A constant droning in the background just below audible level that drives you nuts all night.
Or a buzzing in the skin that doesn’t go away, but isn’t nerves or muscle twitches.
Or, in my case, the longer you lay down and try to relax, the more ‘awake’ you are, to the point of feeling like you are fully awake and alert.
Which is why I learned to just get up and start my day. I don’t like being cranky and on edge, so I’ll just stay up as long as I have to.
When I finally do manage to fall asleep, I’ll twitch like I’m falling...and wake up. And twitch. And wake up.
Well, we are the undead after all.