Why We Sleep

Unlocking the Power of Sleep and Dreams

By Matthew Paul Walker, PhD


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Originally published

Oct 3, 2017


Mental Health


Apr 2, 2020


Apr 21, 2020

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Charlotte Brontë: "a ruffled mind makes a restless pillow" (3)
"human beings are in fact the only species that will deliberately deprive themselves of sleep without legitimate gain" (4)
There are losses to productivity, an hour more of work is not a constant unit of measure for output, especially for more creative endeavors
"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...physical diseases and mental disorders." (4)
"sleep recalibrates our emotional brain circuits" (7)
"a consoling neurochemical bath that mollifies painful memories and a virtual reality space in which the brain melds past and present knowledge, inspiring creativity." (7)
Objective: "revise our cultural appreciation of sleep, and reverse our neglect of it" (8)
Appreciation is such a powerful word. We should love sleep. Crave it. Prioritize it. Put it in the calendar. Set alarms for going to sleep instead of just waking up.
Daylight savings time is a global experiment and we can easily see negative impacts in the data: there are more traffic accidents and heart attacks when people lose an hour. "Most people think nothing of losing an hour of sleep for a single night, believing it to be trivial and inconsequential. It is anything but." (169)
"Emotions make us do things" (247); with sleep disorders and abuse, we lose our ability to have certain strong emotions or control certain strong emotions → inability to act optimally
Within the space of a mere hundred years, human beings have abandoned their biologically mandated need for adequate sleep—one that evolution spent 3,400,000 years perfecting in service of life-support functions. As a result, the decimation of sleep throughout industrialized nations is having a catastrophic impact on our health, our life expectancy, our safety, our productivity, and the education of our children. (340)
Science behind sleep
Circadian rhythm: "signal beamed out from your internal twenty-four-hour clock located deep within your brain" (13)
"controls other rhythmic patterns, too. These include your timed preferences for eating and drinking, your moods and emotions, the amount of urine you produce, your core body temperature, your metabolic rate, and the release of numerous hormones." (14)
Core body temperature drops during times we should be sleeping
Not dependent entirely on daylight (even in full darkness we wake and sleep on a cycle) though it is influenced by the sun
Our natural circadian rhythm is actually slightly greater than 24 hours; "the light of the sun methodically resents our inaccurate internal timepiece each and every day, 'winding' us back to precisely, not approximately, twenty-four hours." (17)
"the brain can also use other external cues, such as food, exercise, temperature fluctuations, and even regularly timed social interaction" for regulating this internal clock (18)
Occurs in the suprachiasmatic nucleus (18)
"The prefrontal cortex controls high-level thought and logical reasoning, and helps keep our emotions in check." When we sleep not in sync with our circadian rhythm, the prefrontal cortex might not be at full operating capacity at the start or end of our days (21)
For example, this is why "night owls" genetically struggle in mornings; "greater ill health caused by a lack of sleep therefore befalls owls, including higher rates of depression, anxiety, diabetes, cancer, heart attack, and stroke" (21)
Melatonin: regulates timing but not generation of sleep; released when the day darkens signaling that it is time to sleep
Adenosine: "chemical substance that builds up in your brain and creates a 'sleep pressure.'" (13)
Builds up ~ logarithmically the longer you go without sleep
"artificially mute the sleep signal of adenosine by using a chemical that makes you feel more alert and awake: caffeine." Caffeine can replace adenosine in receptors in the brain
"Caffeine has an average half-life of five to seven hours." Different people can process it at different rates via enzymes in the liver (they have a more efficient version of the enzyme). (28-29)
We become slower at processing caffeine as we age. (29)
Adenosine still builds up regardless of how much caffeine is in our system → can lead to a crash when the caffeine is cleared out and adenosine fills the receptors
"During sleep, a mass evacuation gets under way as the brain has the chance to degrade and remove the day's adenosine." (33)
The cycle of melatonin generation and the circadian rhythm are what makes dealing with jet lag particularly hard
"For every day you are in a different time zone, your suprachiasmatic nucleus can only readjust by one hour at a time" (25); "the eastward direction requires that you fall asleep earlier than you would normally, which is a tall biological order for the mind to simply will into action." (25)
Travel thus puts us through biological stress which can harm brain cells (26)
Adenosine and the circadian rhythm operate independently, though in an ideally correlated fashion. Adenosine may start building up in the morning, but the circadian rhythm at that time is dipping, enlarging the gap between adenosine levels and the sin curve of the rhythm. The closer the two curves are, the more tired you feel. (34)
If you could go back to sleep around 11am in the morning or if you need caffeine before 12, you probably are not getting enough sleep (35)
Thalamus: blocks perceptual signals coming into the brain during different parts of sleep stopping them from traveling up the cortex
Polysomnography: graphing sleep via brainwave activity, eye movement, and muscle activity (41)
"humans (and all other species) can never 'sleep back' that which we have previously lost" (64) though we do try to, and succeed in part
Sleep is an interesting state. When we look at someone who is asleep, we can instinctually pick up on cues that they are not dead. We also "return to the waking world knowing that you have been asleep." (39)
Phases of sleep
Consciously, we struggle to keep track of time during sleep. Subconsciously, we can actually be pretty precise (like when you wake up right before your alarm goes off); "Unlike the phase of sleep where you are not dreaming, wherein you lose all awareness of time, in dreams, you continue to have a sense of time. It's simply not particularly accurate—more often than not dream time is stretched out and prolonged relative to real time." (41)
Rapid eye movement (REM) sleep: "brain activity [is] almost identical to that when we are awake...and is often described as dream sleep" (42); "plays a role in strengthening [neural] connections" (45)
We learn from signals within our brain rather than external stimuli; "signals of emotions, motivations, and memories (past and present) are all played out on the big screens of our visual, auditory, and kinesthetic sensory cortices in the brain...highly associative carnival of autobiographical themes...building an ever more accurate model of how the world works, including innovative insights and problem solving abilities." (53)
You are completely paralyzed during REM sleep excepting in some (often older) people where dream-related impulses can turn into real-world action (54)
"emotional regions [amygdala, cinagulate cortex] of the brain are up to 30 percent more active in REM sleep compared to when we are awake" (195); when analyzing dreams we see some combination of visual activity + motor activity (signals in brain don't really change, they just aren't propagated to the muscles) + emotional activity
"journaling your waking thoughts, feelings, and concerns has proven mental health benefit, and the same appears true of your dreams" (203)
Edison would nap at his desk holding ball bearings above a pan → "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 fo the creative ideas that were flooding his dreaming mind." (232)
Others including Salvador Dalí engaged in similar practices. It's known as a Hypnagogic Nap.
"Concentrations of a key stress-related chemical called noradrenaline are completely shut off within your brain when you enter this dreaming sleep state" (~ equivalent of epinephrin but for the brain) → "emotional memory reactivation...occurring in a brain free of a key stress chemical" (208). Thus in dreams we can interact with memories and store them in a more useful way: "You have not forgotten the memory, but you have cast off the emotional charge, or at least a significant amount of it...We can therefore learn and usefully recall salient life events without being crippled by the emotional baggage that those painful experiences originally carried." (209)
Explicitly dreaming about painful experiences over time can improve our ability to recover from them mentally (211)
You have to actually dream about something to derive creative benefits
"PTSD patients had higher-than-normal levels of noradrenaline released by their nervous system" → "brain that has not detoxed the emotion from the trauma memory" (212)
We have less hierarchical associativity in logic while in REM sleep (225) → take a more wide-angled lens when considering problems
"Computer files sit like isolated islands. Our human memories are, on the other hand, richly interconnected in webs of associations that lead to flexible, predictive powers." (228) If anything, our ability to index information is helping computers move in this direction as well.
Non-rapid eye movement (NREM) sleep: "a key function...is to do the work of weeding out and removing unnecessary neural connections"
Slow, synchronous, and reliable brain waves (48)
"severing perceptual ties with the outside world" (51) → effectively lose consciousness
"think of each individual slow wave of NREM sleep as a courier, able to carry packets of information between different anatomical brain centers...state of inward reflection—one that fosters information transfer and distillation of memories" (52)

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