Not OP, but I finish every shower with 2-3 minutes of as cold as the faucet will go. I've been doing this every day for about a year. In the winter my water temp is in the mid-40 degrees F. I love this habit. It's always difficult, but I always feel great after doing it.
Wouldn't it be nice if we could measure this and find if there's a point where colder water doesn't provide a benefit, or rapidly diminishing returns?
Wouldn't it be nice if there was evidence that 14C water conferred the same benefits as 4C water?
I know it would be way easier for me to suggest this as a habit to family if I didn't tell them, "yeah you just use water as cold as it goes, even in winter when your pipes are about to freeze".