Seals who sleep with half their brains help explain how humans snooze
REM versus Non-REM sleep
We know there are two types of sleep — REM and non-REM. REM sleep, or rapid eye movement sleep, in humans is associated with dreaming. It is also thought to be the type of sleep we need for learning and mental well-being.
Non-REM sleep, on the other hand, is known as slow wave sleep. In this state, the brain is less active, which saves the brain energy needed for REM sleep.
There is still much to be learned about REM sleep, which occurs near the end of the sleep cycle, specifically to understand its purpose.
Sleeping fur seals
The study looked at sleep patterns in the Northern fur seal, which spends most of its time in water — seven months in water versus five months on land.
It seems they can go great lengths of time — many days or even weeks — without much REM sleep when they're in water. This is surprising because the common thinking is that REM sleep would be required for cognition and learning, which translates into water activities like hunting, predator avoidance, and navigation in fur seals. However, it appears the fur seal does not need REM sleep for any of these functions.
What's happening, according to scientists, is that only half of a fur seal's brain is asleep at any given time when they're in water, similar to dolphins, so that the other side of their brain can stay alert to fend off predators and to make sure the animal doesn't drown.
"The main reason this evolved was to allow them to stay [alert] in the water and not inhale water," says Jerry Siegel, a professor of psychiatry and biobehavioral sciences at University of California Los Angeles. "These are mammals, not fish, and breathing is not a trivial task."
The fact that half of a fur seal's brain is always awake in water means that it's able to keep the brain warm instead of relying on REM sleep, as it does when it sleeps on land.
"It does seem that having unihemispheric sleep makes REM sleep unnecessary," says Siegel.
Is REM sleep important?
Back on land, fur seals flip back to bilateral sleep experienced by land mammals, including alternating periods of non-REM and REM sleep, but only get about 80 minutes of REM sleep per day. What's surprising is they show no ill-effects, or rebound, says Siegel.
He doesn't believe that the fur seals are sleep-deprived in any way, but with this understanding of the link between REM sleep and brain warming, scientists think a very important function of REM sleep is to warm the brain to prepare it for a normal waking temperature, so animals are alert and ready to go fairly quickly.
The fur seals' lack of need for REM sleep make them a little unusual. For example, we need it, and get about 1.5 hours on average a night. The platypus gets about seven hours of REM sleep, and in experiments, rats die if they go too long without it — so there is a mixed bag of REM requirements across mammals.
"[The study] does challenge the idea that REM sleep is essential for life," says Siegel.