I've been trying out alternating 3 minutes cold with 2 minutes hot for 20 minutes taking care to avoid my boys with the hot water(we are trying to get pregnant) I just got a heart rate monitor and wore it last night and my heart rate was 110-120 during the hot water and around 80-90 during the cold. I got a great nights sleep, but that happens after a 20 minute cold shower as well.
OK- day two tracking update- did 3 minutes/2 Minutes Cold hot for a total of 5 times(15 minutes cold) and finished off with 5 minutes cold. My high end was only around 105-115 but my low end was low 80's creeping into the high 70's. On my final 5 minutes I got to the mid 70's on minute 3-4 and then even got down into the high 60's at the end.
random shower note- I use a shower/wand combo I back into the shower head and then use the wand on my front for extra cooling/heating.
Update 3- had a horrendous headache yesterday (Blurry vision, sweating palms, numb fingers, weakness) so decided to skip the alternating (my theory is the alternating may have a pumping effect that may help detox- but this is very unscientific) but did 3 minutes hot, then 20 minutes cold- took my temp after the hot and it was 99.8 and then again after and was 98.2 a difference of 1.6. I found this interesting.
Update 4- Did a 3 minute hot shower/30 minute cold shower after a quick TABATA workout (step ups) and my body temp dropped to 96.2 afterwards. I'd think this would be helpful in a few ways- it activates the brown fat, forces my body to burn more calories to heat myself back up and seemed to help me sleep better.