When you fall asleep, brain waves begin to decrease from 14 Hz in beta waves and reach alpha waves (between 7 and 14 Hz) and then theta (between 4 and 7 Hz) and delta (between 0 and 4 Hz).
Most of your sleep is spent in alpha waves. Hypnosis brings brain activity closer to alpha waves (without falling asleep). In the field of alpha waves, the unconscious mind is opened and ready to receive suggestions.