Once upon a time cafés were for tea, coffee and gossip. Not any more. These days you can keep in touch with the world in an internet café, get a blast of fresh air in an oxygen café and - from today - banish the winter blues at a "light lounge".

Sufferers from seasonal affective disorder (SAD), a recognised mental condition caused by a lack of sunlight during the long nights and short, gloomy days, can drop into the Dana Centre at the Science Museum in London, for a session of intense light therapy. Four specially designed light boxes positioned around a white sofa raise ambient light levels in the lounge to 10 times that of normal room lighting.

Scientists believe a 20-minute session can boost levels of serotonin, a chemical messenger in the brain linked with feelings of happiness and well-being, and slow down the production of the hormone melatonin, which causes drowsiness.

It is estimated that about half a million people suffer from SAD and some 5 million more experience milder versions. Mild forms of SAD cause sleep problems, lethargy, lack of appetite and sex drive and a lack of interest in activities previously enjoyed. Severe forms of SAD can lead to deep depression, with the accompanying feelings of anxiety, hopelessness and despair.

Light therapy has been shown to be effective in 80 per cent of cases. Light boxes have been available to purchase for some time, but this is the first time a "light lounge" has been available free to sufferers, or simply those feeling the effects of the lack of sunlight. The lounge has been opened to coincide with a debate on the science and treatment of winter depression.

Jennifer Eastwood, the founder of the SAD Association, was among the first to receive light therapy and, she says, it has transformed her life. "Without this therapy I would be dead, either by my own hands or just by being terribly, terribly ill because it lowers your immune system to such a degree that you would be really unwell.Ms Eastwood said that light therapy had helped her on to an even keel and had alleviated many of the symptoms characteristic of SAD - such as overeating, oversleeping and a general sense of lethargy.

"I suspect that most work absences during January and February are to do with this. If more employers had this sort of thing we might get more productivity," she said. "If everybody had a coffee break in this sort of environment then they would probably feel a lot better."

She warned that it would take up to five days to feel the effects of light therapy.

The 'winter blues'

* One in 50 people is thought to suffer from seasonal affective disorder (SAD)

* It is more common in women and most commonly starts between the ages of 18 and 40

* As many as one in eight people may experience the milder symptoms of the "winter blues"

* Studies show that SAD becomes more common the farther people live from the equator

* Symptoms usually start in September and are worst during December, January and February

* Lack of bright light entering the eyes seems to affect the severity of the symptoms by preventing the brain's hypothalmus from working properly

* Going on a sunny holiday alleviates the symptoms, but people can feel worse when they return