The highly active REM sleep stage has a number of possible roles. One of the main ones involves memory. One persistent theory is that during REM sleep the brain is reinforcing and organising and maintaining our memories. Old memories are connected to new memories; new memories are activated to help reinforce them and make them more accessible; very old memories are stimulated to make sure the connections to them aren’t lost entirely, and so on. This process takes place during sleep, possibly because there is no external information coming in to the brain to confuse or complicate matters. You
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