Why We Sleep

by Matthew Walker
ISBN: 9781501144318


One of the key takewaways from reading this book is realizing that sleep is important because it is an ancient evolutionary behavior shared by all living creatures (active/inactive phases in life). The book challenges us to consider how modern society has affected an important natural biological function that has evolved over millions of years (e.g. early school and work hours, stereotypes of night owls, stereotypes of sleep with being lazy, technology and digital distractions affecting sleep).

Sleep was the first state of life on this planet, and it was from sleep that wakefulness emerged.

The book explores the science of sleep and contains rich collections of scientific experiments which tests the importance of sleep in memory and health. During sleep, the body and brain undergo many biological events that repair and regulate the body. It is not surprising that insufficient sleep will lead to health problems (Alzheimer, anxiety, depression, stroke, chronic pain, cancer, diabetes, heart attacks, infertility, obesity, immune deficiency etc) since the body loses the opportunity to perform these functions.

The roles of wakefulness towards information processing are laid out in the book as:

  • Reception: experiencing and learning (awake).
  • Reflection: storing and strengthening experiences (NREM).
  • Integration: Integrating all memories and experiences in creative ways (REM).

I feel that this model is eerily similar to how AI and machine learning processes are organized, and the analogy/similarity probes some existential questions around simulation theory:

  • Data Input: Ingesting data from systems and the digital environment.
  • Training: Process data and store new datasets in the right storage format.
  • Learning: Depending on the algorithm, create new nodes and connections based on existing data.

This cycle repeats itself, and the ML models iteratively improves itself, just like the way we improve ourselves daily over sleep.


  1. Keep to a sleep schedule.
  2. Do not exercise near the time of sleep.
  3. Avoid caffeine and nicotine.
  4. Avoid alcoholic drinkgs before bed.
  5. Avoid food/drinks before bed (indigestion and urinating).
  6. Avoid drugs that disrupt sleep.
  7. Do not take naps after 3pm (affects sleep pressure and schedule)
  8. Relax before sleep.
  9. Take a hot bath before bed to drop core body temperature.
  10. Keep the room dark, cool and gadget/clock-free before sleeping.
  11. Have good sunlight exposure to allow daylight to regulate sleep patterns.
  12. If you cannot sleep, don't stay awake in bed. Do something relaxing to relief anxiety before feeling sleepy.


Sleep is largely driven by the interaction of the following mechanisms:

  • Circadian rhythm: Controls core body temperature and daily rhythm of activity and sleep.
  • Melatonin: affects the timing of sleep, but does not generate sleep. It is triggered by lack of sunlight.
  • Adenosine: builds up in the brain and creates sleep pressure.


Sleep is comprised of cycles of NREM and REM sleep, with the prior dominating the earlier phase of sleep. NREM sleep is responsible for transferring various short-term memory to long-term memory. REM sleep is a phase of sleep where the brain is actively 'awake', but motor and logical controls are shut off.

NREM sleep seeks to reduce and preserve neural connections while REM sleep is responsible for stimulating and building new connections. A developing baby in the third trimester is marked with rapid brain development influenced by heavy REM sleep.

Dreams occur in REM sleep and resemble an active brain state but without logic/reasoning restrictions being applied. It allows individuals to synthesize new creative thoughts, memories and experiences, otherwise unavailable in the narrow hyperfocused and myopic awakened state. People lacking REM sleep usually experience psychosis and delirium as the brain tries to refuel its REM-deprived sleep while awake.

REM sleep is what stands between rationality and sanity.


Caffeine competes and inactivates adenosine receptors, therefore blocking sleep pressure. Since adenosine is still constantly building up, a 'caffeine crash' will be experienced at some point.

Alcohol sedates you out of wakefulness, but it does not induce natural sleep. Therefore, the bodily functions and health benefits of natural sleep will not be experienced. Alcohol fragments sleep with brief awakenings, and is a strong suppressor of REM sleep.

During sleep, the brain clears up amyloid and tau proteins that are toxic. When we fall ill, the immune system stimulates the sleep system for the body to rest and recover.

Wakefulness is low-level brain damage, while sleep is neurological sanitation.

Individuals who are consistently sleep deprived will accept this state as the new norm, preventing them from ever recovering the sleep they lost. Less sleep leads to an overactive sympathetic nervous system. This triggers nonspecific states of chronic inflammation, and lead to various health problems (e.g. diabetes, cancer).

Sleep/overnight therapy is gaining popularity as REM-dreaming creates a brain state of low-anxiety (noradrenaline is shut off during dreaming). Sleep allows strengthening important memories and knowledge, forgetting painful and emotional memories. Sleep therapy has been used successfully on PTSD patients by allowing them to gain confidence in incremental sleep, and allowing the sleep mechanisms to slowly repair the brain and affected memories.


Night owls and morning larks sleep behaviors are driven by genetics. The work culture is biased against owls, with emphasis on early work hours and stereotyping late mornings as being irresponsible/lazy.

Humans are biphasic creatures, as evidenced in our circadian rhythm (hence the desire to nap in the afternoon after lunch). However, modern society has shifted our sleep behaviors to being monophasic.