Lots of great advice here already, the only thing I’d add is 4-7-8 breathing. There are lots of guides and explanations online, but essentially you breath in for 4 seconds, hold your breath for 7 seconds, then exhale for 8 seconds.

It takes practice before you can do it and stay relaxed at the same time, so I would suggest practicing during the daytime so when you do it at night it doesn’t fully wake you up. It took about a week of practice before I felt comfortable enough to feel the benefit, but I still use 4-7-8 breathing in lots of contexts four years later.

I think the theory behind it is tied to vagus nerve and helping the body relax and enter rest and digest states.