Blood pressure varies according to seasons and is usually higher in winter than in summer. According to British researchers, sunlight is responsible for this effect. Sunlight alters the nitric oxide (NO) in the skin and thereby reduces blood pressure, scientists write in the "Journal of Investigative Dermatology".

In the study, the team from the universities in Southampton and Edinburgh exposed 24 participants to a tanning light for two sessions of 20 minutes each. In one of the two sessions, the UV rays were blocked. Subsequent examinations showed that the effect of warmth alone did not change the blood pressure, yet it was reduced by UV exposure.

Nitric oxide levels in the body are responsible for this, according to study author Martin Feelisch from the University of Southampton. "NO along with its breakdown products, known to be abundant in skin, is involved in the regulation of blood pressure. When exposed to sunlight, small amounts of NO are transferred from the skin to the circulation, lowering blood vessel tone", he explained.

Reducing blood pressure was also associated with a reduced risk of heart attack and strokes, said Feelisch. Therefore, NO could be an essential factor in cardiovascular disease prevention and complete avoidance of exposure to sunlight may be counterproductive, the researchers said.