Why We Sleep: The New Science of Sleep and Dreams

by Matthew Walker

the shorter your sleep, the shorter your life.

individuals were far more ravenous when sleeping four to five hours a night.

participants just didn’t feel satisfied by food when they were short sleeping.

By muting the chemical message that says “stop eating” (leptin), yet increasing the hormonal voice that shouts “please, keep eating” (ghrelin), your appetite remains unsatisfied when your sleep is anything less than plentiful, even after a kingly meal.

leptin signals satiety and ghrelin instigates hunger

sleep loss increases levels of circulating endocannabinoids,

causes having the munchies

Combine this increase in endocannabinoids with alterations in leptin and ghrelin caused by sleep deprivation and you have a potent brew of chemical messages all driving you in one direction: overeating.

Inadequate sleep is the perfect recipe for obesity: greater calorie intake, lower calorie expenditure.

supervisory regions in the prefrontal cortex required for thoughtful judgments and controlled decisions had been silenced in their activity by a lack of sleep.

Ample sleep can therefore restore a system of impulse control within your brain,

When you are not getting enough sleep, the body becomes especially stingy about giving up fat. Instead, muscle mass is depleted while fat is retained.

short sleep (of the type that many adults in first-world countries commonly and routinely report) will increase hunger and appetite, compromise impulse control within the brain, increase food consumption (especially of high-calorie foods), decrease feelings of food satisfaction after eating, and prevent effective weight loss when dieting.

a marked drop in testosterone relative to their own baseline levels of testosterone when fully rested.

among mid-twenties test subjects limited to five hours of sleep for a week

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.

Males with low testosterone often feel tired and fatigued throughout the day. They find it difficult to concentrate on work tasks, as testosterone has a sharpening effect on the brain’s ability to focus. And of course, they have a dulled libido, making an active, fulfilling, and healthy sex life more challenging.

testosterone maintains bone density, and plays a causal role in building muscle mass and therefore strength,

key aspects of the human reproductive system are affected by sleep in both men and women. Reproductive hormones, reproductive organs, and the very nature of physical attractiveness that has a say in reproductive opportunities: all are degraded by short sleeping.

The less sleep an individual was getting in the week before facing the active common cold virus, the more likely it was that they would be infected and catch a cold.

sleep profoundly impacts your response to a standard flu vaccine.

Those participants who obtained seven to nine hours’ sleep in the week before getting the flu shot generated a powerful antibody reaction, reflecting a robust, healthy immune system. In contrast, those in the sleep-restricted group mustered a paltry response, producing less than 50 percent of the immune reaction their well-slept counterparts were able to mobilize.

It is these cancer-amplifying and -spreading processes that we now know a lack of sleep will encourage,

sleep deprivation will diminish one form of these macrophages, called M1 cells, that otherwise help combat cancer. Yet sleep deprivation conversely boosts levels of an alternative form of macrophages, called M2 cells, which promote cancer growth.

Deprive a mouse of sleep for just a day, as researchers have done, and the activity of these genes will drop by well over 200 percent.

the effects of insufficient sleep on genetic activity are just as striking in humans as they are in mice.

a lack of sleep will cause a drop in high-density lipoproteins (HDLs)—a directional profile that has consistently been linked to cardiovascular disease.

The less sleep an individual obtains, or the worse the quality of sleep, the more damaged the capstone telomeres of that individual’s chromosomes.

Telomeres cap the ends of chromosomes preventing them damage/unravelling

these emotional regions of the brain are up to 30 percent more active in REM sleep compared to when we are awake!

Amygdala and the cingulate cortex

REM sleep can therefore be considered as a state characterized by strong activation in visual, motor, emotional, and autobiographical memory regions of the brain, yet a relative deactivation in regions that control rational thought.

Explains why we have fantastic dreams

Between 35 and 55 percent of emotional themes and concerns that participants were having while they were awake during the day powerfully and unambiguously resurfaced in the dreams they were having at night.

Confirming the importance of the dream state, the better the quality of REM sleep from one individual to the next across that rested night, the more precise the tuning within the emotional decoding networks of the brain the next day.

The outside world had become a more threatening and aversive place when the brain lacked REM sleep—untruthfully so.

By removing REM sleep, we had, quite literally, removed participants’ levelheaded ability to read the social world around them.

problem-solving abilities rocketed up, with participants solving 15 to 35 percent more puzzles when emerging from REM sleep compared with awakenings from NREM sleep or during daytime waking performance!

Based on response times, solutions arrived more instantaneously following an REM sleep awakening, relative to the slower, deliberative solutions that came when that same individual was exiting NREM sleep or when they were awake during the day.

Because prefrontal cortex was disabled before REM sleep, thus removing tendency to second-guess, self-doubt?

the REM-sleep brain was shortcutting the obvious links and favoring very distantly related concepts.

It is both the act of dreaming and the associated content of those dreams that determine creative success.

learning versus comprehension. REM sleep allows your brain to move beyond the former and truly grasp the latter.

Different from solidifying memories, which we now realize to be the job of NREM sleep, REM sleep, and the act of dreaming, takes that which we have learned in one experience setting and seeks to apply it to others stored in memory.

Scientists had gained objective, brain-based proof that lucid dreamers can control when and what they dream while they are dreaming.

the parts of the brain involved in the maintenance of normal wakefulness: the alerting, activating regions of the brain stem and the sensory gate of the thalamus that sits on top, a setup that looks almost like a scoop of ice cream (thalamus) on a cone (brain stem). As the brain stem powers down at night, it removes its stimulating influence to the sensory gate of the thalamus. With the closing of the sensory gate, we stop perceiving the outside world, and thus we fall asleep.

The sleep-wake switch within the hypothalamus has a direct line of communication to the power station regions of the brain stem.

the sleep-wake switch in the hypothalamus releases a neurotransmitter called orexin.

When orexin is released down onto your brain stem, the switch has been unambiguously flipped, powering up the wakefulness-generating centers of the brain stem. Once activated by the switch, the brain stem pushes open the sensory gate of the thalamus, allowing the perceptual world to flood into your brain, transitioning you to full, stable wakefulness.

key factors have powerfully changed how much and how well we sleep: (1) constant electric light as well as LED light, (2) regularized temperature, (3) caffeine (discussed in chapter 2), (4) alcohol, and (5) a legacy of punching time cards.

Humans are predominantly visual creatures. More than a third of our brain is devoted to processing visual information, far exceeding that given over to sounds or smells, or those supporting language and movement.

Compared to reading a printed book, reading on an iPad suppressed melatonin release by over 50 percent at night.

iPad reading delayed the rise of melatonin by up to three hours, relative to the natural rise in these same individuals when reading a printed book.

First, individuals lost significant amounts of REM sleep following iPad reading. Second, the research subjects felt less rested and sleepier throughout the day following iPad use at night. Third was a lingering aftereffect, with participants suffering a ninety-minute lag in their evening rising melatonin levels for several days after iPad use ceased—almost

Using LED devices at night impacts our natural sleep rhythms, the quality of our sleep, and how alert we feel during the day.

create lowered, dim light in the rooms where you spend your evening hours. Avoid powerful overhead lights. Mood lighting is the order of the night.

Alcohol is in a class of drugs called sedatives. It binds to receptors within the brain that prevent neurons from firing their electrical impulses.