things that will keep you from being able to go back to sleep
Dreaming about spiders all over your bed & bedroom--the kind that are just too large to vacuum up properly. (For some reason, one of them attached itself to a box of crayons, which was somehow small enough to fit down the vacuum cleaner's nozzle but too big for the opening into the bag, and...that's when I woke up, thank goodness.)
(Really, it was more of a very mellow nightmare, I suppose.)
Jack O'Neill would find this sort of bad dream almost relaxing, I suspect.