Question: Does winter light from my southern window make vitamin D or prevent seasonal affective disorder?

Answer: Winter sunlight, if there is enough of it, may help your mood but will not make any vitamin D. Window glass lets the longer ultraviolet A rays pass through but blocks the shorter UV-B rays that the skin needs to make vitamin D. Even outdoors in northern latitudes, almost no vitamin D is made in winter."In Boston, you cannot make any vitamin D from November through March, even if you were standing naked in the middle of the city," Dr. Frank C. Garland, a vitamin researcher at the University of California in San Diego, said in 1990.

Todd Hardin, a researcher in the seasonality study at the National Institute of Mental Health, said the light streaming through a southeast window on a sunny winter day was 10 times what a patient would get from the light boxes prescribed to fight seasonal affective disorder, a mild to moderate winter depression.

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Timing (morning) and length of exposure (20 minutes up to several hours) are important, said Hardin, but there is some evidence that bright enough light at any time, even if it is not full-spectrum light, helps. Light therapy, whether natural or artificial, "can't really prevent SAD but can ward off the worst of it," he said. What really keeps it away, he said, is the change of seasons or moving closer to the Equator.

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