Why Do We Sleep?
This is the third installment of a five-part series.
Still, the basic premise that sleep provides a survival advantage, keeping animals out of harm's way, is not entirely persuasive. Wouldn't lying quietly in a burrow or cave, with full awareness of the surroundings, provide the same safety—and conserve nearly as much energy? Why did animals evolve to spend so much time unconscious? And why the massive changes in nervous system function that accompany sleep?
The variation in sleep among animals does not necessarily undermine the case that sleep serves an essential function either. Consider again the baby dolphins. Perhaps they evolved in some way that allows them to buck the usual mammalian pattern and stay awake early on. Perhaps at birth their brains are already more developed than those of other animals that spend large amounts of time in REM. Siegel himself has pointed out a trend that seems to offer indirect evidence for this possibility: The more an animal can move around on its own at birth, the less REM it tends to get. Dolphins are able to paddle about early on, and get little or no REM during this time. Platypuses, on the other hand are born blind and defenseless and are REM champions, spending more time in this state than any other animal. But that suggests that dolphins' relative lack of need for baby sleep does not mean slumber is any less critical in the animals that get a lot of it after birth. (And surely sleep must be important for dolphins later on, since they go to the trouble of sleeping with one hemisphere of the brain at a time.)
Human dreams raise even more tantalizing questions. Evolution covered some serious ground between sleeping platypuses, which experience REM only in their brainstems and almost certainly do not dream, and sleeping people, who have REM throughout the cortex and can leap off cliffs or stand naked in front of classrooms in the course of a night. Harvard sleep researcher J. Allan Hobson has pointed out that if our brains did what they do during dreams while awake, we would be diagnosed as mentally ill. Yet we enter into this cousin of delirious insanity every night. "Do we go mad at night to prevent ourselves from doing so in the day time?" Hobson has asked. "Or do we go mad because the brain temporarily gives up certain of its controls in order to regain them, in better order, when sleep ends?" Dreams are only one aspect of REM, of course. But the expansion of REM into the cortex that makes them possible may have aided the development of more advanced mental processing—including, perhaps, memory consolidation. Maybe we got smarter around the same time we developed full-blown dreams. (And perhaps it's no accident that dreams have provoked some of the world's most celebrated aha moments.)
Still, more evidence—especially of the cellular and genetic sort—is needed to flesh all of this out. So I turn next to a neuroscientist who focuses on how sleep may help to promote changes in brain wiring, strengthening some connections and weakening others. Time to peer down at the neurons themselves, through a small hole cut in the cranium.
Amanda Schaffer is a science and medical columnist for Slate.