ARE you irritable on these endless, dark winter mornings? Do you find it hard to wake up and go to work when the sun hardly rises above the horizon all day? And crucially, is it difficult to pass a sweet shop without popping in for a Mars bar?

If the answer to all of these questions is yes, you may well be suffering from Seasonal Affective Disorder (SAD), a form of depression caused by lack of natural daylight. Research has shown that a lack of light creates a chemical imbalance in the part of the brain that regulates many body functions and this affects our sleep, our moods and our energy levels.

Now recognised by the World Health Organisation as a legitimate illness, SAD is believed to affect - to some extent - anything up to 2.5m people in Britain.

In around a quarter of a million cases the condition is so serious that it can cause depression between October and April. Studies suggest that people who live in northern latitudes are more susceptible to light deprivation than their southern counterparts. This makes the North-East a prime site for SAD.

Despite being more than 300 miles away, Professor Chris Thompson from Southampton University, a psychiatrist and a leading expert on SAD, regularly sees people from the North-East who have been referred by their GPs. His Hampshire clinic is one of only two in the country.

"At one end of the spectrum there are people who have it very severely and those at the other end who have it mildly. However, if you mention the symptoms to almost anyone they say 'yes that happens to me'," says Prof Thompson.

"The typical symptoms include not being able to wake in the morning, depression, loss of memory and difficulty in concentrating," he said.

Irritability and anxiety are also a common feature - as well as a craving for high-energy foods.

"My worst patients are those who can't walk past a sweet shop without buying a Mars bar, which is not popular with their dentist or their doctor because it is easy to gain weight but difficult to lose it," says Prof Thompson, who has studied SAD alongside American experts.

Only 20 years ago, the idea that human behaviour was seasonally affected by the quantity of light available during the day was not scientifically respectable. The first medical man to suggest that there could be a link between depression and light deprivation during winter was a German psychiatrist called Dr Kraeplin, nearly 90 years ago. His theory was largely dismissed and forgotten until a piece of research by an American scientist in the early 1980s.

"We know that animals are responsive to light, there is a yearly rhythm which tells sheep when to breed and birds when to moult and it has mostly to do with what time the sun rises," says Prof Thompson. "The timing of dawn is the key variable. It tells your body what time of year it is."

Until the early 1980s, scientists thought that humans were no longer susceptible to light in the same way as animals, but a scientist called Alfred Lewy carried out a groundbreaking piece of research which showed clearly that humans were responsive to the dawn signal. The condition known as SAD had arrived.

Prof Thompson says: "At the time a scientist called Herbert Kern read Lewy's paper and contacted him. He took in his diary and pointed to the blank pages for every winter. Kern told Lewy he was so depressed during winter that he has no motivation to do anything."

In a bid to treat Herbert Kern's light deprivation, the research team built the world's first light box - now a proven method of treating SAD. Scientists found that by exposing SAD sufferers to a powerful light source many times brighter than normal internal lighting, for between one and two hours a day, the condition could be alleviated over time. Modern light boxes, which are widely available from retailers such as Boots, produce an intensity of light which simulates natural summer sunshine.

Prof Thompson has been studying depression since the late 1970s and in 1984 spent a year at the US National Institute of Health just as the breakthrough on SAD was being made.

Research has shown that the further north in the world you go, the more SAD there is. But beyond a certain point there seems to be a genetic protection against the condition.

"It seems to be a highly genetic condition. If you lived in Iceland, where there is very little light for much of the year, you would have the genes for SAD bred out - or life would be intolerable," Prof Thompson adds. But studies have also suggested that if you were born to the north of where you live - or if your parents were northerners - you are less likely to have SAD than people who have always lived locally.

"For example, a study in Canada has shown that Icelandic immigrants who are living south of where they or their parents were born are less likely to have problems," he adds.

Despite the growing awareness of SAD in the UK there are still only two specialist centres where patients can be referred. "People are referred to me by GPs and sometimes psychiatrists," says Prof Thompson. Often patients read about us and contact us through their GPs. I even get phone calls from Europe."

People can often help themselves by investing in a light box, but Prof Thompson advises people to try before they buy because around three in ten severe SAD sufferers do not respond to light therapy.

"It is worth contacting the SAD Association who can sometimes loan light boxes out," he adds. People who respond best to light therapy are those who have the most classic symptoms: over-sleeping, fatigue and over-eating.

Prof Thompson is also interested in the growing number of winter sun seekers - Brits who head off to The Canaries for a fortnight to avoid the winter gloom.

Despite the international industry that has grown up around SAD and its treatment, the SAD Association is the only self-help group to have survived in the world.

An information pack is available from the SAD Association by sending a SAE with a £5 cheque or postal order to PO Box 989, Steyning, West Sussex, BN44 3HG.

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