system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Can anything capable of sleeping also have the ability to dream?

My dog sometimes moves his legs like he is running in his sleep and will make noises. So I assume that's dreaming, but does this mean any animal that sleeps can also dream? Or is there a certain intellectual aspect/capacity that must be present in order for the process to occur?
gpt
Tough to answer because what does 'sleep' mean? Is a bear hibernating sleeping? The late-fall spider that pretty much shuts down on cold near-zero nights, what is its brain doing and is that 'sleep'? Then you get into weird cases like dolphins that would drown if they went fully asleep so they only sleep with one-half of their brain at a time so they will always be conscious.

We have evidence from both our own eyes and from scientific measurements using brainwave monitors that higher-order animals like dogs and primates dream. That's well-known. But as you go down the intelligence tree, at some point you reach instinct-driven animals that either don't have enough brainpower to need to quiet-time mental recharge/archiving processes like us mammals do when dreaming, or they just don't do it at all (the platypus is an example, although Perry might have nightmares about Doofenschmirtz from time to time). They just kind of shut down during their rest periods instead, with the cut-off point at least partly determined by intelligence levels.