October, three million of us will begin our annual battle with the winter misery known as SAD.
The darker mornings make getting up about as appealing as a cold bowl of porridge and an afternoon snooze certainly more alluring than a healthy walk in the open air. Stodgy foods and chocolate become irresistible and the pounds pile on until the clocks go forward again in spring.
Don't be SAD
The influence of the seasons on mood has been known for centuries. But it wasn't until 1984 that US psychiatrist Dr Norman Rosenthal, himself a sufferer of seasonal depression, identified what he called seasonal affective disorder, or SAD. According to Rosenthal this is a subspecies of depression caused by lack of sunlight for which the best treatment is light therapy.
Most of us probably experience a touch of the blues come wintertime, but for between 2.4 per cent and 3.5 per cent of people symptoms can be more severe.
Psychiatrist Dr John Eagles who sees around 15 to 20 new SAD patients a year at his clinic in Aberdeen observes, "About 80% of the people I see are women. There is some evidence of a familial predisposition and the genetics perhaps suggest that two things might be inherited - a predisposition to depression and a predisposition to seasonal fluctuations in wellbeing, so called 'seasonality'."
With the unremitting gloom of northern winters it is hardly surprising that Danes, Swedes and Norwegians are especially prone to SAD. There are even cafes in Scandinavia where you can eat under a light box.
Light among the gloom
SAD may be triggered by lack of sunlight, but it's still not known exactly what causes it. One longstanding theory is that it is due to a shift in the body's daily biological cycle caused by an excess of the brain hormone melatonin.
Melatonin is produced during darkness and falls at dawn. Night workers and people with jet lag also report similar symptoms, although in the case of jet lag only for a short time.
But melatonin isn't the only factor says Professor Anne Farmer, honorary consultant psychiatrist, who treats SAD patients at her clinic at London's Maudsley Hospital. "The stress hormone, cortisol, produced by the adrenal glands, tends to be higher in people with both 'ordinary' depression and SAD. Noradrenaline, another adrenal hormone, is also implicated, and there are also low levels of the brain chemical, serotonin, the 'happiness hormone," she says.
Dr Eagles adds, "I think all we can say with certainty is that a lack of light in autumn and winter is the cause of recurrent winter depression and it is likely that this is mediated in some way through serotonin."
So how should SAD be treated? Research published in the American Journal of Psychiatry in 2006 shows that light therapy and the anti-depressant drug Prozac, that helps bring serotonin levels back into balance, are equally effective. Some GPs hire out light boxes. However you will usually have to buy one, which can cost around £125.
Another US study by Professor Kelly Rohan of the University of Vermont in America found that cognitive behavioural therapy (CBT), which involves changing your thought patterns to more positive ones, was effective, especially when combined with light therapy.
The good news is that like all types of depression SAD tends to become less common as we get older.