In addition to a lot of great advice here, I'll add the Physiological Sigh. It is a double breath followed by a long exhale.

Huberman discusses (and demonstrates) it here in the context of reducing stress and anxiety.

https://www.youtube.com/watch?v=kSZKIupBUuc

I first heard about it on The Model Health Show where it was mentioned as a natural mechanism for helping one to fall asleep. I can personally attest that it helps.