The correct answer is: consolidating memory.
Researchers assert that sleep between 7-9 hours each day is very important for learning and memory.
REM sleep (a kind of sleep that is characterized by eye movements and occurs at intervals during the night) plays a crucial part in the consolidation of procedural memory.
Researchers believe that many sleep stages are involved in different types of memories.
The evidence indicates that REM sleep could be implicated in declarative memory processes. This type of memory occurs during earliest sleep and deep sleep. Declarative memory is 'what we know', for example, what you did the previous day.
Researchers also focus on sleep and its crucial role in procedural memory, that is to say, how to do something (for example: play chess)