system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we need REM? Why could't we just close our eyes and dream?

Also exactly what happens during REM?
gpt
Imagine the brain as a large warehouse.

Whenever memories arrive, they are shipped to the warehouse and stored. However, each memory shipment contains many parts all bundled together such as the feelings you had when you experienced the memory and the sensory data from the parts of your body. There is not enough time to sort through the shipments when we are awake because there are always new memories that are being made!

Sleep is the period when the memory actually gets sorted. However, the warehouse has to go through some changes before the sorting can start. One example is that the warehouse has to disconnect itself from your muscles so you won't be flailing around during your dreams. Your shipments of memories reduced which is why you can often forget what you were dreaming about.

It's during REM sleep where the memories are actually unpacked and sorted out. Some parts are thrown away while others are moved to long term storage. Your dreams occur when you are slightly conscious to see some of the memories moving around.

If we instantly go to REM sleep and start moving memories, we would get what are known as hallucinations. Basically, the act of moving memories may produce dreams that in turn, generate new incorrect memories. This can actually cause havoc to what you perceive as real or imaginary. The other problem is that moving memories sends signals to our body to move around. That would be rather bad if we start flailing around every time we close our eyes.