Why Do We Need Sleep?

The Answer Continues to Give Science the Slip

In an age when scientists can split the atom, sequence the human genome, and send space probes to the edge of the solar system, we get used to having solid answers to every question of science. We take it for granted that all the great mysteries of nature have been, or are about to be neatly put to rest.

So when we ask a question as basic as, “What is the purpose of sleep?” it’s a bit of a surprise to hear the answer come back: Nobody really knows for sure…

Remember, you heard it here first!

What we know about sleep on a personal level is that we feel really good when we get it, and really crappy when we don’t. We intuitively understand sleep as rest and recuperation for the body and mind. But astoundingly, scientists cannot say for sure that this is actually what sleep is for. There could be more to it than that, less to it than that, or nothing to it at all.

The problem is made considerably more complicated by the nature of sleep itself. We tend to think of sleep as one monolithic thing, a single continuous state we slip into when we leave wakefulness. But there are actually several different stages of sleep, each with its own very unique characteristics. (See How Sleep Works: A Guided Tour.)

Sleep is categorized into two fundamental states called REM and Non-REM — and they are as different from each other as night and day. REM seems to have evolved independently of Non-REM and may have a purpose all its own. Non-REM sleep is further divided into three stages. And each of these stages seems to be doing different things in different ways. It’s quite possible we may need more than one magic-bullet theory to explain sleep.

And here’s a wild thought: Could it be that there is, in fact, no purpose behind sleep? Is it just a cosmic accident that we have it? A biological feature that incorporated itself into us eons ago in a wild roll of the evolutionary dice, without serving any real function? Is sleep there just because it’s there?

Not likely. The animal kingdom is pretty well populated with confirmed sleepers of all kinds; animals that spend large portions of their time — in many cases more than half of it — in the grips of sleep, immobilized and oblivious to their surroundings. This represents a huge investment of time in an apparently unproductive activity, one that often leaves the animal more vulnerable to predators. The principle of natural selection implies that there must be significant advantages in doing this to outweigh the obvious disadvantages. Otherwise, the sleepers would have long ago been out-hustled, out-bred, and ultimately replaced by animals that could go without sleep.

So what in the world does sleep do? Why and how did it first evolve in animals? And why does it continue to be such an indispensable part of our lives today? Let’s run through a few theories…

Conserving energy

Out in the wild, life runs pretty close to the edge. Animals must scramble to find food while doing everything they can to conserve energy. And if caloric intake doesn’t equal or exceed energy expenditure, you’re toast. This is especially critical for warm-blooded animals that must burn large amounts of calories just to maintain a constant body temperature. Cold weather makes things that much more difficult. The smallest animals, such as rodents, have the most trouble here, since the smaller you are, the more surface exposure you have as a proportion to your body mass, and so you lose body heat at a faster rate.

A great way to conserve energy would be to go into a sort of standby mode during certain times of the day (especially at night when the temperature drops off), slowing down metabolism and receding into a state of inactivity. In other words, sleep. Even better if the animal can do this in some sort of shelter or burrow where it can keep itself warm and secure from predators. This also forces the animal to expend its energy more wisely, confining physical activity to the hours that are most profitable for food gathering. The evidence gives some support to this theory: As we look at the sleep patterns of various animals, we find that, in fact, the ones that sleep the most tend to be smaller and live in cold climates.

But why can’t animals achieve this merely by maintaining periods of quiet wakefulness; why do they actually have to go to sleep? It’s been pointed out that the energy savings from full-blown sleep, as opposed to just resting quietly, is only about 5 to 10 percent. But even this marginal reduction just might have been enough to win the favor of natural selection. In the cut-throat environment of the wilderness, 5 to 10 percent could easily make the difference between success and failure.

Keeping out of trouble

Another theory argues that sleep is not so much a biological function as a defense mechanism. A way of forcing the animal to stay quiet and out of sight when that is the best strategy for avoiding danger. For most animals this means nighttime, when wandering around in the dark looking for food would be a Bad Idea, or at least very unproductive. Sometimes the smartest thing you can do is absolutely nothing.

Notice the tradeoff involved here though, since the sensory disengagement that goes with sleep can make the animal an easy mark for somebody’s dinner. It’s a decent strategy if you’re properly sheltered or otherwise concealed. But we’re still bumping into this question of why sleep itself — as opposed to just quiet wakefulness — is necessary. Why sacrifice full awareness and consciousness just to lay low?

Sleep = Restoration

What about the widely held idea that the purpose of sleep is to rest and restore the body after a day’s worth of activity? At first glance, this seems obvious enough to qualify as a duh statement. But in terms of the science, it’s far from open and shut.

For one thing, it’s not certain that our body’s cells and tissues would actually require hours of continuous rest in order to keep going. Our heart and diaphragm muscles certainly don’t need to stop for rest. They work 24/7 for as long as we’re alive, which (with a little luck) could be a hundred years or more.

Can we even equate sleep to rest, or activity to wakefulness? There are periods when we are awake but our bodies are not strictly active, and periods when we are asleep but our bodies are not strictly at rest — during REM sleep, there are parts of the brain that are more active than when we are awake.

If sleep restores the body, you’d expect to need more of it when you get a lot of exercise or exert yourself greatly. But sleep researchers have yet to find any clear link between levels of physical activity and sleep requirements, whether day-to-day or across populations. In fact, it’s often the case that people who are extremely sedentary or bedridden seem to need more than people who are robustly active.

There’s evidence on the other side of the coin too, of course: Most cell growth and restoration does take place at night during the deepest stages of sleep. Individual cells seem to switch from a daytime mode of outward activity, metabolizing fuel into energy, and turn inward to the business of maintenance and repair. Sleep may also be a key factor in how our immune systems fight off infection and disease.

The sleep-as-restoration argument gets stronger when you narrow the focus to one specific part of the body. Which is the subject of our next theory…

Recharging the Brain

The brain is a fantastic piece of biology. An ultra-compact supercomputer capable of processing and storing huge amounts of data, running all the systems of the body with seamless automaticity, and generating in us this mysterious thing called consciousness. We humans can, of course, take pride in a particularly well-developed neocortex, but in any species of animal you care to name, the brain is a wonder of nature.

Our brains, according to one estimate, can process some 20,000 trillion instructions per second, while storing 1,000 terabytes of memory. It will be a few years before IBM can rival that with a computer that fits in a room, let alone a cranium.

Knowing all this, it shouldn’t be a surprise that the brain is a heavy user of energy. The body devotes a large percentage of its calories just to run the thing at minimal levels. In fact, the brain is kind of fussy about energy: It must be provided with an absolutely uninterrupted supply. And while other parts of the body can use a mix of different fuels, depending on what’s available (fats, proteins, carbohydrates, etc.), the brain is limited to only one brand: sugar glucose.

This poses an interesting challenge: The energy requirements of the brain are highly variable. Our brains have to be ready to go from 0 to 60 at any time. Think of all those crucial, split-second moments when your neurons really have to react and perform. Whether you’re swerving to avoid a car accident, or answering a demanding question from your boss, or just catching a Frisbee at the park — in that one, time-stopping instant, your brain has a real problem. It needs energy right now, but it can’t draw enough fuel from your blood vessels and metabolize it in time to do what it needs to do.

So how does the brain cope with these demand spikes? It keeps a nearby fuel reserve in the form of glial cells. Glial cells make up at least half of all cells inside the brain. Their job is to support the operation of neurons (the cells that do all the thinking, remembering, sensing, etc.). The most important thing glial cells do is store energy, absorbing it from blood vessels when the neurons aren’t using it and storing it away for later. Whenever the brain needs a little extra energy, the glial cells are ready and waiting to release it. Think of them as brain batteries.

But batteries need to be kept charged. And this might just be where sleep comes in. By the end of the day, and 16 hours of mental activity, glial cells are likely getting energy depleted. It may be that sleep — and the low level of brain activity that goes with it — is necessary to allow the glial cells to recharge. So when you wake up in the morning, your brain is once again fully charged and ready for another day.

Learning and Memory

We’re still in the brain to investigate one last theory for the function of sleep: its possible role in the consolidation of memory. This is a hot topic in sleep science these days and it is a fascinating one.

Whenever we learn something, experience something, or acquire a new skill, our brains have to go to work processing the new information, filing it away into long-term memory banks, and integrating it with our existing store of knowledge — all in such a way that it will be available for later retrieval. This process is what is meant by “consolidation of memory.”

Memory consolidation doesn’t happen instantly. Once an experience has been initially “encoded” in the brain, the process of consolidating it may last anywhere from a few hours to several days. (In some ways it is a never-ending process, since our existing memories are constantly being updated and reshaped by new experiences and incoming information.)

Many scientists believe that sleep plays an essential role in all of this — for animals as well as humans. It could be that the brain needs to be “offline,” i.e. unconscious, in order for certain stages of memory consolidation to take place.

There’s a substantial body of evidence that this may be true. Several studies have demonstrated that effective learning depends on good sleep. Whenever you learn something, your ability to absorb it and remember it later will be significantly compromised if you don’t get proper sleep within 12 to 24 hours — even if you make up for the lost sleep later on. So there must be something going on during sleep that helps us internalize what we’ve just learned.

Other research suggests how learning during the day changes the contours of sleep at night. A learning experience can trigger marked changes in brainwave patterns during sleep, as well as shifts in the amount of time spent in each stage of sleep.

Scientists have found intriguing patterns of brain activity, using PET scans and fMRI, suggesting that the neuronal memory traces formed by new experiences actually get “reactivated” while we sleep. The same regions of the brain that “light up” while we’re learning to do a task, are seen during sleep to light up again in the same patterns. The brain may actually be using sleep to rewire itself in order to incorporate new learning.

This could give us a partial explanation for why we dream. If our memories get reactivated or played back as they’re being processed during sleep, we might catch occasional reflections of this in the form of dreaming. Some scientists think we might even “practice in our sleep,” using our dreams to sharpen new skills and find solutions to problems.

Can’t Live Without It

We’ve looked at several plausible theories for the function of sleep. Do any of them hold the right answer? It may be possible that each of them, in part, explains some aspect of sleep. It could also be that as we have evolved, sleep has evolved along with us. The purpose of sleep, when it first appeared in animals millions of years ago, may not be the purpose it serves for us now.

Sleep could have started out as a simple means of conserving caloric energy and evolved in more complex directions, supporting brain function and memory. Today, sleep may have a vital function in one species of animal, and a quite different one in another.

We may never really know for sure exactly what sleep does. What we do know is that there are some very real consequences when we forsake it. Sleep deprivation leads to profound cognitive impairments and mood disorders. In the body, its absence can affect very nearly every system. Whatever its purpose, sleep sustains us in virtually every way. We can’t live without it.