Are you feeling sleepier than usual? Are you gloomy and irritable? Do you crave sweets or alcohol in the late afternoon? Okay, hands down everybody. But now, the clinching question: do you feel like that just at this time of year? If the answer is still yes, you are not alone: a third of us are estimated to suffer from “winter blues” - mild depression triggered by lack of light.

But for a few people (up to 3 per cent, according to Mind, the mental health charity) the condition known as Seasonal Affective Disorder (SAD) is a debilitating illness. Bouts of clinical depression return with dreaded regularity as the days shorten in autumn, and persist until spring. Suffering from severe SAD has been described as “feeling half dead, for half of your life”.

SAD has a kind of primeval plausibility. It seems natural to want to cut down on activity and conserve energy when the weather gets cold and the nights lengthen. In the remote past, when human beings regulated their lives according to the hours of daylight and the cycle of the seasons, the ability to slow down in winter may even have been an advantage. If it is an adaptive trait, however, it is one that has become very inconvenient in modern 24-hour societies. (Interestingly, SAD only occurs in countries with clear seasonal variations in the amount and strength of the light - it is unknown in latitudes within 30 degrees of the equator.) Although doctors from ancient Greece to the present have recognised the importance of light to good health, SAD was only described and named in 1984.Since then, various physiological causes have been suggested. Most theories involve the hormone melatonin, and the mechanisms which regulate its manufacture in the brain. Melatonin is produced during the hours of darkness, and causes us to sleep. Light suppresses its production. People with SAD produce much higher levels of melatonin during the winter than people without SAD symptoms. Exposure to bright light can lower melatonin levels, but simply suppressing melatonin does not remove the symptoms, so the hormone is not the sole cause of the illness.

The obvious way to counter mild SAD is to make the most of what little light there is in winter. Spend an hour a day outside during daylight, if possible in the morning. Sit near a window. Paint your house in pale colours to maximise natural light, and take a sunny holiday in winter.

Severe cases of SAD require more drastic treatment. Antidepressants are one option. Another, not available on the NHS but reporting a high rate of success, is bright light therapy, which involves spending an hour or so a day in front of a specially designed lamp up to 10 times as bright as an ordinary domestic light. The light boxes cost between pound;100 and pound;300 (though most are available on a rental or trial basis), and the treatment is time-consuming. Still, if the alternative is feeling half dead half the time, it may be worth a try.

SAD Association: for information send an SAE to PO Box 989, Steyning, West Sussex, BN44 3HG. SAD Lightbox Co Ltd: phone 0800 0741105 for information, prices and rental terms. A Philips Bright Light Box can be ordered through Boots, pound;199