Why We Sleep: The New Science of Sleep and Dreams

by Matthew Walker

Routinely sleeping less than six or seven hours a night demolishes your immune system, more than doubling your risk of cancer. Insufficient sleep is a key lifestyle factor determining whether or not you will develop Alzheimer’s disease. Inadequate sleep—even moderate reductions for just one week—disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic. Short sleeping increases the likelihood of your coronary arteries becoming blocked and brittle, setting you on a path toward cardiovascular disease, stroke, and congestive heart failure. Fitting Charlotte Brontë’s prophetic wisdom that “a ruffled mind makes a restless pillow,” sleep disruption further contributes to all major psychiatric conditions, including depression, anxiety, and suicidality.

Every component of wellness, and countless seams of societal fabric, are being eroded by our costly state of sleep neglect: human and financial alike. So much so that the World Health Organization (WHO) has now declared a sleep loss epidemic throughout industrialized nations.fn2 It is no coincidence that countries where sleep time has declined most dramatically over the past century, such as the US, the UK, Japan, and South Korea, and several in western Europe, are also those suffering the greatest increase in rates of the aforementioned physical diseases and mental disorders.

Tragically, one person dies in a traffic accident every hour in the United States due to a fatigue-related error. It is disquieting to learn that vehicular accidents caused by drowsy driving exceed those caused by alcohol and drugs combined.

When you don’t get enough sleep, one consequence among many is that adenosine concentrations remain too high. Like an outstanding debt on a loan, come the morning, some quantity of yesterday’s adenosine remains. You then carry that outstanding sleepiness balance throughout the following day. Also like a loan in arrears, this sleep debt will continue to accumulate. You cannot hide from it. The debt will roll over into the next payment cycle, and the next, and the next, producing a condition of prolonged, chronic sleep deprivation from one day to another. This outstanding sleep obligation results in a feeling of chronic fatigue, manifesting in many forms of mental and physical ailments that are now rife throughout industrialized nations.

In a disturbing later study, researchers in Australia took two groups of healthy adults, one of whom they got drunk to the legal driving limit (.08 percent blood alcohol), the other of whom they sleep-deprived for a single night. Both groups performed the concentration test to assess attention performance, specifically the number of lapses. After being awake for nineteen hours, people who were sleep-deprived were as cognitively impaired as those who were legally drunk. Said another way, if you wake up at seven a.m. and remain awake throughout the day, then go out socializing with friends until late that evening, yet drink no alcohol whatsoever, by the time you are driving home at two a.m. you are as cognitively impaired in your ability to attend to the road and what is around you as a legally drunk driver. In fact, participants in the above study started their nosedive in performance after just fifteen hours of being awake (ten p.m. in the above scenario).

Operating on less than five hours of sleep, your risk of a car crash increases threefold. 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.

The recycle rate of a human being is around sixteen hours. After sixteen hours of being awake, the brain begins to fail. Humans need more than seven hours of sleep each night to maintain cognitive performance. After ten days of just seven hours of sleep, the brain is as dysfunctional as it would be after going without sleep for twenty-four hours. Three full nights of recovery sleep (i.e., more nights than a weekend) are insufficient to restore performance back to normal levels after a week of short sleeping.

You may find it surprising to learn that vehicle accidents caused by drowsy driving exceed those caused by alcohol and drugs combined.

There is but a fraction of 1 percent of the population who are truly resilient to the effects of chronic sleep restriction at all levels of brain function. It is far, far more likely that you will be struck by lightning (the lifetime odds being 1 in 12,000) than being truly capable of surviving on insufficient sleep thanks to a rare gene.

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.

We discovered that different deep emotional centers in the brain just above and behind the amygdala, called the striatum—associated with impulsivity and reward, and bathed by the chemical dopamine—had become hyperactive in sleep-deprived individuals in response to the rewarding, pleasurable experiences. As with the amygdala, the heightened sensitivity of these hedonic regions was linked to a loss of the rational control from the prefrontal cortex.