The stuff that dreams aren't made of: why wake-state and dream-state sensory experiences differ.
Cognition June 1, 1993 D Symons
Natural selection has shaped sleep so that vigilance for external threats remains possible. During sleep, visual and movement hallucinations (dreams) do not compromise vigilance because closed eyes and immobility mean those senses provide no accurate external information. In contrast, hallucinations involving kinesthesis, pain, touch, warmth, cold, odor, or sound would interfere with monitoring the environment and have been disfavored by evolution. This vigilance hypothesis predicts specific features of dream content and REM sleep brain activity.