I make no apology for extracting the following from the the Seasonal Affective Disorder Association website. They're the professionals, so why re-write the information.
What is Seasonal Affective Disorder (or S.A.D.)?
SAD is a type of winter depression that affects an estimated 7% of the UK population every winter between September and April, in particular during December, January and February.
It is caused by a biochemical imbalance in the hypothalamus due to the shortening of daylight hours and the lack of sunlight in winter.
For many people SAD is a seriously disabling illness, preventing them from functioning normally without continuous medical treatment.
For others, it is a mild but debilitating condition causing discomfort but not severe suffering. We call this subsyndromal SAD or 'winter blues.' It is estimated that a further 17% of the UK population have this milder form of condition.
Most sufferers show signs of a weakened immune system during the winter, and are more vulnerable to infections and other illnesses.
SAD symptoms disappear in spring, either suddenly with a short period (e.g. four weeks) of hypomania or hyperactivity, or gradually, depending on the intensity of sunlight in the spring and early summer.
In sub-syndromal SAD, symptoms such as tiredness, lethargy, sleep and eating problems occur, but depression and anxiety are absent or mild.
SAD may begin at any age but the main age of onset is between 18 and 30 years.
SAD occurs throughout the northern and southern hemispheres but is extremely rare in those living within 30 degrees of the Equator, where daylight hours are long, constant and extremely bright.
Light therapy has been shown to be effective in up to 85 per cent of diagnosed cases. That is, exposure, for up to four hours per day (average 1-2 hours) to very bright light, at least ten times the intensity of ordinary domestic lighting.
Ordinary light bulbs and fittings are not strong enough. Average domestic or office lighting emits an intensity of 200-500 lux but the minimum dose necessary to treat SAD is 2500 lux, The intensity of a bright summer day can be 100,000 lux.
Light treatment should be used daily in winter (and dull periods in summer) starting in early autumn when the first symptoms appear. It consists of sitting two to three feet away from a specially designed light box, usually on a table, allowing the light to shine directly through the eyes.
The user can carry out normal activity such as reading, working, eating and knitting while stationary in front of the box. It is not necessary to stare at the light although it has been proved safe.
Treatment is usually effective within three or four days and the effect continues provided it is used every day. Tinted lenses, or any device that blocks the light to the retina of the eye, should not be worn.
Some light boxes emit higher intensity of light, up to 10,000 lux, which can cut treatment time down to half an hour a day.
Traditional antidepressant drugs such as tricyclics are not usually helpful for SAD as they exacerbate the sleepiness and lethargy that are symptoms of the illness. The non-sedative SSRI drugs such as sertraline (Lustral), paroxetine (Seroxat) and fluoxetine (Prozac) are effective in alleviating the depressive symptoms of SAD and combine well with light therapy.
Other psychotropic drugs e.g. lithium, benzodiazepines have not proved widely useful in the treatment of SAD.
The SAD Association is a voluntary organisation and registered charity founded in 1987 to support and advise people with SAD and inform the public and health professionals about the condition.
SADA is self-financing and receives no government funding.
SADA is the world's longest surviving support organisation for SAD. Membership subscriptions form the financial basis of the charity and help to fund the paid administrator. Enquiries are received from individuals with SAD, health professionals and organisations, employers, educational institutions, local authorities and many more. Since it was set up, SADA has dramatically raised the awareness of SAD in the UK.
SADA's aim is to ensure that SAD is recognised and accepted in every part of the UK, and that those with SAD can maintain a productive life with the support of doctors, employers, family and friends.