Daniel is deeply disturbed. What he has seen has taken so much energy from him that he falls into a deep sleep. The Lord provides him with rest so that he can consciously take in the explanation of this vision. When he is sufficiently rested so that he is able to grasp the revelation, it is Gabriel who awakens him.
18 And when he had spoken to me, I fell into a deep sleep with my face to the ground. But he touched me and made me stand up.