Ice baths are a popular subject on American social media pages.
Kim Kardashian, Harry Styles, Kristen Bell, Lizzo, and other stars have posted about their cold water experiences.
Health claims surrounding ice baths include improved mood, increased energy, weight loss and reduced inflammation.
But the science linked to some of those claims is not very strong.
Here is what medical evidence, experts and fans say about cold water baths, an activity that dates back hundreds of years.
Since June 2020, Dan O'Conor has jumped into Lake Michigan almost daily.
The 55-year-old man lives in the city of Chicago.
O'Conor used the term endorphins - "feel good" hormones released in reaction to pain, stress, exercise and other activities – to talk about his experience.
"The endorphin rush ... is an incredible way to wake up and just kind of shock the body and get the engine going," he said on a recent morning when the air temperature was minus-5 Celsius.
With the lake temperature at 1 degree Celsius, O'Conor did a running jump into the icy gray water.
His first jump came early in the pandemic.
O'Conor had too much alcohol and his wife was angry at him.
She told him to "go jump in the lake."
The common saying is not usually received as a real command.
But, O'Conor acted on it.
He says the water felt good that June day, so he kept jumping in every day following.
As the water grew colder with seasonal change, the good effect was even greater, he said.
"My mental health is a lot stronger, a lot brighter.
I found some zen down here coming down and jumping into the lake and shocking that body," O'Conor said.
Dr. Will Cronenwett of Northwestern University's Feinberg medical school tried cold-water immersion once while visiting Scandinavian friends.
After a sauna, he jumped into the ice-cold water for a few minutes.
He said it was a powerful experience.
But Cronenwett says studying cold water immersion with a kind of scientific study known as a randomized controlled trial is difficult.
This is because it is hard to develop a placebo for cold water baths.
A placebo is something that has no effect.
Placebos are important because scientific tests need to compare the possible effect of something with something else that does not have any physical effect.
There are a few ideas about how cold water immersion affects the mind.
Cronenwett says it makes the part of the nervous system that controls the resting state more active.
Such an effect may increase feelings of well-being.
Cold water immersion also affects the part of the nervous system that controls the fight-or-flight stress reaction.
Cold water treatment on a usual basis may reduce that reaction.
So, Cronenwett said, it might help people feel better able to deal with other stresses in their lives, although that is not proven.
Czech researchers found that cold water immersion can increase blood levels of dopamine — another hormone — by 250 percent.
High amounts have been linked with paranoia and aggression, noted James Mercer of the Arctic University of Norway.
Mercer co-wrote a recent scientific paper that examined studies on the treatment.
Cold water immersion raises blood pressure and increases stress on the heart.
Studies have shown this is safe for healthy people and the effects are only temporary.
But sometimes these effects can cause the heart to beat unevenly and can cause death, Cronenwett said.
People with heart conditions or a family history of early heart disease should talk with a doctor before doing cold water immersion, he said.
Repeated cold-water immersions during colder seasons have been shown to improve how the body reacts to insulin, a hormone that controls blood sugar levels, Mercer noted.
This might help reduce the risk of getting diabetes.
It also might help people with the disease control it.
More study is required to know for sure.
Cold water immersion also activates brown fat.
This tissue helps keep the body warm and helps it control blood sugar and insulin levels.
It also helps the body burn calories.
I'm John Russell.