One trick that makes me sleepy really fast: After I close my eyes, I imagine someone throwing black paint on them. The first coat is kinda gray and has lots of blob and not fully black. Then another coat. And another. Each one gets darker until it's just pure black and I'm usually asleep by then.
For some reason, my brain follows it, and I fall asleep much faster. It works way better for me than box breathing or most other sleep tricks I've seen. Sharing in case someone else finds this useful.