There are these mats you can buy. Advertised as pet cooling mats, they are rubbery plastic with a gel filling of some sort that is cold at room temperature.

Despite being sold as for pets, some of my family have bought them to put in their own bed to be cool during hot summer nights. Others seems to have had this idea too as all the mats for "big dogs" are sold out. I was gifted one (medium size) and while I've not used it over night, my first impression is it's pretty nifty.

What I don't get is why these are for pets. It makes more sense to me that you'd make these either bed sized or pillow sized. Imagine having a perpetually cold pillow. It makes me wonder if these are somehow poisonous or otherwise dangerous for humans.