What can I do to put myself back to sleep when I wake up at 4:00 a.m.?

When I’m feeling a lot of stress, I get insomnia. I am able to fall asleep quite easily, but I end up waking up like two hours before I need to which isn’t a reasonable hour to wake up since I’ve only slept like 5 or 6 hours, and I’m a person that needs at least 8 to function.

My question is this: What are some techniques I can use to fall back to sleep when my mind is running around in my head. I try to distract myself and not think of the thing that is causing me stress, but that hasn’t worked.