I stumbled across some YT vids on Resonant Breathing a few days ago. You breathe a certain, consistent number of breaths per minute. (And yes, there's an app for that.) You have to experiment to find what is comfortable for you. I'm currently at 4 breaths a minute. It's awkward at first because the in-breath feels too brief and the out-breath feels too long. But with practice it starts to feel natural and is very relaxing, focusing, and mind-clearing and when you get into the rhythm of it you really don't need the app at all anymore. Of course you use deep diaphragm breathing.

When I wake in the middle of the night and can't get back to sleep it has helped me to clear my head and fall back asleep. It has also helped me focus much more consistently on my breathing during my meditation sessions with less mind-wandering. I don't know much about it and will do more research but it's very interesting and could be perhaps of great benefit to my meditation practice.