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.
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