In the U.S., a few adventurous folks might take a novel dip in a lake at an organized 'polar bear plunge' on what's usually a frigid New Year's Day. Jumping in a lake in the dead of winter in Sweden, is not a once-a-year event.
Taking an icy dip is a tradition in Sweden that requires no social media advertizing to gather participants. Swedes have long recognized the benefits of a quick, winter swim, and many people do it for that reason. Often. No Facebook invitations needed.
It’s not by accident that the sauna (bastu) houses of yore were often built near lakes. Long ago, it was very practical to bathe in the water after sweating in the sauna. And in the far north, tradition was that if they couldn’t jump in a hole in the lake, people would throw themselves into the snow instead.
Health reasons were the primary reason for the dip in the cold water, but it also known to be invigorating and refreshing. Studies show that when the body is exposed to cold, the outer blood vessels constrict and the blood flows faster through the body. The shock releases adrenaline and endorphins, making the cold bather feel refreshed and exhilarated. There is also an effect on the immune system that appears to be beneficial in fighting infection.
Of course there are risks, and a dip in the lake in the winter should be brief. Icy cold water can jump start heart and lung emergencies, among other concerns, in vulnerable people. And of course staying in the water for too long poses the risk of hypothermia in anyone. When cautious and healthy, however, studies show the positive effects outweigh the bad. So, next time you jump in a cold lake, consider these recommendations:
swimming naked is best (wet bathing suits keep the body cold); never swim alone; do not immerse your head; do not stay in the water longer than 20 seconds.

The more often than not preceding sauna bath has its own benefits: Heat may be good for the heart