When they find the cave to sleep in, what keep Bilbo from falling asleep completely? *

From the text, we can infer that nervousness kept him awake.... and when he was able to fall asleep, he had bad dreams.

It turned out a good thing that night that they had brought little Bilbo with them, after all. For somehow, he could not go to sleep for a long while; and when he did sleep, he had very nasty dreams. He dreamed that a crack in the wall at the back of the cave got bigger and bigger, and opened wider and wider, and he was very afraid but could not call out or do anything but lie and look. Then he dreamed that the floor of the cave was giving way, and he was slipping-beginning to fall down, down, goodness knows where to.