Not every night, but recently my son has called us up in the evening after bedtime because he's thinking about things he doesn't like. Being chased by a dinosaur, for example.
We have a little chat about how he's feeling, sometimes look at a book as a distraction and have a cuddle and he does manage to go to sleep after a bit of reassurance. This happens before he's drifted off, he's not waking up in the night so it's awake thoughts rather than dreams.
I try not to tell him not to worry because personally I don't find that helpful, I say sometimes I feel the same and it's difficult to know what to do, so I'm sympathetic but not really offering a solution.
Just wondered if anyone had experienced similar and had any good tips on how to deal with this. He doesn't seem anxious or worried at any other time.