Sudden changes in temperature cause thermal stress for the body - which has to work harder to maintain its constant temperature. In particular, research has shown this makes it more likely for people to suffer from dangerous blood clots during winter. Study authors, from a hospital in Nice, France, suggested that respiratory tract infections more common in winter might make patients more vulnerable to blood clots. They also suggested that chilly weather might make the blood vessels constrict, making it more likely that blood clots will form. 

People exposed to cold weather are more likely to suffer a heart attack, a recent study revealed. Researchers from Sweden from Lund University in Sweden found that the average number of heart attacks per day was significantly higher when the weather as cold compared to when the weather was warm. On a day-to-day basis it translated to four more heart attacks per day when the average temperature was below zero. It is thought the risk of heart attacks is higher in cold weather because the body responds to feeling chilly by restricting superficial blood vessels. This decreases how warm the skin is and increases blood flow through the arteries. The body also begins to shiver and your heart rate increases to keep you warm. But these responses can add extra stress on your heart.