By Amanda McMillan, Outside

Scientists have known for years that rates of hypertension are higher in the winter and in countries farther from the equator, but they haven’t known why—until now. A new study from the universities of Southampton and Edinburgh suggests that exposure to sunlight plays a large role, by causing nitric oxide in the skin to be absorbed into the blood stream. Blood vessels widen as a result, causing a modest—but potentially life-saving—drop in blood pressure.

To study the sun’s effect on blood pressure, researchers exposed 24 healthy volunteers to ultraviolet light, mimicking the amount of exposure a person might get wearing short sleeves and shorts outside for 30 minutes on sunny day. In a second session, they were exposed to the same amount of light and heat, but no UV rays.

The UV exposure triggered a small decline in blood pressure—about two to five points—while the “sham” exposure did not. While that may not seem like a lot, the authors write that a even tiny drop can go a long way: A 10-point change in diastolic pressure, for example, can slash your risk of cardiovascular disease and stroke in half.

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