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Sarah Browne said she moved to California from the U.S. Midwest for the sun to combat her seasonal affective disorder. (David Royal/AP)

For people with seasonal mood disorders, sunny days are definitely something to smile about.

Laid low in fall and winter by short days and diminished sunlight, people with seasonal affective disorder, or SAD, can suffer debilitating depression and related physical symptoms such as increased fatigue and a craving for high-carbohydrate foods.

For Ingrid Mraz, 45, of Toronto, summer's sunny days offer a short-lived reprieve from SAD, which she began experiencing more than 20 years ago.

While most people with the condition begin feeling their mood darkening around November, Mraz said her SAD season starts manifesting itself in August and begins lifting about the same time as that of fellow sufferers, around March or April.

"My mood is definitely improved," said Mraz of these sunshine-filled days. "I'm not feeling anxious, I'm not feeling so tired, I'm more myself again.

"So this is definitely a better spot to be in than me in February."

An estimated two to six per cent of Canadians will experience SAD at some point in their lifetime, says the Mood Disorders Association of Canada. Another 15 to 20 per cent experience a milder form of SAD, commonly called the winter blues or winter blahs.

"[Seasonal affective disorder] is a subtype of clinical depression, where people only get depressed in the winter and then feel fine in the summer," said psychiatrist Dr. Raymond Lam, director of the Mood Disorders Centre at the University of British Columbia.

Many people with SAD can find relief during dreary winter months with light therapy, which involves staring into a special light-emitting box for a certain period each day. 

Light's lifting effect on mood is mediated through the eyes, not through skin exposure, Lam said Thursday from Vancouver.

Brain's biological clock suspected

"We don't know exactly [what the mechanism is], but the major theory is that it affects the biological clock in the brain," he said. "And that's quite clear that there is a biological clock that controls many of our hormones and 24-hour rhythms, the sleep-wake cycle and also mood.

"So in the summertime, we usually feel better. And, in fact, with some people with bipolar disorder, for example, manic depressive illness, they may overshoot the mark and it's a period of risk for manias in the spring and summer, when there's changing light levels.

"That may be related to disturbances in the biological clock as well, because of the rhythmic nature of bipolar disorder."

The beneficial effects of light absorption have nothing to do with photoreceptors in eyes, but are related to a photoreceptive pigment in melanopsin cells in the retina, which provide a direct pathway to the brain's biological clock, he said. 

'Summer just can't be long enough.' — Ingrid Mraz

Lam, whose research focuses in large part on the effects of light on mood, said the body's internal timekeeper governs the circadian rhythms in virtually all of Earth's inhabitants, from one-celled organisms through to mammals, including humans.

"It just shows how dependent we are on the light-dark cycle, the 24-hour day."

For Mraz, the long hours of sunlight boosting her sense of well-being is more than a fair trade for the discomfort of the current heat wave.

"One of the ways I deal with my SAD, and with everything that's going on in my life, is I try to practise gratitude," she said.

"It may be a really hot day and I'll get into a bit of nitpicking. And a couple of us might be standing around and going: 'Oh, this is hard, this is really oppressive.' But after a while, I don't want to hear myself saying those things anymore, so I have to say: 'You know what, I don't deal well with winter, so let's just try to be happy about this.'

"And before I know it, it's going to be over. We're moving fast-forward towards August, and I don't like that.

 "Summer just can't be long enough."