Answer:
The answer is REM sleep.
Explanation:
The acronym REM stands for Rapid Eye Movement. It usually occurrs after about two hours of sleep.
Dreams take place during this stage, and for this reason, the brain turns off our spinal cord's signals, so that we don't act our dreams out.
Studies have found that a lack of REM sleep (and thus, dreaming) has major negative effects in memory and learning.