Sleep

We sleep so much of our life—fully one-third of our time is spent sleeping—that you would think any human knows all there is to know about it. But the truth is that until very recently, we had no real idea of why sleep was so ubiquitous across all species, or so necessary (deprive any human of sleep for a substantial period of days or weeks and that human will first hallucinate and eventually die). To many, it seemed to be this bothersome waste of time: why sleep when one could be doing so many interesting or profitable things? Indeed, that attitude, especially in advanced industrial nations, is one of the reasons Matthew Walker wrote his recent book, Why We Sleep: Unlocking the Power of Sleep and Dreams (Scribners: 2017). Walker, a professor of neuroscience and psychology at UC Berkeley, considers the lack of sleep in modern society a virtual epidemic that has enormous consequences to our national and global well-being. His book is therefore full of fascinating details and studies informing us why sleep is so necessary, and which kind of sleep is necessary for which functions.

            Begin with what almost everyone now knows: there are two different kinds of sleep for humans—deep or NREM sleep, and REM or rapid-eye-movement sleep. This discovery (by Aserinsky and Kleitman) came in 1952, but what the explosion of neuroscience since then has now revealed is why each type of sleep is necessary, when it usually occurs during the night, and why missing it has the damaging effects it does. First the basics: there are five cycles in the average person’s night of sleep, each lasting about 90 minutes. In each cycle, we go from being awake to REM sleep to four ever-deeper levels of NREM sleep, and then start over, with the average cycles lasting from 11PM to 12:30, 12:30 to 2, 2 to 3:30, 3:30 to 5:15 and 5:15 to 7AM. These 90-minute cycles change in composition through the night, with most of the early cycles consumed by deep NREM sleep and very little REM sleep, but then changing to REM sleep domination later, especially in the last cycle just before we wake for the day. What’s explained here is how the functions of the two types of sleep differ. NREM sleep—marked by slow, synchronous brain waves—does the work of “weeding out and removing unnecessary neural connections” (45). That is, our brains are filled with outside input during the day, and could be overwhelmed if they retained all that information. NREM sleep seems to do what Walker calls the “excavatory” work of ridding the brain of what’s not necessary and storing the parts that are, and it does it early in the night. Then, in the later part of the night, the REM sleep in which we dream plays a role in integrating the new connections that are left. Walker calls this the “etching hand of REM sleep,” which “blends, interconnects, and adds details…to auto-update our memory networks based on the events of the day” (45). REM sleep, in short, is the active time of sleep, with parts of the dreaming brain 30 % more active than when we are awake (though it should be pointed out that the brain is quite active during NREM sleep too). This activity is also the reason that a key part of REM sleep is body paralysis: you can’t move while dreaming because if you could, you might act out your dream and that could prove dangerous, as it sometimes is for sleepwalkers. Walker summarizes the various brain states as follows:

When it comes to information processing, think of the wake state as reception (experiencing and constantly learning the world around you), NREM sleep as reflection (storing and strengthening those raw ingredients of new facts and skills), and REM sleep as integration (interconnecting those raw ingredients with each other, with all past experiences, and, in doing so, building an ever more accurate model of how the world works.) (53).

            Walker underlines the importance of REM sleep to human evolution by theorizing that when primates moved from tree sleeping to ground sleeping, they could do more REM sleep without fear of falling. This meant that the increased dream time fostered both human cognitive intelligence and our ability to navigate socially complex groupings. That is, REM-sleep dreaming increases our ability to “successfully navigate the kaleidoscope of socioemotional signals” characteristic of human culture, and thus “forge(s) the types of cooperative alliances that are necessary to establish large social groups and societies” (74). REM sleep is also the key to human creativity, according to Walker, as the ability of dreams to combine all sorts of irrational elements seems to prove. Finally, for Walker, dreams have a much more specific function than that theorized by Freud. Nurturing to our emotional and mental health, dreams allow us to process emotional themes and concerns too fraught for the daytime brain. That is partly because during REM sleep, the stress-related chemical noradrenaline (or noepinephrine) is shut off. That means the brain can deal with an upsetting memory more calmly, in what Walker calls a “safe dreaming environment.” He even theorizes, based on the work of colleagues, that REM-sleep dreaming accomplishes two goals:

first, to remember the details of those valuable, salient experiences, integrating them with existing knowledge and putting them into autobiographical perspective; and second, to forget, or dissolve, the visceral, painful emotional charge that had previously been wrapped around those memories (208).

If this is true, thought Walker, then these ideas might be extended to PTSD, as his communication with Dr. Murray Raskind, of Seattle Veteran’s Hospital, suggested. Raskind found out by chance that a drug called prazosin he was using to treat patients with high blood pressure had an unexpected side effect: it suppressed noradrenaline in the brain, and thereby “alleviated reoccurring flashback nightmares” (213). This confirmed Walker’s intuition that REM sleep allows the dreaming brain to detoxify painful experiences by reliving them in a more stress-free setting than when awake.

            One other revelation, among many, is worth mentioning. Sleep turns out to be a great space for learning, from specific facts to motor skills. The key neuroscientific fact here is that learning during sleep seems mainly to be fostered by “sleep spindles.” These are pulses that repeat every 100 to 200 milliseconds, moving back and forth between the hippocampus (where short-term memory is stored) and the longer-term memory sites in the cortex. Here is how Walker describes this key discovery:

In that moment, we had just become privy to an electrical transaction occurring in the quiet secrecy of sleep: one that was shifting fact-based memories from the temporary storage depot (hippocampus) to a long-term secure vault (the cortex). In doing so, sleep had cleared out the hippocampus, replenishing this short-term information repository with plentiful free space…the learning of new facts could begin again, anew, the following day (111).

Moreover, learning during sleep aids more than the mental compiling of facts. The “offline learning” of sleep also aids motor memory, such as that required by athletes or musicians. A musician Walker met told him of his experiences in this regard, which happened routinely:

“As a pianist, I have an experience that seems far too frequent to be chance. I will be practicing a particular piece, even late into the evening, and I cannot seem to master it…I go to bed frustrated.  But when I wake up the next morning and sit back down at the piano, I can just play, perfectly” (124).

Numerous experiments, both in Walker’s laboratory and elsewhere, have now proven this capacity of sleep to be correct. In one test of learning odd number sequences, for example, one group was given time off from practicing during the day, with no sleep; the other half was given the same amount of time off, but at night when they could sleep. The group that was given a daytime break without sleep showed no improvement after twelve hours. But the group that had slept overnight “showed a striking 20 percent jump in performance speed and a near 35 percent improvement in accuracy” (125). This same result has been duplicated many times, and most people would probably confirm something like it in their own experience. This essentially means that the old admonition of “sleep on it” has now been confirmed scientifically: it is not practice alone that makes perfect, but that old standby, sleep.

            There is lots more in this invaluable book, but this should give you the idea. Walker is a missionary in our world whose message concerns both the damnation that comes from sleep deprivation (for teenagers, drivers, doctors, workers of all kinds), and the salvation that derives from what should be the simplest of our activities: a good night’s sleep.

Lawrence DiStasi