Sensory deprivation is the intentional removal of stimuli affecting one or all of the five human senses. Frequently used in alternative medicine as a form of relaxation and meditation, sensory deprivation has also been used as a form of interrogation and torture. Sensory deprivation and its effects have been studied and debated by numerous scientists, but no medical or scientific benefits have yet been confirmed. In its simplest form, sensory deprivation may be merely tying a blindfold over someone’s eyes, rendering the sense of sight useless. However, as a form of relaxation, meditation, or even prayer, sensory deprivation usually occurs in a sensory deprivation tank or isolation tank.

The isolation tank, or Restricted Environmental Stimuli Therapy (REST) Tank, was invented by John Lilly in 1954 as a way to test the effects of sensory deprivation. Inside an isolation tank, a person floats in salt water that is the same temperature as the skin in order to deprive the skin of the feeling of hot or cold. The tank is usually without light, reducing the sense of sight, and is often soundproof as well. The sense of smell is frequently reduced in sensory deprivation tanks by eliminating the use of chemicals with odors, like chlorine, to treat the water.

Similar type rooms are also used in sensory deprivation for meditation and alternative healing. A sensory deprivation room may eliminate sight, sound, and smell. The period of sensory deprivation in a tank or room may last up to an hour in a typical session.

Many scientists debate whether sensory deprivation can relax a person to the point of achieving the same results as hypnosis. However, it is believed that long periods of sensory deprivation can result in depression, hallucinations, and severe anxiety. These effects are what constitute sensory deprivation as a form of torture, though not one condoned by any civilized government.