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Zalameda t1_j9g4kej wrote

Every characteristics and features of every lifeform are kept because somehow, at some point, it gave the species members who developed it advantages over the members without such characteristics and features, thus these members are more able to survive, reproduce and pass on it's advantageous coding to further generations. This is evolutionary pressure and natural selection.
Applying this to dreaming, we can hypothesize that dreaming emerged from a variation in the way the brain was hard-wired by the genes that dictate it's development and features throughout the species's member's lifespan; and that dreaming reinforces vital synaptic connections that would go into atrophy in members who were uncapable of dreaming, thus making dreamers brain healthier.
While most dreams make absolutely no sense, it still brings elements of waking experiences, as no one can ever dream of something not experienced in waking state of consciousness. As a vague example, you can't dream of someone with a face you have never seen. If you dream of an unseen face, it will at least be a combination of facial features of people you have seen.

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Mortal-Region t1_j9ggdp9 wrote

In any system that continuously accumulates new memories, there's the problem of how to balance newer memories (e.g., "There's a bear in the cave") with older ones (e.g., "Bears are faster than people"). If nothing is done about this problem, then newer memories will simply crowd out older ones, leading to catastrophic forgetting. You'll know that there's a bear in the cave, but not that bears are faster than people.

Nature's solution is to periodically take the system offline in order to perform a memory integration procedure. At night, recent memories are replayed; they are uploaded from the hippocampus (literally up) and "stirred" into the cortex, where longer-term memories and general knowledge-of-the-world are stored.

This replay procedure is performed during both REM sleep (when dreams occur) and non-REM sleep. It's thought that dreams occur during REM sleep because this is when the replay procedure is performed on the kinds of memories that pertain to immediate conscious awareness. (Non-REM sleep seems to deal more with motor memories.)

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