REM (rapid eye movement) sleep is one of the five distinct stages of sleep; the others being NREM or non-rapid eye movement sleep. REM sleep is significant because of the various physiological changes it affects on a sleeping individual, and the fact that most vivid dreams occur during REM.
During the REM cycle, brain waves are irregular and brief, with large bursts of ocular activity. The REM period of sleep is where the most visually intense dreams happen. Brain activity resembles that of an awake individual, rather than a sleeping one.
Blood flow during REM changes; blood pressure may greatly increase, as may the pulse. Unfortunately, the pulse might increase in an irregular way, and those with cardiac problems often face the greatest risk of heart attack while sleeping during times of REM. Interestingly, it is suspected that the changes in blood flow that REM induces also explain the tendency of the sexual organs of both men and women to engorge.
Other Bodily Changes
The chin and jaw go slack during REM sleep, and the face, fingers and toes may twitch. The large muscles are paralyzed, so that sleepers cannot move their arms, legs or the torso. The body's temperature regulation weakens and shivering and sweating cease. Breathing often becomes irregular.
Known REM Sleep Disorders
RBD (REM behavior disorder) is a well-known and documented REM sleep disorder in which people under REM sleep are not subject to the normal paralysis of REM sleep. Instead, they can physically move their limbs, and respond not to their environment, but the impulses their sleeping brain is being given -- sleepwalking, essentially.