Norepinephrine release is one of the most consistent physiological responses. You can get that from 20 seconds at, like, 40 °F water or 2 minutes at 49 or 50 °F water will release norepinephrine 2-fold over baseline. Or you can be crazy and stay in, like, 50 Fº water for an hour—who does that—and you'll get a five-fold increase.