Yes I tried it. It was my primary method, the secondary one was getting pneumonia and dying from high fever (yeah, dumb, I know).
I've spent 2 hours lying in cold water, everyday (I didn't measure it, but I guess it's 5-10 Celsius?, the tap water temp). I was fully submerged in the bath (below the neck). I've read somewhere that you lose most warm through legs (don't know how much truth is that).
After the cold baths, I've spent an hour sitting outdoors (0 Celsius temp). I've never felt sleepy, tired or anything else, I was just shaking.
Didn't even get sick for weeks I've tried this method, I just coughed for a couple hours and then everything's back to normal.
It probably depends on your physical health state, but indoor hypothermia is dumb. You need a lonely forest or a river with at least minus 5 Celcius temp, if you wanna go for hypothermia.
I wasted a lot of time with this. I wouldn't recommend it. But the cold baths were fun :) idk why, but the stress from being cold even made me laugh for no reason, lol...