What is a common physiological change during REM sleep?

During REM (Rapid Eye Movement) sleep, a common physiological change is an increased heart rate paired with irregular breathing patterns. This stage of sleep is characterized by heightened brain activity, often similar to wakefulness, which corresponds with vivid dreaming. The irregular breathing reflects the brain's high levels of activity and can lead to fluctuations in blood pressure and heart rate, typically causing both to become more variable than during other sleep stages.

As for other processes during REM sleep, muscle tone is significantly reduced to the point of atonia, which prevents the body from acting out dreams, while body temperature regulation may become less effective as the body does not maintain a consistent temperature during this phase. The brain is actually quite active in this state, contrary to the option regarding decreased brain activity.