When sleep researchers have studied humans and other species, they found that only the higher mammals, such as humans, great apes, and bottlenose dolphins experience REM sleep.

Question

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Many mammals, not just higher mammals, experience REM sleep. This includes not only humans, great apes, and bottlenose dolphins but also other species such as rodents and even birds, indicating that REM sleep is more widespread across the animal kingdom.

When Sleep Researchers Have Studied Humans and Other Species, They