First identified in 1984, SAD soon became recognised as a condition that affects up to one million people in Britain, according to the SAD Association (SADA), a voluntary body set up in 1987 to advise sufferers. With a core of 2,250 members, SADA helps many others - it sold more than 5,000 information packs last winter. But, since few people welcome shorter days and grey skies, how do you know if you've got it, or you'd just rather be in the Bahamas?
Begin by looking at your moods - have you noticed a real plunge into depression every winter? Does this start in September and end in April? During December, January and February, are you particularly miserable, with low self-esteem, feelings of hopelessness and guilt? Have you lost your sex drive? Has your brain gone slow? Do you crave bread, potatoes and pasta?
To distinguish between SAD and other forms of depression, says Dr Alison Kelly, an independent consultant psychiatrist, "you need to look at two things: sleepiness and appetite. In ordinary depression, sufferers can't sleep and don't eat. But people with SAD are very sleepy, and they crave carbohydrates."
So what causes it? Research has focused on the way our brains react to natural light. Since the optic nerve is connected to parts of the brain which control moods, appetite and hormones, lack of light during winter has a direct effect on eating, sleeping and sex.
One theory focuses on the pineal gland, which makes melatonin (the hormone that tells us to nod off). Light deprivation means more melatonin, hence more sleep. Other research suggests that SAD is related to an imbalance of the mood-enhan cing hormone serotonin - and can thus be treated with drugs such as Prozac.
You would expect the further north you go, the more people would suffer. In Scotland, for instance, an estimated 50,000 people are afflicted by SAD.But studies in Scandinavia show that the shorter northern day doesn't necessarily result in more cases. Scientists say the reason is possibly genetic - some of us are built to hibernate, others don't need to - so if your family has a history of SAD, your chances of getting it are higher. Others favour cultural explanations: people get used to being moody in winter and don't seek help. The condition seems to affect about 2 per cent of people, whether in Finland or Spain. To reduce your chances of getting it, head for the Equator, where it's virtually unknown.
It's not hard to find teachers who suffer from SAD. Kate Hesketh Moore, SADA's newsletter editor, was once a secondary teacher. "The winter term used to be a killer. The actual teaching wasn't so bad, because the adrenaline rush keeps you going, but I used to collapse in the evenings. Doing marking or preparation became very hard. I used to fall asleep at lunchtime in a crowded staffroom."
Alan West, head of Hardwick primary in Cambridgeshire for 13 years, says: "I used to find that in the run-up to Christmas, I began to suffer stress, with headaches and debilitating tiredness. I was arriving at work in the dark and leaving in the dark, so I never saw daylight. "
For the worst sufferers, winter can be deadly. In February 1997, The Daily Telegraph reported the case of Eddie Baxter, a school caretaker who hanged himself while suffering from winter depression. His daughter, Paula Chambers, says that the onset of cold, dark days "plunged him into the depths of despair from October onwards".
Are teachers more prone to SAD than other professions? "Obviously, it doesn't help to be in a stressful job," says Kate Hesketh Moore. "When SAD symptoms begin to kick in - during December - most teachers are already under stress. Constantly teaching in badly lit classrooms doesn't help either."
One problem is distinguishing SAD symptoms from other factors. Dr Tim Dalgleish, researcher at the Medical Research Council's Applied Psychology Unit, says: "You have to be careful about professions which have seasonal variations in the workload. It may be that a long summer break and a heavy winter workload causes depression, rather than the absence of light."
Bob Wright, a London teacher who started his career in 1971, believes his episodes of winter depression were triggered by the extra strain of running the Christmas fair. "When I was down I could barely speak to people; I would just sit in the corner. " Changes in the national curriculum added to the stress - and low self-esteem among teachers didn't help either.
Thankfully there are a number of ways to treat the condition. "My solution was to change my lifestyle," says Kate Hesketh Moore. "So I resigned! I loved teaching, but the pressure was too much. I resigned constructively - in the summer - and now teach part-time, which is much less strain." Bob Wright took early retirement.
One possible way of alleviating symptoms is phototherapy, using a light box, which delivers 2, 500-l0,000 lux (a light bulb is only 500 lux). "For schools, I would recommend a light box in the staffroom," says Kate Hesketh Moore. "It could be used by staff while they were reading or doing lesson preparation."
Bob Wright combines wearing a light visor for 40 minutes every morning with a low dosage of Prozac, which boosts his serotonin, "the happy hormone", for the winter months.
If you're worried about possible side-effects, a natural option is hypericum, which comes from the herb St John's Wort, now nicknamed "nature's Prozac". Andrea Jarman started taking this in January 1997. "A few weeks later, I felt able to jump on the tube and go and see a show in London." Members of SADA are currently testing hypericum, which is available from most health food shops under the brand name Kira.
Perhaps the best treatment for SAD is a mixture of phototherapy using a light box, supplemented if necessary by a visor or a "natural" alarm clock - which wakes you up by bathing the room in light - and some form of drug or herbal remedy.
But it is not just adults who suffer. Teachers should also be aware, says Steve Hayes, managing director of Outside In - a company that supplies light treatments - of the effect of SAD on children. "Some kids do well in summer, but badly in winter. It should be possible to use test scores to monitor this - and see if any children need medical help." Bob Wright agrees: "I've asked the unions to be more aware of pupils who may suffer winter depression," he says.
One case is that of Tina Benton, aged nine, of Hardwick in Cambridgeshire,who, says her mother Elaine "suffered a personality change in winter. She became incredibly tired, irritable and pale as a ghost. She had dark rings around her eyes." But after treatment with a light box for 20 minutes in the morning and afternoon, "she's such a different child - much more her usual self."
Judging by the 50 per cent year-on-year increase in sales reported by companies such as Outside In, both awareness of Seasonal Affective Disorder and the use of remedies to treat it, are growing rapidly. So if you can't afford to hibernate, the good news is that you don't have to.