https://phys.org/news/2018-06-fur-insight-function-rem.html

"We show that when the fur seal stays in seawater, where it spends most of its life, it goes without REM sleep for days or weeks," Siegel says. "After this nearly complete elimination of REM sleep, it displays minimal or no REM sleep 'rebound' upon returning to baseline conditions. It just resumes its 'on-land' bilateral non-REM and REM sleep pattern."

The findings are consistent with the hypothesis that REM sleep, which has been shown to warm the brain, functions to reverse the reduced metabolism and brain cooling that occurs in bilateral non-REM sleep. Siegel says that this warming of the brain can be seen as preparation for waking, noting that humans and other animals are much more alert when they awaken from REM sleep.