need only four or five hours of sleep. They are referred to as suffering from “healthy insomnia.” Essentially, it comes down to whatever amount of sleep is right for you. When robbed of that, bad things really do happen to your brain.
Sleep loss takes a toll on the body, too—on functions that do not at first blush seem associated with sleep. When people become sleep deprived, for example, their body’s ability to utilize the food they are consuming falls by about one-third. The ability to make insulin and to extract energy from the brain’s favorite source, glucose, begins to fail miserably. At the same time, you find a marked need to have more of it, because the body’s stress hormone levels begin to rise in an increasingly deregulated fashion. If you keep up the behavior, you appear to accelerate parts of the aging process. For example, if healthy 30-year-olds are sleep deprived for six days (averaging, in this study, about four hours of sleep per night), parts of their body chemistry soon revert to that of a 60-year-old. And if they are allowed to recover, it will take them almost a week to get back to their 30-year-old systems.
Taken together, these studies show that sleep loss cripples thinking in just about every way you can measure thinking. Sleep loss hurts attention, executive function, working memory, mood, quantitative skills, logical reasoning ability, general math knowledge. Eventually, sleep loss affects manual dexterity, including fine motor control, and even gross motor movements, such as the ability to walk on a treadmill.
So what can a good night’s sleep do for us?
Sleep on it: benefits of a solid night’s rest
Dimitri Ivanovich Mendeleyev was your archetypal brilliant-but-mad-looking scientist. Hairy and opinionated, Mendeleyev possessed the lurking countenance of a Rasputin, the haunting eyes of Peter the Great, and the moral flexibility of both. He once threatened to commit suicide if a young lady didn’t marry him. She consented, which was quite illegal, because unbeknownst to the poor girl, Mendeleyev was already married. This trespass kept him out of the Russian Academy of Sciences for some time, which in hindsight may have been a bit rash, as Mendeleyev single-handedly systematized the entire science of chemistry. His Periodic Table of the Elements—a way of organizing every atom that had so far been discovered—was so prescient, it allowed room for all of the elements yet to be found and even predicted some of their properties.
But what’s most extraordinary is this: Mendeleyev says he came up with the idea in his sleep. Contemplating the nature of the universe while playing solitaire one evening, he nodded off. When he awoke, he knew how all of the atoms in the universe were organized, and he promptly created his famous table. Interestingly, he organized the atoms in repeating groups of seven, just the way you play solitaire.
Mendeleyev is hardly the only scientist who has reported feelings of inspiration after having slept. Is there something to the notion of “Let’s sleep on it”? Mountains of data say there is. A healthy night’s sleep can indeed boost learning significantly. Sleep scientists debate how we should define learning, and what exactly is improvement. But there are many examples of the phenomenon. One study stands out in particular.
Students were given a series of math problems and prepped with a method to solve them. The students weren’t told there was also an easier “shortcut” way to solve the problems, potentially discoverable while doing the exercise. The question was: Is there any way to jump-start, even speed up, the insight into the shortcut? The answer was yes, if you allow them to sleep on it. If you let 12 hours pass after the initial training and ask the students to do more problems, about 20 percent will have discovered the shortcut. But, if in that 12 hours you also allow eight or so hours of regular sleep, that figure triples to about 60 percent. No matter how many times the experiment is run, the sleep group consistently outperforms the non-sleep group about three to one.
Sleep also has been shown to enhance tasks that involve visual texture discrimination (the ability to pick out an object from an ocean of similar-looking objects), motor adaptations (improving movement skills), and motor sequence learning. The type of learning that appears to be most sensitive to sleep improvement is that which involves learning a procedure. Simply disrupt the night’s sleep at specific stages and retest in the morning, and you eliminate any overnight learning improvement. Clearly, for specific types of intellectual skill, sleep can be a great friend to learning.
Why we sleep
Consider the following true story of a successfully married, incredibly detail-oriented accountant. Even though dead asleep, he regularly gives financial reports to his wife all night long. Many of these reports come from the day’s activities. (Incidentally, if his wife wakes him up—which is often, because his financial broadcasts are loud—the accountant becomes amorous and wants to have sex.) Are we all organizing our previous experiences while we sleep? Could this not only explain all of the other data we have been discussing, but also provide the reason why we sleep?
To answer these questions, we turn to a group of researchers who left a bunch of wires stuck inside a rat’s brain—electrodes placed near individual neurons. The rat had just learned to negotiate a maze when it decided to take a nap. The wires were attached to a recording device, which happened to still be on. The device allows scientists to eavesdrop on the brain while it is talking to itself, something like an NSA phone tap. Even in a tiny rat’s brain, it is not unusual these days to listen in on the chattering of up to 500 neurons at once as they process information. So what are they all saying?
If you listen in while the rat is acquiring new information, like learning to navigate a maze, you soon will detect something extraordinary. A very discrete “maze-specific” pattern of electrical stimulation begins to emerge. Working something like the old Morse code, a series of neurons begin to crackle in a specifically timed sequence while the mouse is learning. Afterward, the rat will always fire off that same pattern whenever it travels through the maze. It appears to be an electrical representation of the rat’s new maze-navigating thought patterns (at least, as many as 500 electrodes can detect).
When the rat goes to sleep, its brain begins to replay the maze-pattern sequence. Reminiscent of our accountant, the animal’s brain repeats what it learned that day. Always executing the pattern in a specific stage of sleep, the rat repeats it over and over again—and much faster than during the day. The rate is so furious, the sequence is replayed thousands of times. If a mean graduate student decides to wake up the rat during this stage, called slow-wave sleep, something equally extraordinary is observed. The rat has trouble remembering the maze the next day. Quite literally, the rat seems to be consolidating the day’s learning the night after that learning occurred, and an interruption of that sleep disrupts the learning cycle.
This naturally caused researchers to ask whether the same was true for humans. The answer? Not only do we do such processing, but we do it in a more complex fashion. Like the rat, humans appear to replay certain learning experiences at night, during the slow-wave phase. Unlike the rat, more emotionally charged memories appear to replay at a different stage in the sleep cycle.
These findings represent a bombshell of an idea: Some kind of offline processing is occurring at night. Is it possible that the reason we need to sleep is simply to shut off the exterior world for a while, allowing us to divert more attention to our cognitive interiors? Is it possible that the reason we need to sleep is so that we can learn?
It sounds compelling, but of course the real world of research is much messier. Some findings appear to complicate, if not fully contradict, the idea of offline processing. For example, brain-damaged individuals who lack the ability to sleep in the slow-wave phase nonetheless have normal, even improved, memory. So do individuals whose REM sleep is suppressed by antidepressant medications. Exactly how to reconcile these data with the previous findings is a subject of intense scientific debate. Newer findings in mice suggest that the brain uses the time to clean house, sweeping away the toxic molecules that are a byproduct of the brain doing its thinking. With more time and more research, we’ll gain a greater understanding of what the brain is doing as we sleep—and why.
For now, a consistent concept emerges: Sleep is intimately involved in learning. It is observable with large amounts of sleep; it is observable with small amounts of sleep; it is observable all the time. It is time we did a better job of observing its importance in our