A trait is ego-syntonic when the person experiences it as simply "who they are," not a problem — which is why personality disorders like NPD rarely bring someone to therapy on their own. Ego-dystonic traits feel wrong to the person who has them.
You're the one losing sleep, wondering if you're the problem — that distress is ego-dystonic. They sleep fine. From the inside, nothing is wrong with them, which is exactly the difficulty.