As summer, for all it was, shifts into autumn and days get shorter and darker, around two million Britons will begin to wish they could pull the duvet over their heads until spring. As sufferers of seasonal affective disorder, or Sad, they will typically feel lethargic, depressed and have cravings for carbohydrate foods, with symptoms intensifying between December and February.

Precisely what causes Sad is unclear, although it centres on the way light triggers messages to the hypothalamus in the brain, which controls sleep, sex drive, appetite and mood. For those affected, diminishing daylight hours cause these functions to slow down or deteriorate.

By far the most the most common treatment is light therapy. But we are not talking any old light. An average light bulb produces 200-500 units of light energy (or lux) whereas patients with Sad are advised to sit in front of a light box that emits about 10,000 lux from a fluorescent lamp. Most will need at least 45 minutes' exposure a day.

Studies at Columbia University in New York showed that dawn simulators - devices that gradually increase bedroom light every morning - can be helpful as they fool the body into thinking it has a summer's day ahead. The same scientists found sitting in front of negative ion generators - gadgets that cleanse air of pollutants - to be beneficial. One US study showed cognitive behavioural therapy to be as effective as light box therapy for patients with Sad.

Diet and regular exercise can be helpful. Shun carbohydrates and eat low-fat protein foods instead. A small study found supplements of vitamin D, produced naturally by sunlight, might help, but there is no confirmed link. Nothing, though, beats a bright day, which provides a whopping 100,000 lux. Spot any winter sunshine and grab it while you can.