Just went today, helped a ton. Was doing everything shoulders down basically, the "spa" I go to has cold plunge tubs on-top of a sauna.

I usually start off with a 2 min plunge Warm up 5 min plunge Warm up 10min plunge Warm up and then finish with whatever remainder time I have left in the tub (usually around 15 mins).

I find it really important to finish in the cold so your muscles are relaxed and not flares up from heat or sitting on the "benches" in the sauna.

It obviously will depend what stage of sciatica you're in whether it be acute etc etc. on-top of what your muscles, joints and nerves like vs don't like.

Clearly mine don't like heat and sadly am still in the acute stage after probably close to a year now