Why We Sleep: The New Science of Sleep and Dreams

by Matthew Walker

Why did evolution decide to outlaw muscle activity during REM sleep? Because by eliminating muscle activity you are prevented from acting out your dream experience. During REM sleep, there is a nonstop barrage of motor commands swirling around the brain, and they underlie the movement-rich experience of dreams. Wise, then, of Mother Nature to have tailored a physiological straitjacket that forbids these fictional movements from becoming reality, especially considering that you’ve stopped consciously perceiving your surroundings. You can well imagine the calamitous upshot of falsely enacting a dream fight, or a frantic sprint from an approaching dream foe, while your eyes are closed and you have no comprehension of the world around you. It wouldn’t take long before you quickly left the gene pool. The brain paralyzes the body so the mind can dream safely.

Why we are paralysed during REM sleep

If you compare the electrical depth of the deep NREM slow brainwaves on one half of someone’s head relative to the other when they are sleeping at home, they are about the same. But if you bring that person into a sleep laboratory, or take them to a hotel—both of which are unfamiliar sleep environments—one half of the brain sleeps a little lighter than the other, as if it’s standing guard with just a tad more vigilance due to the potentially less safe context that the conscious brain has registered while awake. The more nights an individual sleeps in the new location, the more similar the sleep is in each half of the brain. It is perhaps one reason why so many of us sleep so poorly the first night in a hotel room.

Why our sleep is worse the first night somewhere new

Perhaps it was practice, with sleep, that makes perfect?

In ways your waking brain would never attempt, the sleeping brain fuses together disparate sets of knowledge that foster impressive problem-solving abilities. If you ponder the type of conscious experience such outlandish memory blending would produce, you may not be surprised to learn that it happens during the dreaming state—REM sleep.

Get behind the wheel of a car when having slept just four hours or less the night before and you are 11.5 times more likely to be involved in a car accident.

A structure located in the left and right sides of the brain, called the amygdala—a key hot spot for triggering strong emotions such as anger and rage, and linked to the fight-or-flight response—showed well over a 60 percent amplification in emotional reactivity in the participants who were sleep-deprived.

It was as though, without sleep, our brain reverts to a primitive pattern of uncontrolled reactivity. We produce unmetered, inappropriate emotional reactions, and are unable to place events into a broader or considered context.

With a full night of plentiful sleep, we have a balanced mix between our emotional gas pedal (amygdala) and brake (prefrontal cortex). Without sleep, however, the strong coupling between these two brain regions is lost. We cannot rein in our atavistic impulses—too much emotional gas pedal (amygdala) and not enough regulatory brake (prefrontal cortex). Without the rational control given to us each night by sleep, we’re not on a neurological—and hence emotional—even keel.

“The best bridge between despair and hope is a good night’s sleep.”fn7

In the Northern Hemisphere, the switch to daylight savings time in March results in most people losing an hour of sleep opportunity. Should you tabulate millions of daily hospital records, as researchers have done, you discover that this seemingly trivial sleep reduction comes with a frightening spike in heart attacks the following day. Impressively, it works both ways. In the autumn within the Northern Hemisphere, when the clocks move back and we gain an hour of sleep opportunity time, rates of heart attacks plummet the day after. A similar rise-and-fall relationship can be seen with the number of traffic accidents, proving that the brain, by way of attention lapses and microsleeps, is just as sensitive as the heart to very small perturbations of sleep.

When short sleeping, the very same individuals ate 300 calories more each day—or well over 1,000 calories before the end of the experiment—compared to when they were routinely getting a full night of sleep.

Following the buffet, Van Cauter and her team once again quantified what participants ate, and how much they ate. Despite eating almost 2,000 calories during the buffet lunch, sleep-deprived participants dove into the snack bar. They consumed an additional 330 calories of snack foods after the full meal, compared to when they were getting plenty of sleep each night.

We are now observing these effects very early in life. Three-year-olds sleeping just ten and a half hours or less have a 45 percent increased risk of being obese by age seven than those who get twelve hours of sleep a night. To set our children on a pathway of ill health this early in life by way of sleep neglect is a travesty.

With a genuine lack of malice, I proceed to inform them that men who report sleeping too little—or having poor-quality sleep—have a 29 percent lower sperm count than those obtaining a full and restful night of sleep, and the sperm themselves have more deformities.