A CRUCIAL question will be on the lips of many a hypochondriac after yet another week of British winter: do I really suffer from Seasonal Affective Disorder, or am I just a moany old git? Seasonal Affective Disorder (SAD) is a clinically diagnosed disorder that severely affects a small minority of people. Yet it has become something which, like wheat allergies and sensitive skin, everyone thinks they have.

Steve Hayes, director of Outside In, the body clock company, says, "An EU-funded project looking at outcomes of depression in different areas interviewed 3,000 people. It found that 1.8 per cent of people met the strict clinical criteria for SAD; a further 7 to 8 per cent met less stringent criteria."

So you probably haven't got it, but you may be on the continuum. Experts draw this continuum extending from the people who become suicidal with lack of light, all the way down to those who feel a bit gloomy when it's grey again.

SAD has a chemical basis. Dr Ann Macaskill, health psychologist at Sheffield Hallam University, who has done work on SAD, explains: "The most likely hypothesis is that it is a chemical imbalance caused by lack of light. Natural light stimulates the pineal gland to secrete a natural endorphin." Some people are more affected than others.

Tests have shown that it is nothing to do with warmth. A skiing holiday is just as good as a holiday in the Bahamas for solving the problem. Possibly even more so, because the level of light may be be intensified by reflective effect of the snow.

The problem with SAD, however, is that many of the symptoms are similar to normal depression and to "I'm-run-down-because-of-the-cold-weather- and-no-holiday-lurgies" (that's the technical term) and even to what Shirley MacLaine in Steel Magnolias called "being in a very bad mood for 40 years".

SAD people...

1. Start feeling a sinking feeling when the days draw in and feel better come the spring. Well, almost everyone I know feels like this, except one old friend who liked winter because she could wear thick jumpers that entirely disguised her body fat.

2. Still feel tired after a long sleep. Yes, I do.

3. Have flu symptoms: aching limbs, etc, without the head cold - yep, certainly.

4. Have cravings for carbohydrates - well yes, but then I have them all year round.

5. Are known for leaving the lights on. Me again, but I'm also known for being an appalling mess. Am I SAD or just sad? You begin to understand the complications.

The most successful treatment is daily exposure to a light box, so it's a good means of sorting out the real sufferers from the grumps. The Journal of The American Medical Association said that "light therapy is one of the most successful and practical results of basic research in biomedical rhythms".

Light boxes flood the back of your eyes with simulated daytime, outdoor- strength light. The sufferer sits in front of them for between 30 minutes and two hours depending on the bulb's strength. This artificially stimulates the underactive pineal gland and you feel jolly again.

Handily, Outside In does a three-week "satisfaction or return" offer with their light boxes. After getting your light box, your question will be answered. SAD sufferers should start feeling better by the end of a week. If you don't feel better, I'm afraid you are a moany old git. In either case, follow the instructions below.

Advice for all:

1. SAD sufferers: contact Outside In, tel: 01954 211 955. Light boxes start from pounds 100.

2. Moany old gits: when feeling lethargic, force yourself to do things. Get outside. See friends and have a jolly old time. Give yourself little treats. Take brisk walks.