Human sleep consists of different stages which can be distinguished by inspecting the recordings of electrical activity from sensors placed on someone's scalp, a method called electroencephalography. While dreams can occur during all sleep stages, we are dreaming most vividly in a sleep stage known as "REM sleep" because of the occurrence of Rapid Eye Movements. During REM sleep, all body muscles (with the exception of the eye muscles, obviously) are paralyzed by neural structures in the brain stem, which prevent us from acting out our dreams. Accordingly, complex movements like sleepwalking normally occur only during non-REM sleep stages. In such cases, a small part of the brain appears to be awake while the rest of the brain is asleep. This seems to be true for sleep talking as well: producing speech requires the planning and execution of rapid sequences of muscle movements, hence it will most likely occur in non-paralyzed, non-REM sleep stages.

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