The fourth stage in the sleep cycle is the Rapid Eye Movement Stage. In this phase, your eyeballs scurry in different directions under the closed eyelids. The eye movements may or may not be due to intense dreaming. It cannot be clearly stated yet. This is the stage where the brain is most active, and the muscles of the body become more immobilized. REM is the stage when dreaming happens most often and with the most intensity.
Heart rates increase, and breathing becomes faster, but the muscles of the body are most relaxed. This is really important as otherwise, our body would react as per our dreams, and that can cause many accidents. In the first cycle, the REM sleep is short, but it gets longer in the later cycles and can extend up to one hour. Hence there are more chances for you to remember the dreams that you see in the latter part of the night. REM sleep phases are highest in childhood and are extremely important for the developmental functions of the brain.