Healthy sleep is divided into four stage cycles. REM (rapid eye movement) sleep is stage 4 deep sleep.
During this time the brain processes and synthesises memories, emotions and activities that are crucial for learning and higher level thought.
It is important to get REM sleep. A lack of REM sleep results in slower cognitive and social processing, problems with memory, and difficulty concentrating.
So if you want to improve memory, concentration and cognition, it is important to get 7-8 hours sleep a night.