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Autumn 2006 | Volume 29, Number 4 | Features

Why Do We Sleep?

The answer may surprise you

Sleep is an activity that occupies a whopping one-third of our time on the planet. That is why it is so extraordinary to contemplate that until about 10 years ago, absolutely no one had any idea why we needed to sleep. Even now, while there still is not complete agreement about sleep’s role in the human experience, there is growing evidence that the reason we need to sleep is not just so that the body can physically recuperate from the day’s activity, but so that the brain can learn.

The Incredible Sleeping Brain

One of the first things you have to overcome if you ever get a chance to listen in on a living brain while it is slumbering is disbelief. The reason is that the brain does not appear to be asleep at all. Instead, the organ is almost unbelievably active during “rest,” with legions of neurons crackling electrical commands to each other in constantly shifting patterns, actually displaying greater rhythmical activity during sleep than when the brain is wide awake.

The reason for this activity has to do with the tug-of-war nature of the sleep cycle itself. Your sleep/wake tendencies can be thought of as the byproduct of a constant battle between two opposing drives in your body. One of these drives, the Circadian Arousal System, is trying to keep you awake and alert all the time. The other, the Homeostatic Sleep Drive, is trying to make you drowsy so that you stay unconscious all the time. These two drives are locked in a daily warfare with a cycle of victory and surrender so predictable you can actually graph it.

An interesting consequence of this sleep/wake conflict occurs in the early afternoon and is sometimes called the “nap zone.” Usually experienced sometime between 2 and 4 p.m., it is a form of extreme, though temporary, drowsiness. It can be nearly impossible to get anything complex done at this time; if you choose to attempt to push through (which is what most of us try), you can spend much of your afternoon fighting a gnawing tiredness.

The whole concept of “siesta” — a phenomenon institutionalized in many other cultures — likely came as an explicit reaction to the nap zone. Most scientists believe that a long sleep in the night and a short nap during the midday represents human sleep behavior in its most naturally genetic, highest quality form.

Though sleep cycles are burned deep into the genetic fabric of the human brain, they are subject to regulatory forces, some of which are also genetic in origin. For instance, approximately one in 10 humans is what is formally designated in the scientific literature as an “early chronotype” or “lark.” Larks report being most alert around noon and feel most productive a few hours before they eat lunch. They often wake at 6 a.m., get drowsy in the early evening, and go to bed (or feel like they want to go to bed) around 9 p.m.

About two in 10 people exist at the other end of the sleep spectrum. They are called “late chronotypes” or “owls.” In general, owls report being most alert around 6 p.m., rarely want to go to bed before 3 a.m., and rarely want to wake before 10 a.m. The behaviors of larks and owls are very specific and potentially genetic. The rest of us have chronotypic behavior that falls somewhere between the patterns of larks and owls.

Sleep and Learning

So what, then, do sleep and its regulatory functions have to do with learning? Although the earliest data establishing a link between sleep and learning focused on the effects of sleep deprivation, the opposite is now abundantly clear: Healthy sleep can produce a significant learning boost in certain tasks.

One study stands out in particular. The experiment involved giving students a series of math problems and providing them with a method to solve the problems. Unbeknownst to these students, there was also an easier, “shortcut” way to solve the same problems, which potentially could be discovered while doing the exercise.

During the initial training, however, students never discovered the easier method. So the question for researchers was: “Is there any way to jumpstart, even speed up, their insight?”

The answer is “yes,” but only if you allow them to sleep on it. When you let 12 hours pass after the initial training, and then ask the students to do more problems, approximately 20 percent actually discover the shortcut. But, if in that 12 hours you also allow eight or so hours of regular sleep, the number of students discovering the shortcut triples to approximately 60 percent. No matter how many times the experiment is run, the sleep group consistently outperforms the non-sleep group to the tune of approximately three to one.

This is just one example of the phenomenon. Sleep enhancement has been shown for tasks that involve visual texture discrimination, motor adaptations testing, motor sequencing, and other cognitive skills. The type of learning most sensitive to sleep improvement is a category called procedural learning (not surprisingly, this involves learning a procedure). The effect can also be shown in the negative. Simply disrupt the night’s sleep at specific stages and then retest in the morning. No overnight learning improvement will be observed.

Even the simple act of taking a nap can show cognitive benefit. One study conducted by researchers at NASA showed that a 26-minute nap improved a pilot’s performance by more than 34 percent. Another study showed that a 45-minute nap produced a similar boost in cognitive performance, a boost that lasted more than six hours.

These data and others like them represent a consistent and increasingly well-characterized finding: Sleep is intimately involved in learning. It is this consistent finding that caused some researchers, now almost 10 years ago, to ask a deeper question about rats and mazes, specifically having to do with leaving a bunch of electrodes deliberately stuck inside some rats’ brains.

Rats, Mazes, and a Revolutionary Discovery

It is possible these days to put electrodes inside a living brain and listen in on the neural chatter while it is thinking. Even in a tiny rat’s brain, it is not unusual to listen to up to 500 different neurons at once. And just what are they saying? If you listen in while the rat is acquiring new information, for instance while it navigates a new maze, a very discrete “maze-specific” pattern of electrical stimulation begins to emerge. Working something like the old Morse Code, these neurons begin to crackle in a specifically timed sequence. Once acquired, the rat will always fire off that pattern when negotiating the maze.

Interestingly, if the electrodes stay in place when the rat goes to sleep, something very mysterious can be observed. The rat begins to replay the maze pattern sequence. Always executed in a very specific stage of sleep, the rat repeats the pattern over and over again during the night. If the rat is awakened during that stage, the rat has trouble remembering how to navigate the maze the next day. The rat is consolidating the day’s learning the night after that learning occurred. An interruption of that sleep disrupts this learning cycle.

Does something like that happen in humans? You bet it does, depending upon the type of learning being measured. Humans appear to replay certain types of daily learning experiences at night. Some of the best research has been done with spatial memory tasks, but it has also been shown for other types of learning as well. Interestingly, the replay of the day’s learning is done in a highly compressed format, and even appears to be happening in the same stage of sleep as the rats’.

These findings represent a bombshell of an idea. The data seem to suggest that some kind of offline processing is occurring at night. And that begins to address the question posed in the title of this article: Why do we sleep? Is it possible that the reason why we need to sleep is simply to shut off the exterior world for a while, allowing us to divert more attentional resources to this processing? The answer to that question may be “yes” and represents the first real clue as to why sleep is so important. We may need to sleep so that we can learn.

Lessons for the Real World

As is true with any good scientific discourse, these data have been met with some controversy. Increasingly, however, it is becoming clear that one of the functions of sleep involves the need for our really big brains to review what they have learned during the day. This isn’t a trivial need and is demonstrable in the negative. The effects of sleep deprivation are thought to cost U.S. businesses more than $100 billion a year.

What if schools and businesses took the sleep needs of their students and employees seriously? Here are three possibilities:

CHRONOTYPES. What if we began to match chronotypes to educational experiences? Teachers are just as likely to be late chronotypes as their students. Why not put them together at the time of the day when their teaching/learning would be maximized? A similar case could be made in the work world. What if we began to match chronotypes to work schedules? Given that 20 percent of the workforce is already at sub-optimal productivity in the current 8–5 model, what if that model were permanently broken up?

NAPS. Given the data about the powerful effects of short bursts of sleep on human cognition, is it time to take the nap zone seriously in schools and offices? Do you recall the NASA research? “What other management strategy will improve people’s performance 34 percent in just 26 minutes?” exclaims Mark Rosekind, the NASA scientist who conducted the research.

SLEEPING ON IT. Lastly, if we took the data about sleep and the ability of a good night’s rest to upgrade the insight rate, schools and businesses might tackle the most intractable issues at miniretreats. After being presented with the problem, students or employees would not start coming to conclusions, or even begin sharing ideas, before they had had an intervening eight hours of sleep. Would they experience the same increase in problem-solving rates demonstrated in a laboratory setting?

Notice that these ideas are posed as questions, not as prescriptions. This is done deliberately, because I have no idea whether anything I just mentioned here would work. Beyond the laboratory, there is very little research that tells us how we might harness the wild bucking bronco of the human sleep schedule. Thus, we don’t know if creating a chronotypically conscious learning environment or workplace would boost test scores or productivity. We don’t know if creating a school or workspace sensitive to the nap zone would give a lift to learning and productivity. We have no idea if, in a real-world setting, letting a group “sleep on it” would actually do anything more than waste time.

Why don’t we know? Because, as I say so often, brain scientists, educators, and business professionals haven’t gotten together to do such research. And that was exactly why The Brain Center for Applied Learning Research was created at Seattle Pacific University. In the meantime, we are left with a simple brain rule — that sleep is important to the learning process — and an opportunity to find out how that might apply to the real world of education.

—By John Medina Director of the Seattle Pacific university Brain Center for Applied Learning Research
illustrations By John lavin

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Copyright © 2006 Seattle Pacific University. General Information: 206-281-2000