In REM, like in Light sleep, our brainwaves are similar to when we are awake. Our dreams in REM often form stories that we remember in the morning, as opposed to the fleeting images or memories that characterize Light sleep dreams. During REM our bodies are actually paralyzed, meaning that we are completely motionless except for our eyes. If you see a sleeping person staying very still while their eyes move underneath their eyelids, they’re probably in REM.
When we are in REM, our bodies lose thermoregulation, which means that our bodies cannot regulate our temperatures. In effect, our bodies behave like cold-blooded animals, since our bodies change temperature based on how hot or cold our sleep environment is.