One study linked the length of REM sleep to the body temperature of animals

According to a new study by UCLA Professor Jerome Siegel, groups of warm-blooded animals with higher body temperatures had shorter periods of rapid eye movement (REM) time, while animals with lower body temperatures had more rapid eye movement sleep

The study, published in The Lancet Neurology, shows that there is a previously unobserved relationship between body temperature and REM sleep, a period of highly active sleep in the brain, Siegel said

Birds have the highest body temperature of any warm-blooded animal, reaching 41 degrees, while they have the least amount of REM sleep, at just 0.

Brain temperature drops during non-REM sleep and then rises

Compared to other thermostatic animals, the rembouncular sleep duration in humans is neither high nor low, "which undermines some popular belief that REM sleep plays a role in learning or mood regulation," he said

Siegel's research was funded

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Sleep function: an evolutionary perspective