Perhaps most importantly, how do you know if you’re getting enough sleep?

we humans have a similar, internally generated circadian rhythm.

It was from this darkness that Kleitman and Richardson were to illuminate a striking scientific finding that would define our biological rhythm as being approximately one day (circadian), and not precisely one day.

Richardson, in his twenties, developed a sleep-wake cycle of between twenty-six and twenty-eight hours in length. That of Kleitman, in his forties, was a little closer to, but still longer than, twenty-four hours.

Yet daylight isn’t the only signal that the brain can latch on to for the purpose of biological clock resetting, though it is the principal and preferential signal, when present. So long as they are reliably repeating, the brain can also use other external cues, such as food, exercise, temperature fluctuations, and even regularly timed social interaction.

the two governing forces that regulate your sleep—the twenty-four-hour circadian rhythm of the suprachiasmatic nucleus and the sleep-pressure signal of adenosine—communicate

They are not coupled; though, they are usually aligned.

deep NREM sleep, which predominates early in the night, is to do the work of weeding out and removing unnecessary neural connections.

In contrast, the dreaming stage of REM sleep, which prevails later in the night, plays a role in strengthening those connections.

Place your finger between your eyes, just above the bridge of your nose. Now slide it up your forehead about two inches. When you go to bed tonight, this is where most of your deep-sleep brainwaves will be generated: right in the middle of your frontal lobes. It is the epicenter, or hot spot, from which most of your deep, slow-wave sleep emerges.

REM sleep has also been called paradoxical sleep: a brain that appears awake, yet a body that is clearly asleep.

the sensory gate of the thalamus once again swings open during REM sleep.

signals of emotions, motivations, and memories (past and present) are all played out on the big screens

When it comes to information processing, think of the wake state principally 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

As the geneticist Theodosius Dobzhansky once said, “Nothing in biology makes sense except in light of evolution.”

What if sleep is so useful—so physiologically beneficial to every aspect of our being—that the real question is: Why did life ever bother to wake up?

Adopt this perspective, and we can pose a very different theory: sleep was the first state of life on this planet, and it was from sleep that wakefulness emerged.

After all, absence of evidence is not evidence of absence.

When a theme repeats in evolution, and independently across unrelated lineages, it often signals a fundamental need.

That humans (and all other species) can never “sleep back” that which we have previously lost

Sleep with both sides of the brain, or sleep with just one side and then switch. Both are possible, but sleep you must. Sleep is non-negotiable.

Throughout developed nations, most adults currently sleep in a monophasic pattern—that is, we try to take a long, single bout of slumber at night, the average duration of which is now less than seven hours.

Both these groups take a similarly longer sleep period at night (seven to eight hours of time in bed, achieving about seven hours of sleep), followed by a thirty- to sixty-minute nap in the afternoon.

Pre-industrial tribes, such as the Hadza in northern Tanzania or the San of Namibia, sleep in a biphasic pattern in the hotter summer months, incorporating a thirty- to forty-minute nap at high noon.

these tribespeople will fall asleep two to three hours after sunset, around nine p.m. Their nighttime sleep bouts will come to an end just prior to, or soon after, dawn.

The practice of biphasic sleep is not cultural in origin, however. It is deeply biological. All humans, irrespective of culture or

I posit that both have been beneficially and causally shaped by the hand of sleep, and specifically our intense degree of REM sleep relative to all other mammals: (1) our degree of sociocultural complexity, and (2) our cognitive intelligence.

REM sleep exquisitely recalibrates and fine-tunes the emotional circuits of the human brain

REM sleep increases our ability to recognize and therefore successfully navigate the kaleidoscope of socioemotional signals

the coolheaded ability to regulate our emotions each day—a key to what we call emotional IQ—depends on getting sufficient REM sleep night after night.

From this REM-sleep-enhanced emotional IQ emerged a new and far more sophisticated form of hominid socioecology across vast collectives, one that helped enable the creation of large, emotionally astute, stable, highly bonded, and intensely social communities of humans.

Alcohol is one of the most powerful suppressors of REM sleep that we know

The changes in deep NREM sleep always precede the cognitive and developmental milestones within the brain by several weeks or months, implying a direction of influence:

Of concern is that administering caffeine to juvenile rats will also disrupt deep NREM sleep and, as a consequence, delay numerous measures of brain maturation and the development of social activity,

But non-volitional, non-negotiable, and strongly biological they are. We parents would be wise to accept this fact, and to embrace it, encourage it, and praise it, lest we wish our own children to suffer developmental brain abnormalities or force a raised risk of mental illness upon them.

As parents, we are often too focused on what sleep is taking away from our teenagers, without stopping to think about what it may be adding.

Based on scientific evidence, a new policy has rightly been suggested by my colleague Dr. Mary Carskadon: “No child needs caffeine.”

These three key changes are: (1) reduced quantity/quality, (2) reduced sleep efficiency, and (3) disrupted timing of sleep.

As you enter your fourth decade of life, there is a palpable reduction in the electrical quantity and quality of that deep NREM sleep. You obtain fewer hours of deep sleep, and those deep NREM brainwaves become smaller, less powerful, and fewer in number.

Any individual, no matter what age, will exhibit physical ailments, mental health instability, reduced alertness, and impaired memory if their sleep is chronically disrupted.

Evening light suppresses the normal rise in melatonin, pushing an average adult’s sleep onset time into the early-morning hours, preventing sleep at a reasonable hour.

Those who were awake throughout the day became progressively worse at learning, even though their ability to concentrate remained stable

the more sleep spindles an individual has at night, the greater the restoration of overnight learning ability come the next morning.

Of broader societal relevance, the concentration of NREM-sleep spindles is especially rich in the late-morning hours, sandwiched between long periods of REM sleep.

Like a computer hard drive where some files have become corrupted and inaccessible, sleep offers a recovery service at night.

Analogous to looping your favorite songs in a repeating playlist at night, we cherry-pick specific slices of your autobiographical past, and preferentially strengthen them by using the individualized sound cues during sleep.

Not so, it seemed. Perhaps it was practice, with sleep, that makes perfect?

The increases in speed and accuracy, underpinned by efficient automaticity, were directly related to the amount of stage 2 NREM, especially in the last two hours of an eight-hour night of sleep (e.g., from five to seven a.m., should you have fallen asleep at eleven p.m.).

Even the ability of the body to cool itself during physical exertion through sweating—a critical part of peak performance—is impaired by sleep loss.

And so, therefore, we continue to need and rely upon our NREM sleep for refining and maintaining those motor movements.

Sleep provides a nighttime theater in which your brain tests out and builds connections between vast stores of information. This task is accomplished using a bizarre algorithm that is biased toward seeking out the most distant, nonobvious associations, rather like a backward Google search.

YOU DO NOT KNOW HOW SLEEP-DEPRIVED YOU ARE WHEN YOU ARE SLEEP-DEPRIVED

With chronic sleep restriction over months or years, an individual will actually acclimate to their impaired performance, lower alertness, and reduced energy levels.

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.

seeking, risk-taking, and addiction. Sleep disturbance is a recognized hallmark associated with addictive substance use.IV

practices, I pointed a scolding finger directly at the faculty, myself included. I suggested that if we, as teachers, strive to accomplish just that purpose—to teach—then end-loading exams in the final days of the semester was an asinine decision. It forced a behavior in our students—that of short sleeping or pulling all-nighters leading up to the exam—that was in direct opposition to the goals of nurturing young scholarly minds.

As many have said about such stoic institutions: theories, beliefs, and practices die one generation at a time. But the conversation and battle must start somewhere.

As with a fine-dining experience, it is far more preferable to separate the educational meal into smaller courses, with breaks in between to allow for digestion, rather than attempt to cram all of those informational calories down in one go.

In other words, if you don’t sleep the very first night after learning, you lose the chance to consolidate those memories, even if you get lots of “catch-up” sleep thereafter.

is not like the bank. You cannot accumulate a debt and hope to pay it off at a later point in time. Sleep for memory consolidation is an all-or-nothing event.

night. The glial cells of the brain were shrinking in size by up to 60 percent during NREM sleep, enlarging the space around the neurons and allowing the cerebrospinal fluid to proficiently clean out the metabolic refuse left by the day’s neural activity. Think of the buildings of a large metropolitan city physically shrinking at night, allowing municipal cleaning crews easy access to pick up garbage strewn in the streets, followed by a good pressure-jet treatment of every nook and cranny. When we wake each morning, our brains can once again function efficiently thanks to this deep cleansing.

Phrased differently, and perhaps more simply, wakefulness is low-level brain damage, while sleep is neurological sanitation.

From this cascade comes a prediction: getting too little sleep across the adult life span will significantly raise your risk of developing Alzheimer’s disease.

A more radical and converse prediction that emerges from these findings is that, by improving someone’s sleep, we should be able to reduce their risk of developing Alzheimer’s disease—or at least delay its onset.

Take away the bedrock of sleep, or weaken it just a little, and careful eating or physical exercise become less than effective, as we shall see.

Unhealthy sleep, unhealthy heart. Simple and true.

The less you sleep, the more you are likely to eat. In addition, your body becomes unable to manage those calories effectively, especially the concentrations of sugar in your blood.

When your sleep becomes short, you will gain weight.

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

We found that a full night of sleep repairs the communication pathway between deep-brain areas that unleash hedonic desires and higher-order brain regions whose job it is to rein in these cravings. Ample sleep can therefore restore a system of impulse control within your brain, putting the appropriate brakes on potentially excessive eating.

When you are not getting enough sleep, the body becomes especially stingy about giving up fat.

Sleep fights against infection and sickness by deploying all manner of weaponry within your immune arsenal, cladding you with protection. When you do fall ill, the immune system actively stimulates the sleep system, demanding more bed rest to help reinforce the war effort.

sleep profoundly impacts your response to a standard flu vaccine.

Left switched on without a natural return to peaceful quiescence, a nonspecific state of chronic inflammation causes manifold health problems, including those relevant to cancer.

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

Not sleeping enough, which for a portion of the population is a voluntary choice, significantly modifies your gene transcriptome—that is, the very essence of you, or at least you as defined biologically by your DNA.

the amygdala and the cingulate cortex,

both of which help generate and process emotions. Indeed, these emotional regions of the brain are up to 30 percent more active in REM sleep compared to when we are awake!

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. Finally,

I am in no way suggesting that reviewing your dreams yourself, or sharing them with someone else, is a waste of time. On the contrary, I think it is a very helpful thing to do, as dreams do have a function,

Indeed, journaling your waking thoughts, feelings, and concerns has a proven mental health benefit, and the same appears true of your dreams.

But Stickgold did find a strong and predictive daytime signal in the static of nighttime dream reports: emotions.

Sleep, and specifically REM sleep, was clearly needed in order for us to heal emotional wounds.

A testable prediction emerged: if I could lower the levels of noradrenaline in the brains of PTSD patients during sleep, thereby reinstating the right chemical conditions for sleep to do its trauma therapy work, then I should be able to restore healthier quality REM sleep.

Accurately reading expressions and emotions of faces is a prerequisite of being a functional human being, and indeed, a functional higher primate of most kinds.

a dream-starved brain cannot accurately decode facial expressions, which become distorted. You begin to mistake friends for foes.

The REM-sleep dreaming brain was utterly uninterested in bland, commonsense links—the one-step-to-the-next associations. Instead, the REM-sleep brain was shortcutting the obvious links and favoring very distantly related concepts. The logic guards had left the REM-sleep dreaming brain.

Wonderfully eclectic lunatics were now running the associative memory asylum.

Using that wide-angle dream lens, we can apprehend the full constellation of stored information and their diverse combinatorial possibilities, all in creative servitude.

The very same benefit was found after daytime naps of sixty to ninety minutes that also included REM sleep.

Our participants went to bed with disparate pieces of the jigsaw and woke up with the puzzle complete. It is the difference between knowledge (retention of individual facts) and wisdom (knowing what they all mean when you fit them together).

REM sleep is capable of creating abstract overarching knowledge and super-ordinate concepts out of sets of information.

“How can I understand and connect that which I have recently learned with that I already know, and in doing so, discover insightful new links and revelations?” Moreover, “What have I done in the past that might be useful in potentially solving this newly experienced problem in the future?”

Edison would allegedly position a chair with armrests at the side of his study desk, on top of which he would place a pad of paper and a pen. Then he would take a metal saucepan and turn it upside down, carefully positioning it on the floor directly below the right-side armrest of the chair. If that were not strange enough, he would pick up two or three steel ball bearings in his right hand. Finally, Edison would settle himself down into the chair, right hand supported by the armrest, grasping the ball bearings. Only then would Edison ease back and allow sleep to consume him whole. At the moment he began to dream, his muscle tone would relax and he would release the ball bearings, which would crash on the metal saucepan below, waking him up. He would then write down all of the creative ideas that were flooding his dreaming mind. Genius, wouldn’t you agree?

It remains unclear whether lucid dreaming is beneficial or detrimental, since well over 80 percent of the general populace are not natural lucid dreamers.

However, this argument makes the erroneous assumption that we have stopped evolving. It is possible that lucid dreamers represent the next iteration in Homo sapiens’ evolution.

sleep-state misperception, also known as paradoxical insomnia.

patients will report having slept poorly throughout the night,

The sleep recordings indicate that the patient has slept far better than they themselves believe, and sometimes indicate that a completely full and healthy night of sleep occurred.

The problem is that some people confuse time slept with sleep opportunity time.

We know that many individuals in the modern world only give themselves 5 to 6.5 hours of sleep opportunity, which normally means they will only obtain around 4.5 to 6 hours of actual sleep.

Beyond longer commute times and “sleep procrastination” caused by late-evening television and digital entertainment—both

(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.

Even a hint of dim light—8 to 10 lux—has been shown to delay the release of nighttime melatonin in humans.

But we do stare at LED-powered laptop screens, smartphones, and tablets each night, sometimes for many hours, often with these devices just feet or even inches away from our retinas.

One of the earliest studies found that using an iPad—an electronic tablet enriched with blue LED light—for two hours prior to bed blocked the otherwise rising levels of melatonin by a significant 23 percent.

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 like a digital hangover effect.

Maintaining complete darkness throughout the night is equally critical, the easiest fix for which comes from blackout curtains.

The politically incorrect advice I would (of course never) give is this: go to the pub for a drink in the morning. That way, the alcohol will be out of your system before sleep.

Nightly alcohol will disrupt your sleep, and the annoying advice of abstinence is the best, and most honest, I can offer.

To successfully initiate sleep, as described in chapter 2, your core temperature needs to decrease by 2 to 3 degrees Fahrenheit, or about 1 degree Celsius.

It is no evolutionary coincidence that we humans have developed the pre-bed ritual of splashing water on one of the most vascular parts of our bodies—our face, using one of the other highly vascular surfaces—our hands. You may think the feeling of being facially clean helps you sleep better, but facial cleanliness makes no difference to your slumber.

The need to dump heat from our extremities is also the reason that you may occasionally stick your hands and feet out from underneath the bedcovers at night due to your core becoming too hot, usually without your knowing.

A bedroom temperature of around 65 degrees Fahrenheit (18.3°C) is ideal for the sleep of most people, assuming standard bedding and clothing.

Instead, the hot bath invites blood to the surface of your skin, giving you that flushed appearance. When you get out of the bath, those dilated blood vessels on the surface quickly help radiate out inner heat, and your core body temperature plummets. Consequently, you fall asleep more quickly because your core is colder.

Most of us are unaware of an even greater danger that lurks within the alarm clock: the snooze button. If alarming your heart, quite literally, were not bad enough, using the snooze feature means that you will repeatedly inflict that cardiovascular assault again and again within a short span of time.

Waking up at the same time of day, every day, no matter if it is the week or weekend is a good recommendation for maintaining a stable sleep schedule if you are having difficulty with sleep.

Squeezed by the vise grips of an electrified night and early-morning start times, bereft of twenty-four-hour thermal cycles, and with caffeine and alcohol surging through us in various quantities, many of us feel rightly exhausted and crave that which seems always elusive: a full, restful night of natural deep sleep.

In addition, patients must (1) establish a regular bedtime and wake-up time, even on weekends, (2) go to bed only when sleepy and avoid sleeping on the couch early/mid-evenings, (3) never lie awake in bed for a significant time period; rather, get out of bed and do something quiet and relaxing until the urge to sleep returns, (4) avoid daytime napping if you are having difficulty sleeping at night, (5) reduce anxiety-provoking thoughts and worries by learning to mentally decelerate before bed, and (6) remove visible clockfaces from view in the bedroom, preventing clock-watching anxiety at night. One

All twelve suggestions are superb advice, but if you can only adhere to one of these each and every day, make it: going to bed and waking up at the same time of day no matter what.

Sleep appears to be a natural analgesic, and without it, pain is perceived more acutely by the brain,

7. Don’t take naps after 3 p.m.

8. Relax before bed.

9. Take a hot bath before bed.

10. Dark bedroom, cool bedroom, gadget-free bedroom.