A hallucination is an experience similar to a normal perception but with the difference that sensory stimulus is either absent or too minor to explain the experience satisfactorily. Certain abnormal mental conditions produce hallucinations, as does taking of hallucinogenic drugs. Hallucinations may also result from exhaustion or fever, or may be experienced while falling asleep (hypnogogic) or waking (hypnopompic), and also by individuals under hypnosis. A negative hallucination is lack of perception despite adequate stimulus. Mass hallucination is hallucination shared by the members of a group; it may particularly result from mass hypnosis.