Hypnosis is a process of achieving an altered state of consciousness. It's similar to daydreaming. With your conscious mind occupied or turned off, the messages from the hypnotist go into your subconscious. The effect is that new attitudes and behaviors can be suggested so deeply that they can become as integrated as if you'd been that way for years. For example:
It becomes natural to reach for the water bottle instead of a cigarette. The idea of buying ice cream now turns your stomach. The extra hour or so is just enough time to get to the gym instead of wasting time on the computer.
The quick answer is one to six sessions for most.
The number of sessions you'll need depends on a variety of factors. Generally, deeper subjects need fewer sessions. Simpler issues can be resolved in fewer sessions. New behaviors that you can practice more frequently require fewer sessions. People under a lot of stress often require more sessions.
For a little better idea, I have included the average number of treatments and their timing in the section "What does it do?" if you click on the issue with which you'd like help.
