Why We Sleep: The New Science of Sleep and Dreams

by Matthew Walker

Caffeine has an average half-life of five to seven hours. Let’s say that you have a cup of coffee after your evening dinner, around 7:30 p.m. This means that by 1:30 a.m., 50 percent of that caffeine may still be active and circulating throughout your brain tissue. In other words, by 1:30 a.m., you’re only halfway to completing the job of cleansing your brain of the caffeine you drank after dinner.

If you are trying to stay awake late into the night by drinking coffee, you should be prepared for a nasty consequence when your liver successfully evicts the caffeine from your system: a phenomenon commonly known as a “caffeine crash.”

brainwave activity, (2) eye movement activity, and (3) muscle activity. Collectively, these signals are grouped together under the blanket term “polysomnography” (PSG), meaning a readout (graph) of sleep (somnus) that is made up of multiple signals (poly).

The practice of biphasic sleep is not cultural in origin, however. It is deeply biological. All humans, irrespective of culture or geographical location, have a genetically hardwired dip in alertness that occurs in the midafternoon hours.

post-prandial alertness dip

the tree-to-ground reengineering of sleep was a key trigger that rocketed Homo sapiens to the top of evolution’s lofty pyramid.

For fact-based information—or what most of us think of as textbook-type learning, such as memorizing someone’s name, a new phone number, or where you parked your car—a region of the brain called the hippocampus helps apprehend these passing experiences and binds their details together. A long, finger-shaped structure tucked deep on either side of your brain, the hippocampus offers a short-term reservoir, or temporary information store, for accumulating new memories. Unfortunately, the hippocampus has a limited storage capacity, almost like a camera roll or, to use a more modern-day analogy, a USB memory stick. Exceed its capacity and you run the risk of not being able to add more information or, equally bad, overwriting one memory with another: a mishap called interference forgetting.

E. Joseph Cossman: “The best bridge between despair and hope is a good night’s sleep.”

First, if we feed a waking brain with the individual ingredients of a problem, novel connections and problem solutions should preferentially—if not exclusively—emerge after time spent in the REM dreaming state, relative to an equivalent amount of deliberative time spent awake. Second, the content of people’s dreams, above and beyond simply having REM sleep, should determine the success of those hyper-associative problem-solving benefits. As with the effects of REM sleep on our emotional and mental well-being explored in the previous chapter, the latter would prove that REM sleep is necessary but not sufficient. It is both the act of dreaming and the associated content of those dreams that determine creative success.

Edison was a habitual daytime napper. He understood the creative brilliance of dreaming, and used it ruthlessly as a tool, describing it as “the genius gap.”

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?

Most of the thermic work is performed by three parts of your body in particular: your hands, your feet, and your head. All three areas are rich in crisscrossing blood vessels, known as the arteriovenous anastomoses, that lie close to the skin’s surface. Like stretching clothes over a drying line, this mass of vessels will allow blood to be spread across a large surface area of skin and come in close contact with the air that surrounds it. The hands, feet, and head are therefore remarkably efficient radiating devices that, just prior to sleep onset, jettison body heat in a massive thermal venting session so as to drop your core body temperature. Warm hands and feet help your body’s core cool, inducing inviting sleep quickly and efficiently.

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.