Dream
When he does manage to sleep, Danny's dreams are rarely pleasant.
It's not like he never has good dreams. But the thing is, those are the dreams that he tends to forget easily. He'll wake up well-rested and know he either didn't dream or had a good dream, but he'll never be able to recall specifics.
The nightmares, though? They haunt him every night. Some are recurring – Phantom being ripped from Danny, Phantom and Plasmius fusing, Dan killing his family – and others are different from each other by just small little details – the many ways Vlad might manage to kill his dad, what might happen to any of his friends or his town if he isn't careful enough or strong enough or fast enough-
But he never tells anyone. For one, it would just make his friends and family upset. But for another thing, talking about them makes them feel more – more real.
And to be honest, Danny's lived through enough nightmares already. He doesn't need any more coming true.