Dr. Mark Harper recalls his first cold-water swim in the south of England 20 years ago. It was August, but the initial jolt from the plunge took his breath away.

The shock to his system lasted a minute or two until he was “recombobulated and able to think about something other than the cold,” Harper says. A surprise sensation soon replaced his discomfort.

“I remember getting out of the water the first time and feeling so good,” Harper, an anesthesiologist who has since researched the potential risks and rewards of taking a nippy dunk . “I wasn’t expecting that.”

Claims about the benefits of cold-water immersion date back centuries. Thomas Jefferson, the principal author of the Declaration of Independence and the third American president, wrote toward the end of his life about usin

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