[zhynn]

my well water varies between 4C and 10C from the ground, and I don't full-cold shower, but I do finish my shower cold, with as cold as it can go. It is bracing, but as the parent says, mentally fantastic. I find that it works as a quick meditative reboot (I can't follow any thought threads while the cold water is full on, so it interrupts my main mental loops). It is easier to achieve a "new day" state where it feels like you have permission to begin again. Now, that's just an illusion, just as the particular day of a new year is an illusion, but the "new day" effect is very powerful for behavior modification.