📚 Notes for: Why We Sleep 👤 Author: Matthew Walker 📅 Notes created: Apr 21, 2026

📝 SUMMARY ──────────

• Two main factors determine when you want to sleep or be awake.
  • Signal beamed out from our internal 24-hour clock.
  • Chemical substance that builds up in our brain and creates “sleep pressure”. Longer we are awake more chemical builds up.
• 24-hour rhythm is also known as circadian rhythm
• 24-hour biology clock sitting in the middle of the brain is called suprachiasmatic.
• Suprachiasmatic nucleus communicates its repeating signal of night and day to your brain and body using a circulating messenger called melatonin.
• Melatonin simply provides the official instruction to commence the event of sleep to other regions of brain but does not participate in the sleep itself.
• Adenosine is the chemical that builds in the brain every waking minute. It increases the desire to sleep along with the circadian rhythm. It is also known as sleep pressure.
  • We can artificially mute adenosine by using caffeine.
  • Caffeine has half-life of 7-8 hours. It takes much longer for the brain to clear caffeine.
  • Caffeine is removed by an enzyme that is generated by the liver.
  • Aging also alters the speed of caffeine clearance; and thus, more sensitive we become in later life to caffeine’s sleep-disrupting influence.
• 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 these raw ingredients with each other, with all past experiences, and, in doing so, building an ever more accurate model of how the world works, including innovative insights and problem-solving abilities).
• Cetaceans like dolphin's sleep is unihemispheric, meaning they will sleep with half a brain at a time. One half of a brain must stay awake to maintain life-necessary aquatic movement.
• Birds can do it too. When birds are alone, one half of the brain and its corresponding (opposite-side) eye must stay awake, maintaining vigilance to environmental threats. As it does so, the other eye closes, allowing its corresponding half of the brain to sleep.
  • Things get even more interesting when birds group together. In some species, many of the birds in a flock will sleep with both halves of the brain at the same time. How do they remain safe from threat? The answer is truly ingenious. The flock will first line up in a row. With the exception of the birds at each end of the line, the rest of the group will allow both halves of the brain to indulge in sleep. Those at the far left and right ends of the row aren't so lucky. They will enter deep sleep with just one half of the brain (opposing in each), leaving the corresponding left and right eye of each bird wide open. In doing so, they provide full panoramic threat detection for the entire group, maximizing the total number of brain halves that can sleep within the flock. At some point, the two end-guards will stand up, rotate 180 degrees, and sit back down, allowing the other side of their respective brains to enter deep sleep.
  • Humans have very mild version of unihemispheric sleep. If you compare the electrical depth of the deep NREM slow brainwaves on one half of someone's head relative to the other when they are sleeping at home, they are about the same. But if you bring that person into a sleep laboratory or take them to a hotel-both of which are unfamiliar sleep environments-one half of the brain sleeps a little lighter than the other, as if it's standing guard with just a tad more vigilance due to the potentially less safe context that the conscious brain has registered while awake. The more nights an individual sleeps in the new location, the more similar the sleep is in each half of the brain. It is perhaps one reason why so many of us sleep so poorly the first night in a hotel room.
  • Unihemispheric sleep is only possible during NREM stage.
• As unifying as sleep is across animal kingdom, yet within the species there is a remarkable diversity in amount (example time), form (example half-brain, whole brain) and pattern (monophasic, biphasic, polyphasic)
• Humans spend 20-25% of their sleep in REM stage when compared to 9% across all other primates (apes)
• Sleep, specifically REM sleep fuels creativity.
  • While NREM sleep helps transfer and make safe newly learned information into long-term storage sites of the brain; REM sleep takes these freshly minted memories and begins colliding then with the entire catalog of your life’s autobiography. These mnemonic collisions during REM sleep spark new creative insights as novel links are forged between unrelated piece of information.
  • This is why at times we wake up with new solutions to previously intractable problems or even be infused with radically new and original ideas.
• Sleep across our lifespan
  • Prior to birth, humans spent all their time in a sleep like state, much of which resembles REM sleep.
• Autistic children. Exhibit 30 to 50% deficit in the amount of REM sleep they obtain relative to non-autistic children.
  • Considering the role REM sleep plays in establishing the balanced mass of synaptic connections within a developing brain, there is now keen interest in discovering whether or not REM sleep deficiency is a contributing factor to autism.
  • Existing evidence in humans is simple correctional.
• Alcohol is one of the most powerful suppressors of REM sleep that we know of.
• Older adults have 3 nighttime issues.
  • Loss of deep sleep - quality and quantity - due to process called atrophy - some parts of the brain that start losing neurons much earlier and faster than other parts of the brain. This deterioration usually starts at middle-frontal region of the brain (few inches above bridge of the nose), same region responsible for deep NREM sleep.
  • Early sleep onset and offset - due to change in circadian rhythm.
  • Sleep fragmentation - waking up in between sleeps - weakening gallbladder causing bathroom breaks.
• Deep sleep is critical for cementing new memories and retaining new facts.
• Poor memory and poor sleep-in old age are significantly interrelated.
• Newly learned facts are stored in hippocampus which has limited storage.
  • Sleeping helps move the information into permanent store freeing up memory storage in hippocampus.
  • Stage 2 NREM helps with this.
• NREM wins out in providing superior memory retention savings relative to late night REM-rich sleep.
• Learned facts are retrieved from different areas of brain based on where we have slept or not. Before sleeping it is retrieved from hippocampus (temporary warehouse); after the sleep information is retrieved from neocortex (long term storage site)

Sleep is constantly modifying the information architecture of the brain at night.

• Brain continues to improve our skill memory (playing pool, riding bike, …) in absence of any further practice during sleep.
• Practice does not make you perfect; practice followed by a night of sleep leads to perfection.
• 30 years of intensive research answers
  • After 16 hours of being awake, brain begins to fail.
  • After 10 days of just 7 hours of sleep, the brain is as dysfunctional as it would be after without sleep for 24 hours.
  • 3 full nights of recovery are insufficient to restore performance.
  • Human mind cannot accurately sense how sleep-deprived it is when sleep deprived.
  • Amygdala- a key hot spot for triggering strong emotions such as anger and rage, are linked to the fight or flight response - showed well over a 60% amplification in emotional reactivity in the participants who were sleep deprived.
  • Striatum- hot spot for impulsivity and reward, is bathed with chemical dopamine- also becomes hyperactive when sleep deprived.

Lack of sleep sets you on a path towards diabetes and leads you to obesity is now well understood and incontrovertible.

• Chronic sleep deprivation is now recognized as one of the major contributors to the escalation of type 2 diabetes.
• Lepton signals sense of feeling full and Ghrelin in contrast triggers strong sensation of hunger.
  • Inadequate sleep decreased concentration of the satiety-signaling hormone lepton and increased levels of hunger-instigating hormone ghrelin.
• 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. Lean and toned is unlikely the outdone of dieting when you are cutting sleep short.
• There is a clear and linear relationship of sleep with infection rate. In the experiment conducted, those sleeping 5 hours or less on average, infection rate was almost 50% when compared to participants who slept 7 hours or more, whose inject rate was just 18%.
• REM sleep can be considered as a state characterized by activation in visual, motor, emotional, and autobiographical memory region of the brain, yet a relative deactivation in regions that control rational thought.
  • Using MRI, we can guess the type of dream a person has, based on the regions that were activated during REM sleep - would it be visual or motoric or awash with emotions or completely irrational and bizarre.
• In an experiment 35-55 percent of emotional themes and concerns that participants were having while they are awake during the day powerfully and unambiguously resurfaced in the dreams they were having at night.
• Function of dream
  • Nursing of emotional and mental health
  • Problem solving and creativity.
    • 2 hypotheses
      • 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 equivalent amount of deliberative time spent awake.
      • Content of people’s dreams, above and beyond simply having REM sleep, should determine the success of those hyper-associative problem-solving benefits.
• REM sleep dreaming accomplishes two critical goals.
  • Sleeping to remember the details of those valuable salient experiences, integrating them with existing knowledge and putting them into autobiographical perspective.
  • Sleeping to forget, or dissolve, the visceral, painful emotional charges that had previously been wrapped around those memories.
• Higher quantity of noradrenaline causes lower REM sleep quality which in turn increases PTSD patient's symptoms. Prazosin helps get bigger quality REM sleep for these patients.
• REM sleep plays a role in in understanding facial cues. In its absence, we can no longer distinguish one emotion from another with accuracy. Tuning V of the brain had been changed, rudely pulled all the way up from the base and fattened into a horizontal line, as if brain was in state of hypersensitivity without the ability to map gradations of emotional signals from the outside world. Brain’s emotional navigation system had lost its true magnetic north of directionality and sensitivity.

Have a problem? “Sleep on it” Interestingly this phrase or something close exists that in most languages, indicating that the problem-solving benefits of dream sleep is universal, common across the globe.

• Dreaming takes an approach of interrogating our recent autobiographical experience and skillfully positioning it within the context of past experiences and accomplishments, building a rich tapestry of meaning.
  • 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 these newly experienced problems in the future.
• 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.
• National sleep foundation website - http://sleepfoundation.org
• Sleep disorders
  • Somnambulism refers to sleep (somuns) disorders that involve some form of movement (ambulation), like sleep walking, talking, …
    • Most people believe these events happen during REM sleep as an individual is dreaming, and specifically acting out ongoing dreams. However, all these events arise from the deepest stage of non-dreaming sleep.
    • While we do not yet fully understand the cause of somnambulism episodes existing study evidence suggests that an unexpected spike in nervous system activity during deep sleep as one trigger.
  • Insomnia- suffering from an inadequate ability to generate good sleep quality or quantity despite allowing oneself adequate opportunity to get sleep.
    • 2 subtypes
      • Onset which is difficulty falling asleep
      • Maintenance or difficulty staying asleep.
    • Two most common triggers of chronic insomnia are.
      • Emotional concerns or worry.
      • Emotional distress or anxiety
    • Fatal familial insomnia condition where a person cannot go to sleep and eventually dies.
      • It is caused by a genetic defect causing PrNP (prion protein) to mutate and spread like virus. Protein begins targeting and destroying certain parts of the brain, resulting in rapidly accelerating form of brain degradation. It made holes in thalamus.
• Thalamus- the sensory gate within the brain that must close shut for wakefulness to end and sleep to being.
• In humans, balance between wakefulness and sleep appears to be around sixteen hours of total wakefulness and between seven and nine hours of total sleep.
• The light receptors that communicate “daytime” to the suprachiasmatic nucleus are most sensitive to short-wavelength light within the blue spectrum - the exact sweet spot where blue LEDs are most powerful. As a consequence, evening blue LED light has a more harmful impact on human nighttime melatonin suppression than the warm, yellow light from old incandescent bulbs, even when their lux intensities are matched.
• iPad enriched with blue light, used 2 hours prior to bed blocked otherwise raising melatonin level by 23%
• Alcohol affects prefrontal cortex, which controls our impulses and restrains our behavior. Alcohol immobilizes this part of the brain first. As a result, we “loosen up”, becoming less controlled and more extroverted.
• Alcohol does not induce natural sleep. Even moderate amount of alcohol in the afternoon and or evening deprives us of dream sleep. The pent-up REM-sleep pressure erupts forcefully into waking consciousness, causing hallucinations, delusions and gross disorientation.
• Thermal environment, specifically the proximal temperature around your body and brain, is perhaps the most underappreciated factor determining the ease with which you will fall asleep and the quality of sleep you will obtain.
• To successfully initiate and maintain sleep into the night, your core temperature needs to be decreased by 2-to-3-degree Fahrenheit; also, why it's easier to sleep in a cold room than hot.

Splashing water on face a night has sleep-inviting powers, water hot or cold, helps dissipate heat from the surface of the skin as it evaporates, there by cooling the inner body core.

• Ideal optimal temperate for good sleep is 65 degrees Fahrenheit. Differs a bit based on age, gender, …
• Hot bath helps us sleep faster. Contrary to popular belief which if because you feel warm and toasty. Hot bath invites blood to the surface of your skin, giving you that flushed appearance. When you get out of 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. Hot baths prior to bed can also induce 10 to 15 percent deeper NREM sleep in healthy adults.
• Sleeping pills attack the same region of brain as alcohol and so do not provide natural sleep.
• Cognitive behavioral therapy for insomnia (CBT-I) - Obvious method involves reducing caffeine and alcohol intake, removing screen technology from the bedroom and having cool bedroom. In addition, patients must
  • Establish a regular bedtime and wake up time, even on weekends.
  • Go to bed only when sleepy and avoid sleeping on the couch early/mid-evenings.
  • Never lie awake in bed for significant time period; rather, get out of bed and do something quire and relaxing until the rude to sleep returns.
  • avoid daytime napping if you are having difficulty sleeping at night.
  • Reduce anxiety-provoking thoughts and worries by learning to mentally decelerate before bed.
  • Remove visible clock faces from view in the bedroom, preventing clock-watching anxiety.
• Try not to exercise before bed. Body temperature can remain high for an hour or 2 after physical excretion.
• REM sleep is what stands between rationality and insanity. Missing 3 nights of REM sleep can cause us to become anxious, moody, and start to hallucinate.
• Sleep and education
  • One study tracked more than 5000 Japanese students and discovered that those individuals who are sleeping longer obtained better grades across the board.
  • Schools in US that have shifted the start time by an hour had noticed significant jump in grades.
  • Added reason to make sleep a top priority in education and lives of our children concerns the link between sleep deficiency and epidemic of ADHD (attention deficit hyperactivity disorder)
    • Adderall and Ritalin are the common drugs for ADHD. They both contain a stimulant that prevents sleep and keep the brain of an adult or child wide awake. This is the last thing a child needs.