What characterizes the REM phase of sleep?

The REM (Rapid Eye Movement) phase of sleep is characterized by increased brain activity, which is similar to that of being awake. During this stage, individuals often experience vivid dreams. The irregular heart rate and muscle movement are notable features; the heart rate can vary in this phase, contrasting with the more stable rates seen in other stages of sleep. While the body is mostly in a state of atonia—where most voluntary muscles are inactive—some small muscle twitches or movements can occur. This combination of active brain function and fluctuating physiological responses is why REM sleep is distinct from other sleep stages, which tend to involve deeper relaxation and slower heart rhythms. This distinguishes it from other sleep stages where muscle activity is minimal or absent.