This winter could see an epidemic in cases of seasonal affective disorder (SAD), following the abysmal summer that was the wettest since records began. The summer is traditionally a time of respite for those susceptible to SAD, but mental health experts have warned that with sufferers reporting winter symptoms throughout the season, the effect of the prolonged bad weather will make the winter months even more of a struggle than usual.

At least one in 20 people suffers from SAD-related bouts of depression, with a further one in eight affected by the more nebulous winter blues. Bright light, ideally sunlight, is the best treatment for those who suffer from this debilitating mental health condition. But when summer does not provide this, the cumulative lack of good quality sunlight could have a devastating effect.

Dr Natasha Bijlani, a mental health expert at the Priory, in south-west London, said: "I'm sure this unpleasant summer may have an effect. Poor weather that's associated with a lack of sunlight has an effect on some sufferers, so I think those already susceptible will be even more vulnerable this winter."

The summer is ordinarily a time when we spend a greater proportion of our day outdoors, and can expect exposure to sunshine even after finishing a working day. But with unprecedented levels of rainfall, relentlessly overcast days, and a lack of prolonged sunlight, many people have reported that they have suffered SAD symptoms over the summer months.

Eryl Price, communications manager at Outside In, a company that specialises in supplying light therapy lamps to SAD sufferers, said they had seen unprecedented sales this summer. "We started looking at our sales in June, and they were three times as much as they normally would be for that month. By August they were four times as much as the same time the previous year, and equivalent to what we would ordinarily expect in November."

For those who have suffered in the summer months, this winter will be more difficult than ever. A spokesperson for Outside In said: "With the summer we have had, most of us would have spent a great length of time indoors, more than usual for this time of year and simply not gained the light levels we need."

Specialist Dr Trisha Macnair said that a lack of existing research on summer cases of SAD made it difficult to determine for sure what effect it would have on the following winter. But she reckons that for those already showing signs of depression, it could mean a difficult winter. She said: "There's enough evidence to suggest that some people are particularly sensitive to the effects of the light. If people went into autumn already depressed, they might go into a depressive cycle, which would make their chances of getting back up again through the winter months much slighter."

Since SAD is associated with the shorter daylight hours of winter, it has led many scientists to believe the condition would not raise its head in the summer months. However, this assumption has been challenged this year, as many people have begun to exhibit the symptoms of depression ordinarily brought on by winter.

Dr Bijlani called for more research into the effects of poor summers on those with the condition. She stressed that since quality of light, as well as daylight hours, were known to be factors in SAD, it was important to find out whether further preventative steps needed to be taken.

She said: "There haven't been any studies into the effect of bad summers that I'm aware of but I think someone really needs to look into this now."

'Normally I feel better after the summer': Doreen Grainger, 54, Teacher

Mrs Grainger, who works for a primary school in Kilsyth, near Glasgow, has suffered from SAD for years, and says this summer has not provided the break she had hoped for.

"By the beginning of August, I was feeling really down, which I don't normally get at that time of year," she said. "I began to feel the way I normally do in November." After a few weeks of depression, Mrs Grainger decided it must be down to the wintry weather. "Even when I was outside it was dull, and there were times when it felt like night-time. I decided to start using my light box, which I never usually do in summer, and that made it better."

But she believes the real problems will come this winter, when SAD sufferers have not had time to recover. "Normally I feel refreshed after the summer holidays, but not this year. It was always wet and dull, and downright awful. If you don't have the good feelings sunshine gives you, you will be further on the road to depression when winter comes."