If you fall asleep during hypnosis, you do lose some of the potential benefits of the session as your subconscious mind does become less receptive to suggestions by the hypnotherapist.
The best state of internal focus is when you are feeling deeply relaxed to the point of heavy drowsiness. It’s like being half awake and half asleep. Being in a deep trance-like state is like daydreaming. In this state you are able to access your greater imagination and your mental imagery is very vivid – and you are more susceptible to suggestion in this state.
Once you are in a deep, dreamless sleep you are unconscious with very slow brainwave activity you are less likely to benefit from the session.