While amnesia may occur in very rare cases, people generally remember everything that transpired while they were hypnotized. However, hypnosis can have a significant effect on memory. Posthypnotic amnesia can lead an individual to forget certain things that occurred before or during hypnosis. However, this effect is generally limited and temporary.
Ernest Hilgard, who developed the "neodissociation" theory of hypnotism, hypothesized that hypnosis causes the subjects to divide their consciousness voluntarily. One part responds to the hypnotist while the other retains awareness of reality. Hilgard made subjects take an ice water bath. None mentioned the water being cold or feeling pain. Hilgard then asked the subjects to lift their index finger if they felt pain and 70% of the subjects lifted their index finger. This showed that, even though the subjects were listening to the suggestive hypnotist, they still sensed the water's temperature.
Braid made a rough distinction between different stages of hypnosis, which he termed the first and second conscious stage of hypnotism; he later replaced this with a distinction between "sub-hypnotic", "full hypnotic", and "hypnotic coma" stages. Jean-Martin Charcot made a similar distinction between stages which he named somnambulism, lethargy, and catalepsy. However, Ambroise-Auguste Liébeault and Hippolyte Bernheim introduced more complex hypnotic "depth" scales based on a combination of behavioural, physiological, and subjective responses, some of which were due to direct suggestion and some of which were not. In the first few decades of the 20th century, these early clinical "depth" scales were superseded by more sophisticated "hypnotic susceptibility" scales based on experimental research. The most influential were the Davis–Husband and Friedlander–Sarbin scales developed in the 1930s. André Weitzenhoffer and Ernest R. Hilgard developed the Stanford Scale of Hypnotic Susceptibility in 1959, consisting of 12 suggestion test items following a standardised hypnotic eye-fixation induction script, and this has become one of the most widely referenced research tools in the field of hypnosis. Soon after, in 1962, Ronald Shor and Emily Carota Orne developed a similar group scale called the Harvard Group Scale of Hypnotic Susceptibility (HGSHS).
Therefore, Braid defined hypnotism as a state of mental concentration that often leads to a form of progressive relaxation, termed "nervous sleep". Later, in his The Physiology of Fascination (1855), Braid conceded that his original terminology was misleading, and argued that the term "hypnotism" or "nervous sleep" should be reserved for the minority (10%) of subjects who exhibit amnesia, substituting the term "monoideism", meaning concentration upon a single idea, as a description for the more alert state experienced by the others.
Psychiatric nurses in most medical facilities are allowed to administer hypnosis to patients in order to relieve symptoms such as anxiety, arousal, negative behaviours, uncontrollable behaviour, and to improve self-esteem and confidence. This is permitted only when they have been completely trained about their clinical side effects and while under supervision when administering it.
The Federal Dictionary of Occupational Titles describes the job of the hypnotherapist: "Induces hypnotic state in client to increase motivation or alter behavior patterns: Consults with client to determine nature of problem. Prepares client to enter hypnotic state by explaining how hypnosis works and what client will experience. Tests subject to determine degree of physical and emotional suggestibility. Induces hypnotic state in client, using individualized methods and techniques of hypnosis based on interpretation of test results and analysis of client's problem. May train client in self-hypnosis conditioning. GOE: 10.02.02 STRENGTH: S GED: R4 M3 L4 SVP: 7 DLU: 77"