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A lot of the times speedsters have a certain something about them when they sleep that's caused by their powers. For Barry it's sleep (speed)talking, for Wally it's exsessive figiting, but Bart has a particular case.
Bart tends to get very emotional and when he feels extreme emotion his powers react with him and he starts to vibrate uncontrolably. This is normally a problem. But When he's asleep, he feels safe and happy, so much that his powers react but it's normally a small controlled vibration but enough that when he breaths it comes out choppy.
I repeat: BART ALLEN PURRS IN HIS SLEEP
It was noticed, and recorded durring one of the very first core four sleepovers by Tim, our notorius Insomniac, that overheard.
Just when everyone thought Bart couldn't get any cuter, that video 'mysetiously' surfaced on the phone of everone they knew.