This is the second installment of a five-part series.
For a dose of concentrated skepticism, I called Jerry Siegel, a maverick professor of psychiatry at the University of California at Los Angeles. He has been jousting with Stickgold since 2001, when he wrote a bruising takedown in Science of the evidence that memory consolidation depends on REM sleep.
"The literature's all over the place in terms of what stage of sleep and what type of memory" is supposedly affected, says Siegel. "The evidence isn't converging. It's contradictory."
Will sleep turn out to play some role in boosting memory? I ask.
"Maybe, but the evidence isn't there. And I doubt it will be an essential role," he says.
Siegel has turned his attention to early egg-laying mammals, like the platypus and echidna (otherwise known as the spiny anteater). He has probed the relationship between these and other animals' snoozing habits and their ecological niches to theorize about the ancient origins of sleep and its evolutionary progression. Not surprisingly, he ends up telling a different story than the memory camp about why we spend up to a third of our lives in slumber.
Human sleep is divided into stages of non-REM and REM, which alternate in a repeating cycle throughout the night. During non-REM sleep, the brain's activity is highly synchronized, with large groups of neurons firing simultaneously. "Imagine a stadium full of people, all quiet, then all shouting at the top of their lungs together, then quiet, then shouting," one sleep researcher told me. During REM, on the other hand, groups of neurons fire at many different frequencies, and brain activity overall resembles that during waking. Also, blood pressure rises, the body's muscles are paralyzed (except for those that control breathing and darting eyes), and men typically get erections (leading another researcher to quip that these are the antennae for catching dreams).
What original functions might non-REM and REM sleep have served? Researchers have long speculated that non-REM plays a housekeeping role, removing a toxin or replenishing a substance depleted during waking. Siegel and his colleagues observed that sleep-deprived rats show damage to some brain-cell membranes, damage that sleep may serve to prevent or repair. As for REM sleep, researchers once believed that it was a recent evolutionary development—absent, for instance, in early egg-laying mammals like the platypus and echidna. In the late 1990s, however, Siegel and colleagues in Australia demonstrated that these throwback animals do experience a form of REM sleep, although only in their brainstems. Siegel argues that REM's original function may have been to stimulate the brainstem, perhaps to warm the brain physically or to ready it for waking.
Siegel also argues that the sleep habits of different animals evolved to help them adapt to their ecological niches. The way they slept varied according to what dangers they faced, when they needed to hunt for food, what kinds of shelter were available to them. In contrast to the usual evolutionary assumptions, he suggests that sleep is not dangerous or maladaptive at all. Rather, it provides a period of safety for animals—keeping them immobile, tucked away in a cave or a burrow, hidden from predators, and less likely to be injured or get into fights.
Consider that different animals have different time slots during the day for productive foraging. Bats, for instance, come out at dusk, when the flies they eat are abundant. "The rest of the time, bats are better off sleeping," Siegel says. "Their reproductive success is going to be diminished if they are active at any other time of the day." Newborn dolphins, on the other hand, do not appear to sleep for more than a minute at a time and may not sleep at all (in contrast to the heavy slumber of most mammalian babes). Siegel suggests that this pattern emerged because dolphins cannot retreat to a safe spot while they're learning to survive in the open ocean. He also connects how much a species sleeps to how time-intensive its foraging needs are. Carnivores, which eat periodic high-calorie meals, tend to sleep more than herbivores, which must spend more time grazing. (Omnivores like us fall somewhere in between.)
Viewed through this lens, memory consolidation does not look like the key function of sleep. Siegel notes that animals can take care of all manner of physiological business (besides eating and having sex) as they while the hours away in slumber. They can secrete hormones, repair cells, or shore up their immune systems. Humans, for one, may engage in various kinds of emotional processing; sleep may "knit up the raveled sleeve of care," as Shakespeare put it. We and other species may consolidate memories. But for Siegel, the variation—and especially the unusual ability of some marine mammals to do without consolidated sleep for periods of time—calls into question the essential nature of any one of sleep's functions.