Chronic bad sleeper all my life for various reasons, (I'm in my 60s), 3.30am and I are close friends.
I came across this recently, and what do you know, it's really helpful!
It doesn't always work, but it does more often than not. For me, that's really saying something.
Lie down to go to sleep and then do the following, with your eyes closed.
Look up. Look down.
Look to the left. Look to the right.
Circle your eyes in one direction. Circle your eyes in the other direction.
Repeat.
Often it takes no more than three times, one second you're thinking, the next you're asleep.
Persevere if it doesn't work the first time you try it. Despite being sceptical, I've found it a very useful weapon in my armoury. Something to do with the vagus nerve apparently.
If it doesn't work after a few repetitions, stop and do something else for a while.