Somniloquy, commonly referred to as sleep-talking, is a parasomnia in which one speaks aloud while asleep. It can range from simple mumbling sounds to loud shouts or long, frequently inarticulate, speeches. It can occur many times during a sleep cycle and during both NREM and REM sleep stages, though, as with sleepwalking and night terrors, it most commonly occurs during delta-wave NREM sleep or temporary arousals therefrom.[1]
When somniloquy occurs during rapid eye movement sleep, it represents a so-called "motor breakthrough" of dream speech: words spoken in a dream are spoken out loud. Depending on its frequency, this may or may not be considered pathological. All motor functions are disabled during healthy REM sleep and therefore REM somniloquy is usually considered a component of REM behavior disorder.