I use contrasting t - a sauna and then a cold plunge and then back to the sauna. Unfortunately, I cannot (yet) fully submerge myself in the cold plunge.

Since high temperatures - a sauna, a steam bath, a hot salt room - calm racing thoughts, hypomania, and anxiety, it would not surprise me at all to hear that cold showers escalate your hypomania. Why would it be different, if I may ask? Cold showers are highly invigorating, so...? I guess my question is - why does it come as a surprise to you?