The turning back of the clocks tonight marks the descent into winter, bringing with it shorter days, darker evenings, and a condition that a rapidly increasing number of people now dread all year: seasonal affective disorder – SAD.

Figures reveal that up to four million people in the UK may now be affected by SAD – up from 500,000 a decade ago. That, even with a recession descending, has sparked a spending boom among those desperate to find a way to lift the gloom.

Light boxes and timed devices that mimic the effects of sunrise are selling fast, while increasing numbers are signing up for recommended exercise classes. Some are even turning to more dramatic means. Prescriptions of antidepressants have tripled since the early Nineties, and it is thought many of those turning to them believe their condition is driven – or at least made worse – by seasonal factors.

This month the Government announced a £173m programme to improve access to mental health clinics. Meanwhile, the number of doctors prescribing exercise to cure depression has risen four-fold since 2005. "GPs are noticing many more people coming in with symptoms related to SAD," said Sarah Jarvis, a family doctor. "Partly that's because we're more aware of it, partly there's less stigma attached to it, and partly more people are getting it."

John Lewis has reported that sales of SAD light boxes, or "dawn simulators", were up 63 per cent compared to a year ago and the light box manufacturer Lumie said its sales were up 35 per cent. The idea behind them is that people wake up best to light rather than sound. Dawn simulators mimic the effect of the rising sun. Set on a timer, they coax the body into consciousness gradually.

Some researchers believe SAD is a myth. Vidje Hansen, a Norwegian, is professor of psychiatry at the University of Toronto, and has published a paper, What is this thing called SAD?. Professor Hansen said that when he conducted research in Norway, where there is no sunlight for two months of the year, he found "no correlation between depressive symptoms and the amount of light".

But ever since Norman Rosenthal, of the American National Institute of Mental Health, began to investigate the winter blues in 1984, many scientists have thought otherwise. Dr Rosenthal was curious as to why he felt sluggish during the winter. Steve Field, chairman of the Royal College of GPs, said: "SAD does appear to be a real condition".

SAD is thought to be linked to the way light stimulates the part of the brain called the hypothalamus, which controls sex drive, sleep, mood and appetite.