Hypnosis sleep is a interesting state. Its very suprising to people that experience hypnotherapy for the first time, that they actually dont fall asleep.
People always say at the end of the first hypnotherapy session "im not sure if i was hypnotised, i heard everything".
Thats because hypnosis sleep is not actually sleep. It certanly looks like deep sleep to an observer, rather its a extremely relaxed state where the mind is very much engaged and involved in to what is going on. Its a state of hightened focus, imagination and sugestibility that allows the hypnotherapist to comunicate with your subconcious mind.
How useful would it be to actually have people fall asleep while in hypnotherapy? Not usefull at all as the person in sleep, especially deep sleep, won't be responsive and retain the helpful suggestions a hypnotherapy is designed to provide.
Hypnotherapy sleep generally operates in the state that resembles the REM State, where the brain paterns and activity is very similar to the one of a person thats awake, but the person experiencing this sleep state is cleary unresponsive and can appear to look like they are in a deep sleep.
After a hyonotherapy session a client will feel extremely rested, will retain memory of all and everything they hear during hypnotherapy. To help relate hypnosis sleep to something similar that a person who has not experienced a hypnotherapy session might be able to relate to, imagine a state of extremely deep, really deep meditation or perhaps a state of vivid daydreaming.
So in your first hypnotherapy session don't be suprised to realise you will hear everything and be in controll at all times.