I used to struggle to get to sleep and hated those who could do it anywhere. They'd say, "just clear your mind", which wasn't helpful. It could be other things, but I figured out that my mind was always busy, sometimes from stress, sometimes from excitement. For me, it's extreme focus. Often, I'll put my mind to work on a complicated problem I'm having at work or home. If I have nothing, my go to is to see how far I can get calculating the binary digits (1, 2, 4, 8, 16, 32, 64...) or the Fibonacci sequence in my head. I don't get very far and I'm out.