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With clocks going back, millions of British workers will be heading home this evening in dark. Depressing? Yes, but there could be a good reason for why we get winter blues.

When Jenny Simmonds had her first bout of SAD 20 years ago, doctors said it was postnatal depression. But then her spirits slumped again following winter.

"She was depressed two years after birth," says husband Jon. "I'm no doctor, but I thought, 'this isn't postnatal'. It happened again next year, and next. For 10 years we were told it was a chemical imbalance."

Eventually, couple heard about seasonal affective disorder. Jenny got a name for her condition, and began looking for ways to deal with it, that didn't involve an annual sojourn at her local hospital.

Seasonal affective disorder, or SAD, is now a recognised psychiatric condition. Miserable weather, a bunged up nose every two weeks, not forgetting a seasonal rise in household burglaries, well, it's hardly a recipe for fun, is it?

And what dark days do is mess with balance of chemicals that affect a person's moods.

The lack of decent daylight leads to a drop in body's levels of serotonin and a rise in melatonin. Put bluntly, serotonin makes us livelier, melatonin makes us sleepy.

"How do birds know when to leave country?" asks Jon Simmonds. "In autumn, trees change colour, animals hibernate, yet we're expected to carry on as normal!"

It's estimated that 3% of UK adults suffer SAD to a clinically significant degree. Some of those will be affected to such an extent that going to hospital is only option. A further 10% will endure milder symptoms.

It all adds up to millions of Brits spending dark, moody months suffering loss of libido, anxiety, irritability and social withdrawal. But what must it be like for those who live even further north, where winter days are even shorter? Surprisingly, it's not nearly so bad.

Countries such as Iceland or Finland, which are endowed with snow and have little history of mass immigration, may not get many hours of sunlight, but chances are they'll get clear skies and plenty of light reflecting off snow.

It helps explain why British folk find rejuvenation not only in winter holidays to far-flung sun-drenched beaches, but also from visiting a frosty mountain top for a week's skiing.