During REM sleep the vast majority of us experience muscle atonia, which is the paralysis of our muscle tone. Since REM sleep is usually accompanied by vivid dreams in which we move around in a dream world, it is likely that this muscle atonia exists to stop us from moving around and acting out our dreams. However, for some people, muscle atonia during REM sleep is lost. When this occurs, the individual is experiencing REM sleep behaviour disorder (RBD). They act out their dreams, which are often aggressive and violent in nature, which is clearly a big problem for people who share a bed with their partner. People with RBD often go on to develop a neurodegenerative disorder, so it is important to have RBD checked out as it can be an early marker for a more serious problem. It should not be confused with sleep-walking or somnambulism, which is non-REM sleep disorder.