Many people experience 'winter blues' as the clocks change and days become shorter. From commuting to and from work in the dark to cold, wet weather and more time spent indoors, autumn and winter can feel like a dreary time of year. 

However, for a smaller percentage of the population, the change in the seasons can mean a plunge into a form of depression known as 'seasonal affective disorder'. 

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When was seasonal affective disorder (SAD) first described?

The term "seasonal affective disorder" was first used in 1984 by the American psychiatrist and scientist, Norman Rosenthal.

Rosenthal described the condition as a “syndrome characterised by recurrent depressions that occur annually at the same time each year”.

His initial findings were based on a study of 29 patients - 27 of whom had bipolar disorder - who all noticed an improvement in their mood when they travelled south to regions with longer daylight.

Eleven of these patients were trialled on bright light therapy. All showed some beneficial response within three to seven days, but 10 of the 11 patients relapsed when the light was discontinued.

READ MORE: Have these Scots researchers for a cure for SAD? 

What causes SAD?

Most people affected by SAD experience symptoms in winter. The exact causes are unclear, but reduced exposure to sunlight is associated with a decline in serotonin levels - the body's "feel good" hormone.

In addition, production of melatonin - the body's sleep hormone - can be disrupted by the shorter, darker winter days.

As the human body clock uses sunlight to regulate sleep, appetite and mood, this can lead to depression and tiredness which appears to affect some individuals much more severely than others.

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What are the symptoms of SAD?

People who experience SAD in winter can suffer from hypersomnia (excessive tiredness during the day and sleeping longer than usual at night), increased appetite, and a craving for carbohydrates.

It is also associated with feelings of sadness and anxiety; a lack of energy; difficulty concentrating; loss of libido; not wanting to see other people; and being unable to enjoy activities that usually bring pleasure.

READ MORE: Ten of the best winter boltholes for Scots foodies

Can SAD occur at other times of year?

Yes. While the vast majority of people report SAD symptoms in the winter and autumn months, a smaller percentage of people describe seasonal depression associated with spring and summer.

This tends to be characterised by insomnia and a loss of appetite.

Who is susceptible to SAD?

Anyone can experience SAD, but it appears to be roughly three times more common in women than men.

You are also more likely to develop SAD if you have a relative who suffers from seasonal affective disorder or another form of depression.

It is estimated to affect around 25% of people with bipolar disorder (manic depression).

How prevalent is SAD?

It is estimated that 6% of the UK population are somewhere on the SAD "spectrum". This rises to 9% in Scotland, including 3% who experience more serious symptoms.

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Are there treatments for SAD?

Research suggests that around 60-80% of people with SAD can benefit from light therapy, which tricks the body clock.

This works best using white fluorescent lights behind a plastic diffusing screen, which filter out ultraviolet rays.

READ MORE: Scottish celebrities reveal their favourite winter comforts 

Is SAD controversial?

There is scientific disagreement on whether SAD is a distinct form of depression. It is not included as a separate condition in the Diagnostic and Statistical Manual of Mental Disorders.

A 2006 study of 34,000 US adults found no evidence of seasonal or light-dependent increases in depression scores.