it's true that your mind wanders more, and you can't keep yourself from thinking of the things you try to avoid throughout the day, when you stay up too late.
but once you've stayed up all night, and forgotten to sleep at all, it all sort of goes away, leaving you with the thoughts of things that really matter.
like the warmth of his arms around you, and the sound of his voice when he tells you you're beautiful, and the sincerity you feel when he whispers i love you, and the smile on his face when he looks at you.
and these are the thoughts, with the comfort that they give you, which allow you to slowly start drifting off to sleep.