Tetrataenite, an iron-nickel alloy.

I’ve been using it for roughly 5 years so it’s probably an old version but it does exactly what it says it does. Ie. It’s a water regulated mattress pad with a thermostat so it stays at whatever temperature you set. I have to clean it out now and then with hydrogen peroxide and add water every month or so. The heater stopped working at some point a few years ago, but I never need heat anyway and the cooling system works fine. I also set the cooling system well above my bed and that helps the circulation. But it was like $600 or something. I usually set it somewhere in the 60s and I’m in my happy place.

Just to respond to your need for better sleep… For me, good sleep is far more about temperature, darkness, and a nice weight. So I use a weighted blanket, eye covers, and the expensive but awesome chilipad so I don’t try to sleep in a pool of sweat.