Why do some people talk in their sleep?

During deep sleep and REM sleep, the body is normally paralysed to stop us acting out dreams. Sometimes this paralysis is incomplete, and the speech muscles activate briefly, producing mumbles, words or even sentences. It is most common in children and tends to decrease with age. Stress, fever, sleep deprivation and alcohol can increase episodes. Sleep talking is generally harmless and the speaker usually has no memory of it. What is said rarely makes coherent sense and does not reliably reveal inner thoughts, despite the popular belief.