Why we sleep

February 08, 2020 · 14 mins read By Matthew Walker

I. This thing called sleep

1. To sleep

  • We don’t sleep enough.
  • Lack of sleep has serious health effects.
  • We don’t understand why we sleep. It’s an evolutionary mystery that sleep survived.
  • Point of the book: present the author’s last 20 years’ of sleep research:
    • Effects on brain
    • Effects on body

2. Caffeine, Jet lan and Melatonin

Circadian rythm: ~24h rythm that regulates multiple processes in the body:

  • Sunlight (and other “zeitgeber” signals) sync it with the day/night cycle.
  • Regulates e.g. body temperature and other processes.
  • Exists independently of wether or not you get sleep.
  • Differs between people (“night owls” vs “morning larks”).
    • Genetic factors.
    • a.k.a. chronotype.
  • Regulated using melatonin.
  • Melatonin controls the timing of sleep. Not the quantity.
  • Jet lag: circadian rythm can be ajusted by ~1h/day

Sleep pressure: build up of adenosine to “trigger” sleep

  • Builds up when you’re awake, evacuated during sleep.
  • Receptors are blocked by caffeine.
    • Caffeine peaks 30min after ingestion
    • But has a half life of 5-7h! Genetically influenced
    • Most common cause of “insomnia”
  • Not enough sleep: leftover adenosine. “Sleep debt”.

The two cycles are not coupled. They are usually aligned.

Sleep cycles

3. Defining and generating sleep

Sleep characteristics:

  • Loss of external awareness (senses still work but signals are blocked in the brain).
  • Loss of conscious time sense. On a subconscious level we keep it (e.g. waking up 5 minutes before an important alarm)

Types of sleep:

  • NREM: (1-2: light NREM sleep, 3-4: deep NREM sleep).
  • REM sleep a.k.a. paradoxical sleep

Cycles of ~90 min. More REM-intense on later cycles of the night: the cycles are asymetrical.

We don’t know why we have cycles and why they are not symetrical.

Sleep brain wave patterns:

  • Awake ~ REM: incoherent, noisy signal, fast, chaotic.
  • Deep NREM: coherent, slow.

Awake: reception of signals NREM: Reflection (storing and strengthening signals) REM: integration

During REM, the body is paralized.

4. Ape beds, dinosaurs and napping with half a brain

In animals:

  • All forms of life “sleep” in one way or another.
  • There is no direct way to predict how much sleep a species gets: mystery.
  • Not all species experience REM. All mammals do.
  • Some species sleep with half a brain at a time. Shows the lengths to which nature will go to preserve sleep.

In early humans:

  • Hunter gatherers sleep in biphasic sleeps: 7-8h at night (2-3h after sunset) and ~1h nap in the afternoon.
  • Biphasic sleep is rooted in biology. E.g. our feeling alssep after lunch.
  • Still present in some cultures (e.g. “siesta”)

Humans are special: they have much more REM than other animals

5. Changes in sleep across life span

Before birth:

  • Most of the infant’s time is sleep.
  • NREM early on, a lot of REM just before birth.
  • REM is a “fertilizer” for neurons. Lays the path for many neurons to be able to connect later.
  • Disturbing REM in a featus impairs brain growth.
  • Alcohol disturbs REM. Both during pregnancy and nursing.


  • Infants start life with a polyphasic sleep: short naps, then awake.
  • As they get older, the circadian rythm sets in place.
  • Starts as 50/50 REM/NREM. Then REM declines over time


  • REM sleep during childhood creates a lot of neural pathways. NREM sleep during adolescence prunes them.
  • Adolescents have a later circadian rythm. They tend to sleep later and wake up later.
  • Circadian rythm slides back during early adulthood.

Midlife and old age

  • Need as much sleep as before but less able to generate it: getting less sleep.
  • Deep NREM sleep becomes harder to achieve. Losing up to 90% by age 70.
  • Sleep becomes more fragmented. People start waking up more often during the night (bathroom etc.)
  • Circadian rythm shifts back: early rise/early sleep.

II. Why should you sleep?

6. On the many benefits of sleep

For the brain:

  • Sleep before learning: sleep helps transfer memories from short term to long term memory:
    • Short term memory has a limited capacity to store information.
    • Sleep clears out short term memory and makes for better learning.
    • This is done by the “sleep spindles”.
  • Sleep after learning:
    • Deep NREM sleep consolidates memories.
  • Sleep to forget: memories “tagged” for forgetting are erased after sleep.
  • Sleep to enhance motor skills.

7. Sleep deprivation and the brain


Sleep depravation affects attention ~ as much as being legally drunk.

  • When sleep deprived: microsleeps happen. Loss of consciousness for ~ 2 seconds.
  • Symptoms start to show even after a single 4-6h sleep night.
  • 10 days of 6h sleep: same as 24h without sleep.
  • People are not aware of lost capabilities.
  • Sleeping more afterwards does not fix it.
  • Drowsy driving:
    • kills more than drunk driving.
    • is “more dangerous”: drunk drivers are slow to react. Drowsy drivers don’t react.

Can naps help?

Naps momentarily increase basic concentration if sleep deprived. But do not affect more complex function of the brain.

Emotional irregularity

  • Sleep gives emotional stability.
  • Both for healthy subjects and subjects with mental illnesses.

Sleep to remember

  • Sleep deprivation prohibits the formation of new memories.
  • All nighters for students are an awful way to learn durably.

Sleep and Alzheimer’s

Less sleep and Alzheimer’s function in a vicious cycle. There is a definite relationship between the 2, more complex than just causal. Note: Refer to book for more information.

8. Sleep deprivation and the body

For the cardiovascular system

  • Sleep deprivation (<6h sleep) linked to shorter life span, cardiac arrests and more.
  • Sympathetic nervous sytem:
    • responsible of triggering the “fight or flight” response
    • It is supposed to be active for minutes to hours.
    • Without sleep it is continuously active.
    • Triggers a high alert state that strains the cardiovascular system.
    • Deep NREM sleep resets it.

For diabetes

Sleep deprivation makes cells less responsive to insulin. This can lead to type 2 diabetes.

For diet

  • Short sleep increases hunger and appetite, and decreases impulse control.
  • This leads to more food consumption.
  • It also prevents effective weight loss under dieting (loss of muscle vs fat). This is caused by a hormone imbalance (leptin & ghrelin, cf. book)

For the reproductive system

  • On men, sleep deprivation causes a drop in testosterone. This leads to a feeling of fatigue, lower libido, sperm count and more.
  • On women, drop in follicular releasing hormone, irregular menstrual cycles…
  • People look less attractive when sleep deprived.

For the immune system

  • Sleep deprivation increses the likelihood of getting sick (see described experience with the flu).
  • Sleep helps boost the effect of vaccines.
  • Sleep loss is catastrophic for the development of cancers.

III. How and why we dream

9. REM-sleep dreaming

Your brain on dreams

  • MRI scans show:
    • Activation of visual, motor, emotional and autobiographical parts of the brain.
    • Deactivation of rational thoughts.
  • Prelimiary studies can’t see the content of people’s dreams by analyzing MRI.

    Meaning and content of dreams

  • Dreams were very mysterious throughout history, especially the origins of dreams.
  • Freud was the first to situate them in the brain (mind)
  • He also had a non scientific interpretation of dreams as unconscious unfullfilled wishes. This could not be confirmed or refuted by science, which led to the demise of the theory.
  • We now know that dreams do not reflect recent past events. They do however reflect the emotional concerns a person is going through.

10. Dreaming as overnight therapy

  • When recalling emotional memories, the events are recalled but the emotion is damped. This allows us to have a memory without being overwhelmed.
  • This process of stripping emotion from memory occurs during the REM sleep/dreams.
  • When dreaming, emotions and memory parts of the brain are active, but in absence of noradrenaline (stress hormone). This is a kind of “safe environment”.
  • PTSD patients can’t create this safe environment because the noradrenaline levels stay high. Thus they dream with the same emotions as the original events.
  • Dreaming also has a function of helping decode human emotions when awake.

11. Dream creativity and dream control

  • Dreaming phases of REM make the brain able to form more creative connections than awake or non REM.
  • This can lead to solving problems once thought to be hard, by making new connections.
  • Dream content matters: only when dreaming about a problem we can make these novel connections,
  • Lucid dreaming: it is a thing, but we don’t know much more than that.

IV. From sleeping pills to society transformed

12. Sleep disorders and death caused by no sleep

  • Somnambulism: brain alseep while body awake.
  • Insomnia: inability to sleep.
  • Narcolepsy: daytime sleepiness + sleep paralysis + cataplexy (muscles suddenly collapsing)
  • Fatal familial insomnia: inability to generate sleep at all. Deadly.

13. What’s stopping you from sleeping?

  • Artificial light: delays the release of melatonin. Blue light does that more.
  • Alcohol:
    • It’s a sedative: induces loss of consciousness but not sleep.
    • They look the same from outside but the mechanisms are not the same.
    • In particular, alcohol suppresses REM sleep!
    • It also makes the night’s sleep fragmented (but we don’t remember the fragmentation)
  • Body temperature:
    • The body temperature drops during sleep.
    • Sleeping in an environment with colder temperature makes it easier to fall asleep.
    • Hot showers make blood flow to the surface, which reduces core body temperature, making it easier to sleep.
  • Alarm clocks are an unnatural way to wake up.

14. Sleeping pills and therapy

  • Sleeping pills sedate (different than induce sleep). Same as alcohol: they look the same from outside but involve different processes in the brain.
  • Rebound insomnia: drug tolerance builds up. When people stop taking pulls, they suffer from insomnia as part of withdrawal. Than makes them go back to pills.
  • There are health risks associated with sleeping pills, but no direct causation shown.
  • Alternative to sleeping pills: Cognititive Behavioral Therapy for Insomnia (CBT-I)
  • Most helpful advice to have a good sleep: sleep and wake up at the same time, no matter what context.
  • Diet and exercise:
    • Sleep improves exercise and exercise improves sleep.
    • Effect on diet still not well understood.

15. Sleep and society

  • Lack of sleep is a global health epidemic in the West.
  • Sleep at the workplace: underslept employees are less valuable on many metrics. Cf. book for details.
  • Many companies still have a culture that undervalues sleep. Notable exceptions are starting to emerge, cf. Google, Nike, NASA.
  • Lack of sleep is often used as a torture instrument.
  • Early school start times are responsible for a large sleep deprivation in children.
  • Sleep deprivation in children can have the same symptoms as ADHD and can cause wrong diagnoses.
  • Sleep and healthcare: Looking into how medical training deprives the medical students from their sleep and how this can affect results of their training.

16. A new vision for sleep in 21st century

  • Change should come at different levels:
    • Individual
    • Educational
    • Organizational
    • Governemental
    • Societal
  • Individual transformation: vision of how a future with wearables and domotics can make the environment adapt to the circadian rythm.
  • Educational: we need to educate more poeple about the importance of sleep.
  • Organizational and higher, see book.

Appendix: Twelve Tips for Healthy Sleep

  1. Stick to a sleep schedule
  2. Exercise is great, but not too late.
  3. Avoid caffeine and nicotine.
  4. Avoid alcohol before bed.
  5. Avoid large meals and beverages late.
  6. If possible, avoid medicines that delay or disrupt sleep.
  7. Don’t take naps after 3pm.
  8. Relax before bed.
  9. Take a hot bad before bed.
  10. Dark bedroom, cool bedroom, gadget-free bedroom.
  11. Get sunlight exposure during the day.
  12. Don’t lie in bed awake. Anxiety about falling asleep makes it worse.