Dreams are associated with a state called REM sleep, the abbreviation standing for rapid eye movement. The REM state is strongly correlated with sexual arousal. Experiments have been performed in which sleeping subjects are awakened whenever REM state emerges, while members of a control group are awakened just as often each night but not when they're dreaming. After some days, the control group is a little groggy, but the experimental group - the ones who are prevented from dreaming - is hallucinating in daytime. It's not that a few people with a particular abnormality can be made to hallucinate in this way; anyone is capable of hallucinations.

— Carl Sagan

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