Why We Sleep

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Why do we sleep? What evolutionary sense does it make to put our guard down and be so vulnerable for such a large part of a lifetime? One of the best ways to approach the question of what sleep is for is to see what happens when we don't do it. In study after study, sleep deprivation inevitably leads to a telltale decrease in performance on a range of attention and memory tasks. It has been associated with a weakened immune system and negative moods.

One of the first things you see when you sleep deprive people is they get cranky. They become difficult. They don't want to interact with other people.

Clearly, there's something that sleep is doing that improves one's memory, that improves one's health, and that leads to the reverse of all these declines.

Brain imaging has given us a leg up. We can now see certain regions of the brain that are more active in one state of sleep compared to another state of sleep. And there have been studies where people have learned a particular type of task, and when they perform that task, a certain region of their brain is activated. And then during sleep, using brain imaging, you can see those same regions of the brain being reactivated, as if the brain is replaying its memories from the day before, and presumably, using that replay as a time to process those memories. So it looks like one of the main functions of sleep is to give us a time where sensory input is cut off so we can use those parts of the brain that are usually used explicitly for processing incoming information and use it to go over the events from the day and see how they fit together.

So to understand what sleep is for and how it works, we can begin by looking at the stages of sleep in a typical night, and what might be going on at each of these stages. Jill has been kind enough to go to bed tonight with electrodes placed on her head. The electrodes will produce EEG recordings—records of the large scale electrical activity from millions of neurons firing together throughout her brain. At night, it can seem as though the mind just shuts off, closes its doors, turns out the lights. And to our conscious awareness, much of sleep seems like this—blank and empty.

Maybe one of the most important things that sleep research in the last 10 years has made very clear is that steep just isn't a period where the brain shuts off.

It is transitioning into a series of repetitive stages known as the sleep cycle. In the theater of the mind, it is time for a new show that comes in five stages.

The very first stage that you enter when you're going to sleep, not surprisingly, is called Stage 1 sleep. It actually only lasts a few minutes, and you actually start having what are called hypnagogic dreams—hypnagogic just means at the start of sleep—with replaying just little snippets from your day, and saying, oh, this was important. We should think about that tonight. Or, this is really interesting. We should try to figure out what that meant tonight.

And so it looks like Stage 1 sleep, from a functional point of view, is identifying what the work is to do during the rest of the night. And that only lasts for a few minutes. And then you seem to sink into a slightly deeper state of sleep, which is called Stage 2.

And in Stage 2 you start to see little brief events in the EEG. They're called K complexes or sleep spindles. They're brief periods of synchronous activity where it looks like huge numbers of nerve cells in the brain are all firing at the same time. And those sleep spindles, again, are thought to be involved in information processing, and possibly in the taking of recent memories of actual events and starting to move them out into a part of the brain where they get integrated and sorted out by their meaning and their category. So it's taking a half hour in that Stage 2 sleep before you descend into deep sleep.

And deep sleep—which is Stages 3 and 4 of non-REM sleep—is characterised by these very large, slow oscillations in the EEG. It's almost as if the cells in your brain are firing. [FIRING SOUND EFFECT] All together. So the brain is using these synchronous firing—these slow waves—as a way to figure out who wants to fire together to who should get wired together.

And all of that can take maybe 45 minutes to get down into that deepest part of sleep. And then you might spend a half hour there. And then, surprisingly, you come back up. You come back up from Stage 4 to Stage 3, up to Stage 2.

And then instead of going up to non-REM Stage 1, you go into REM sleep. And REM is characterized by these bursts of rapid eye movements. It's characterized by a very fast EEG that's almost impossible to distinguish from the waking EEG. And it's characterized by muscle paralysis. So your body is paralyzed when you're in REM sleep, because otherwise you act out those dreams and your dreams are most intense and most common in REM sleep.

Dreams seem to have some relevance to our lives—that they're not just random dreams—that we dream about things that are important to us. We dream about them in crazy ways, but there's almost always an identifiable theme or concept in that dream that seems to relate to our actual waking life.

Freud believed dreams were messages from our unconscious—signs of repressed desires and feelings that made themselves known in the theater of the dreaming mind. While there is no proof of any one theory, the field has now moved more towards a theory known as the activation-synthesis model, where the brain is responding to internally generated symbols and then imposing a narrative meaning on them. In this theory, our conscious dreaming state is constantly stitching together images and feelings generated from the unconscious brain. This takes into account an understanding of what the brain might be doing while asleep—sorting through the memories of the day and consolidating what is deemed important, fitting these new memories into existing networks of thought and memory, in hopes preparing for future events.

It might be that those dreams are just a window into that memory processing. And it looks like the kind of processing that dreaming gives us access to is the big picture processing, where the brain is asking a very different question. And that question is, what else have I ever done in my life that this seams relevant to? So what does this go with, and how can I put these things together?

And what's interesting is that when it puts it out there into the part of your brain that you see in your dreams—when it puts it out, front stage center—that camera—you watching from the audience—you can't see the actual memories in the background. All the memories are off stage. So when people dream about things, they never replay the actual memories. And it's almost like when you dream—when you observe your dreams—what you're doing, really, is you're observing your brain exploring the possible uses of this experience that you actually had in your life.

But it turns out that we're dreaming almost the entire night long, more intensely more commonly in REM sleep than non-REM. And so you have this REM cycle that's about 90 minutes along that goes throughout the night.

So it looks like each of these sleep stages is serving a different role in memory processing. To ask which stage of sleep is most important for more memory processing is the same mistake as saying, well, if I could only get one vitamin, which vitamin should I get? You actually need all the vitamins, because they serve different functions. And you need all those stages of sleep because they serve different functions.

So how much sleep do we need? The answer varies by age and individual needs. Newborns sleep the most. They average around 16 hours a day, lapsing in and out of sleep. The progression of needing less and getting less sleep moves through childhood—where a typical six-year-old will sleep 11 to 12 hours—into adulthood—where it all averages out to about 7 to 7 1/2 hours a night.

Generally, over an entire lifetime, we sleep about an hour for every two that we're awake. But some people aren't so lucky. Insomnia is the most prevalent among the family of sleep disorders.

Primary insomnia—which is what most people who complain about insomnia are suffering from—appears to have, basically, a psychological origin. And the current view is that it's almost an anxiety disorder.

And once it is there, it's a vicious feedback loop. Insomnia can be exacerbated by worrying about insomnia. While some turn to sleeping pills for remedy, there can be undesirable side effects. At times, the best idea is to simply do something else.

We're learning that sleep—which we all are so used to that we take for granted—is more important than we ever thought it was. And it's only through this research that we come to understand what its medical functions are, as well as its psychological functions.

Having cycled through a dazzling array of different patterns of brain activity, and a few dreams here and there, it's time for Jill to wake up. [ALARM] Jill, you can remove the electrodes now. Jill!

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