(
)
The Secret
World of Sleep
The Surprising Science
of the Mind at Rest
by Penelope A. Lewis
Palgrave Macmillan
Birds do it. Bees do it. Even fruit flies do it. But one of the great mysteries of the universe remains: Why do we sleep?
University of Manchester scientist Penelope A. Lewis attempts an answer in her new, research-driven book “The Secret World of Sleep.” She shows how sleep not only makes us fitter, happier and more productive — but it also improves memory, creativity and learning.
Any doubts as to the importance of sleep are erased when we go without it. Lack of sleep has been linked to higher cancer rates, weight gain, high blood pressure and a greater risk of heart attacks. A recent study even showed that poor sleepers have worse skin and more wrinkles than their restful counterparts.
Wake up on the wrong side of the bed and you might find yourself on the wrong page of “The Diagnostic and Statistical Manual of Mental Disorders,” writes Lewis.
“Sleep-deprived people are more easily frustrated, intolerant, unforgiving, uncaring and self-absorbed than they would be if they were properly rested.”
All these elements tip “perfectly normal people over the edge into clinically relevant zones, so that, if tested on that particular day, they could be classified as depressed or even psychopaths.”
Function with a sleep-deprived brain, and you will behave as if you’re under the influence of alcohol. A hand-eye coordination study showed that each hour awake sets a person back as much as .004% alcohol concentration. In other words, every five hours spent awake equates to one Heineken. At 20 hours, you’re exceeded the legal alcohol limit to drive.
So, what happens while we doze to keep this all in check?
It’s clear that our minds are active during slumbers. Neurons fire almost as often when we are asleep as when awake and consume nearly as much energy during the five stages of sleep.
When you get drowsy and close your eyes, electrical signals from your brain slow down and brain waves get longer, prompting you to enter deep, non-REM sleep. During this time, sleep spindles, or “little bursts of frenetic activity” begin to appear. As you go even deeper, you enter slow wave sleep, where many areas of the brain begin to act “together in a coordinated, but slow, fashion.” Finally, REM sleep — a time when your most bizarre and emotional dreams appear — takes over. Rapid eye movement begins, and your brain activity resembles how we are during drowsy wakefulness.
“This complex dance of sleep stages impact[s] memory, mood and decision-making,” Lewis writes. “Why should sleep deprivation lead to a meltdown in some of these systems?”
Take memory. Procedural memory skills, like riding a bike, seem to improve after sleep. And classical memories, like recalling what you had for breakfast or reciting a list of words, have been shown to be retained better after a night’s rest — or even after a nap as short as six minutes, Lewis writes.
Neuroimaging studies have shown that the hippocampus, or the part of the brain critical for memory storage, is less active when you learn something new after a poor night’s rest.
“Sleep does something to memories,” Lewis writes. “Sleep appears to actively process memories in a way that noticeably strengthens them.”
Simply believing something is important helps strengthen new memories after sleep, studies show. When people are asked to learn two new sets of information — one a list they were told they were going to be tested on and the other they weren’t — improvements in performance were only observed for items that they believed they were going to be tested on after a night’s rest.
“Sleep seems to be working hard to make sure we keep hold of the most important memories, and respond appropriately to stimuli we come across during the day,” she writes.
Sleep not only seems to strengthen certain memories, it also engages in what Lewis calls a “spring cleaning” to filter out the unimportant things weighing us down.
The active downscaling of synaptic connections in the brain during sleep is the subject of a cover article in this month’s Scientific American magazine.
While 20% of the body’s energy goes to the 3-pound mass in skulls, about two-thirds of that energy is devoted to making synaptic connections, a strain that is “unsustainable,” according to sleep scientists Giulio Tononi and Chiara Cirelli at the University of Wisconsin. This downscaling makes room for new learning and removes the unimportant information and “noise.”
“The brain cannot go on strengthening and maintaining revved-up synapses both day and night for the whole of an individual’s lifetime,” write Tononi and Cirelli in Scientific American. “Sleep is the price we pay for the brain’s plasticity — it’s ability to modify its wiring in response to experience.”
Or, as Lewis puts it in her book, “like every good cellar or attic, the brain needs a regular clean-out.”
And the clean-out not only makes room for new connections, but it seems to strengthen old ones, too. The combination of melding new information with the old is another possible advantage of sleeping.
So with lack of sleep, not only is memory creation impaired, but so is the ability to make logical deductions.
In one experiment, people were given three words and asked to supply a fourth that links them. One such example would be: “sixteen, heart, and tooth.” Subjects did far better on figuring out the fourth word (“sweet”) after a 90-minute nap than they did without rest.
“Sleep-deprived people come up with fewer original ideas and also tend to stick to old strategies that may not continue to be effective,” writes Lewis.
Conversely, during sleep there seems to be a greater number of creative connections made. In a study of 35 professional musicians, they not only heard more music in their dreams than non-performers, but a good chunk of this music — around 25% of it — was music they had never heard before.
This creative process during sleep is common. Mary Shelley famously wrote about coming up with her idea for Frankenstein in a dream, and authors such as Robert Lewis Stevenson, Charlotte Brontë and Edgar Allan Poe all claim to have come up with plot points while at rest. The periodic table and the light bulb were both invented overnight, too.
“The sleeping brain is somehow freed of constraints and can thus create whole sequences of free associations. This is not only useful for creativity, but also is thought to facilitate insight and problem solving as well,” Lewis writes.
It’s also believed that dreams might also serve as a “virtual reality simulation” (otherwise known as the simulation hypothesis) where we practice events in dreams to prepare for real-life situations. That’s why so many dreams — around 70% for most of us — are threatening.
Interestingly, though, we don’t need sleep to retain negative information. That we can do without a full night’s rest.
When sleep deprived people were shown a list of words containing unpleasant motifs (“murder,” “rape,” “death”), positive words (“beautiful,” “happy,” “love”) and neutral ones (“chair,” “plant,” “building”), they were able to remember the negative words equally well if they had rested or not. However, the positive and neutral words were recalled less well without sleep.
Perhaps this is related to the fact that those who go without sleep tend to see the world through a more negative filter. In fact, without sleep, “we are more likely to perceive perfectly neutral facial expressions as negative, and we are less able to appreciate humor,” Lewis writes.
There is evidence that specific regions of the frontal lobe that usually help to filter out negative emotions are impaired by lack of sleep.
People who forego sleep aren’t just crankier. They also tend to take more risks, studies show. Brain response studies have suggested that when we engage in risky behavior — say, cheating on a spouse — the reward systems are active, but the brain’s “punishment systems” don’t show normal responses in sleep-deprived brains.
“This suggests that the brain’s system for punishment and reward is temporarily out of balance when people are sleep deprived,” Lewis writes.
Caffeine doesn’t adjust for insomniac’s lack of morality — it might help you be more attentive, but studies have shown it does not relieve the judgment issues, which implies that sleep might play a role in adjusting mood and behavior during waking hours.
Dreams, too, even help regulate mood. One study of divorced women showed that those who often dreamed of an ex were “better adapted to divorce.” In another study, people who went to bed without water but dreamed of drinking reported to feel less thirsty upon waking, providing evidence that dreams might even influence more complex physical states.
Though many of the reasons why we sleep — and what it does to our minds — remain a mystery, shut-eye most certainly exists for a reason.
“Surely something that is so widespread across the animal kingdom, yet so dangerous and time-consuming must serve an important function,” Lewis writes. “It is certainly valuable enough to have evolved through natural selection.”