Why We Sleep: The New Science of Sleep and Dreams

by Matthew Walker

‘A top sleep scientist argues that sleep is more important for our health than diet or exercise’ The Times

Part 1 THIS THING CALLED SLEEP

Chapter 1 To Sleep …

Too little sleep swells concentrations of a hormone that makes you feel hungry while suppressing a companion hormone that otherwise signals food satisfaction.

Worse, should you attempt to diet but don’t get enough sleep while doing so, it is futile, since most of the weight you lose will come from lean body mass, not fat.

As one sleep scientist has said, “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.”

Sleep reforms the body’s metabolic state by fine-tuning the balance of insulin and circulating glucose.

Sleep further regulates our appetite, helping control body weight through healthy food selection rather than rash impulsivity.

Plentiful sleep maintains a flourishing microbiome within your gut from which we know so much of o...

Instead, we are now forced to wonder whether there are any biological functions that do not benefit by a good night’s sleep.

A scarcely believable truth began to emerge—nobody actually knew the clear reason why we needed sleep, and what it does. I could not answer my own question about dementia if this fundamental first question remained unanswered. I decided I would try to crack the code of sleep.

Chapter 2 Caffeine, Jet Lag, and Melatonin

There are two main factors that determine when you want to sleep and when you want to be awake.

The first factor is a signal beamed out from your internal twenty-four-hour clock located deep within your brain.

The second factor is a chemical substance that builds up in your brain and creates a “sleep pressure.”

It is no coincidence that the likelihood of breaking an Olympic record has been clearly tied to time of day, being maximal at the natural peak of the human circadian rhythm in the early afternoon.

Daylight is the most reliable, repeating signal that we have in our environment.

Any signal that the brain uses for the purpose of clock resetting is termed a zeitgeber, from the German “time giver” or “synchronizer.”

The twenty-four-hour biological clock sitting in the middle of your brain is called the suprachiasmatic (pronounced soo-pra-kai-as-MAT-ik) nucleus.

The suprachiasmatic nucleus uses this reliable light information to reset its inherent time inaccuracy to a crisp twenty-four-hour cycle, preventing any drift.

Figure 1: Typical Twenty-Four-Hour Circadian Rhythm (Core Body Temperature)

If I were to keep you awake all night, your core body temperature would still show the same pattern.

Temperature is just one of many twenty-four-hour rhythms that the suprachiasmatic nucleus governs.

The prefrontal cortex controls high-level thought and logical reasoning, and helps keep our emotions in check.

An adult’s owlness or larkness, also known as their chronotype, is strongly determined by genetics.

However, night owls are not owls by choice. They are bound to a delayed schedule by unavoidable DNA hardwiring. It is not their conscious fault, but rather their genetic fate.

Consequently, the group as a whole is only collectively vulnerable (i.e., every person asleep) for just four rather than eight hours, despite everyone still getting the chance for eight hours of sleep. That’s potentially a 50 percent increase in survival fitness.

Your suprachiasmatic nucleus communicates its repeating signal of night and day to your brain and body using a circulating messenger called melatonin.

But melatonin has little influence on the generation of sleep itself: a mistaken assumption that many people hold.

Just signals the darkness.

Melatonin is the voice of the timing official that says “Runners, on your mark,” and then fires the starting pistol that triggers the race. That timing official (melatonin) governs when the race (sleep) begins, but does not participate in the race.

In this analogy, the sprinters themselves are other brain regions and processes that...

Once sleep is under way, melatonin slowly decreases in concentration across the night and into the morning hours.

With dawn, as sunlight enters the brain through the eyes (even through the closed lids), a brake pedal is applied to the pineal gland, thereby shutting off the release of melatonin.

The absence of circulating melatonin now informs the brain and body that the finish line ...

For every day you are in a different time zone, your suprachiasmatic nucleus can only readjust by about one hour.

You may have noticed that it feels harder to acclimate to a new time zone when traveling eastward than when flying westward.

At this very moment, a chemical called adenosine is building up in your brain. It will continue to increase in concentration with every waking minute that elapses.

One consequence of increasing adenosine in the brain is an increasing desire to sleep. This is known as sleep pressure, and it is the second force that will determine when you feel sleepy, and thus should go to bed.

You can, however, artificially mute the sleep signal of adenosine by using a chemical that makes you feel more alert and awake: caffeine.

Caffeine works by successfully battling with adenosine for the privilege of latching on to adenosine welcome sites—or receptors—in the brain.

Levels of circulating caffeine peak approximately thirty minutes after oral administration.

system. In pharmacology, we use the term “half-life” when discussing a drug’s efficacy. This simply refers to the length of time it takes for the body to remove 50 percent of a drug’s concentration. Caffeine has an average half-life of five to seven hours.

Also be aware that de-caffeinated does not mean non-caffeinated. Depending on the decaffeination method and the bean that is used, one cup of decaf can have between 3 to as high as 10 percent

Caffeine is removed from your system by an enzyme within your liver,fn8 which gradually degrades it over time. Based in large part on genetics,fn9 some people have a more efficient version of the enzyme that degrades caffeine, allowing the liver to rapidly clear it from the bloodstream.

Others, however, have a slower-acting version of the enzyme. It takes far longer for their system to eliminate the same amount of caffeine. As a result, they are very sensitive to caffeine’s effects. One cup of tea or coffee in the morning will last much of the day, and should they have a second cup, even early in the afternoon, they will find it difficult to fall asleep in the evening.

Phoebe

Aging also alters the speed of caffeine clearance: the older we are, the longer it takes our brain and body to remove caffeine, and thus the more sensitive we become in later life to caffeine’s sleep-disrupting influence.

They are two distinct and separate systems that are ignorant of each other. They are not coupled; though, they are usually aligned.

Figure 4: The Two Factors Regulating Sleep and Wakefulness

During sleep, a mass evacuation gets under way as the brain has the chance to degrade and remove the day’s adenosine. Across the night, sleep lifts the heavy weight of sleep pressure, lightening the adenosine load.

When these two processes trade places in the morning hours, wherein adenosine has been removed and the rousing volume of the circadian rhythm is becoming louder (indicated by the meeting of the two lines in figure 6), we naturally wake up (seven a.m. on day two, in the figure example).