Brain waves are small and irregular, with big bursts of eye activity. The brain wave activity at this time resembles waking more than it does sleeping.

The four NREM phases are characterized by progressive relaxation. But during REM phase, the body's activity perks up considerably.

Blood pressure may increase drastically.

Pulse rates increase in an irregular way

The sleeper with cardiac problems faces the greatest risk of heart attack at this time.

Breathing becomes irregular and oxygen consumption increases.

The chin is slack during REM sleep

The face, toes and fingers may twitch.

A man experiences penile erections; a woman experiences clitoral engorgement.

The sleepers' large muscles are literally paralyzed. They cannot move their torsos, arms, or legs.

The body seems to have abandoned its effort to regulate its temperature during the REM phase.

Shivering and sweating cease at this time, and the body's temperature drifts gradually toward the temperature of its environment.