Nepali Wanderer

Introduction

We spend one-third of our lives sleeping. I know it’s fun to sleep and all, but one-third is a huge number. This substantial portion of our lives dedicated to sleep suggests it must have some evolutionary benefits. (In line with: “Everything in biology makes sense when viewed through the lens of evolution.”)

Matthew Walker divides his book into four parts, and I will follow the same structure in my summary.

Part I: What is Sleep? Different States of Sleep. Mechanisms that Regulate Our Sleep.

Our body has two states: awake and asleep. Sleep itself is divided into two broad states: Deep Sleep and Dream State (REM sleep). Deep Sleep further subdivides into four stages, illustrating the complexity of sleep studies. Three main criteria help us understand these states: eye movement, brain waves, and muscle movement.

When we are awake, our brain waves are chaotic and random, and our eye and muscle movements reflect our activities. In contrast, during sleep, these criteria change. Sleep cycles between two main stages: Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM). NREM corresponds to Deep Sleep, characterised by regular brain waves and relaxed muscles. REM, associated with dreaming, features chaotic brain waves similar to wakefulness but with paralysed muscles to prevent us from acting out our dreams.

But why do these phases exist, and why in a cyclical manner? While theories abound, Walker proposes that the awake state is when we receive external signals. During Deep Sleep (NREM), we reflect on these signals and selectively store them in long-term memory. NREM blocks sensory pathways to the brain, while REM allows internal signals — our memories and emotions — to enter, reinforcing and modeling them during dreams. The muscle paralysis during REM ensures we don’t physically act out our dreams.

Two mechanisms regulate sleep: the circadian rhythm (that is, our body’s internal clock), and adenosine levels in the brain. These mechanisms operate independently but together regulate sleep. This independence (ignorance of each other) explains phenomena like jet lag, where our circadian rhythm only is disrupted, and the effect of caffeine, which blocks adenosine pathways, temporarily preventing the feeling of tiredness.

Interestingly, individual sleep patterns vary genetically. Walker links this to our prehistoric days when diverse sleep cycles in a group enhanced overall survival by ensuring someone was always alert. This insight suggests that instead of stigmatising the night owls for being lazy and doing it by their choice, we should accommodate different peak times for productivity.

Sleep patterns also change over a lifetime. In the womb, we are mostly asleep, with REM dominant towards the end. Then comes Childhood (dominated by NREM), adolescence (dominated by REM), and old age (when sleeping gets difficult). The author also talks about sleep patterns in other animals, as sleep is seen in almost all species, even in invertebrates. But there is variation in sleep patterns, in terms of duration (like 18 hours of sleep for tigers and lions), no sleep during emergencies (like birds), and different levels of REM (dream) and Non-REM.

Interestingly, since humans dream way more than other species, the author highlights the evolutionary benefit dreams must have had in human dominance over other animals.

Part II: Why Do We Sleep? Memory Retention, Skill Learning, and the Effects of Sleep Deprivation.

Sleep profoundly affects information filtering, memory retention, and skill learning. NREM sleep transfers contents from short-term to long-term memory, ensuring information is stored and freeing up working memory space. It also selectively retains important information. Additionally, sleep also improves our skill learning by making our motor routines ingrained at the instinctive level. (There is no such thing as muscle memory, as everything has to be stored in the brain.) Sleep also aids creativity through diverse neuronal connections during REM sleep.

Lack of sleep not only diminishes energy and performance but also impairs memory and emotional stability. Walker links sleep deprivation to various diseases, including Alzheimer’s, obesity, diabetes, and even cancer. Apart from affecting blood sugar levels and pressure, lack of sleep leads to increased hunger, thereby increasing our affinity towards empty less nutrient calories. If you’re on a diet, it’s even worse, as lack of sleep makes you lose muscle mass instead of fat. This all leads to the whole cycle of diseases of civilisation: obesity-diabetes-cardiovascular diseases. Lack of sleep also leads to the reduction of your testosterone level, and you know the rest of the story from here. It also increases your risk of cancer, and if everything wasn’t worrying enough already, it could even lead to random genetic changes and alterations!

Part III: What is a Dream? Why Do We Dream? How Much Do We Understand Dreams?

Dreams occur during REM sleep. All brain regions, except those related to rationality, are active during REM, leading to bizarre dreams. Contrary to the belief that dreams are residual daily flashbacks, they are chaotic but often emotionally linked. Techniques like pattern-matching MRI have enabled scientists to decipher dream content to some extent.

(The author picks out the Japanese technique of reading dreams using labeled data and pattern-matching machine learning techniques. You collect the content/labels of the dream experienced by the person with his/her brain MRI images during the dream. From a large enough sample, you associate different labels with different image patterns seen in MRI. The technique is good enough to know a person saw a car in his/her dream, but not detailed enough to know which car.)

Dreams serve several purposes. They help cleanse emotions by releasing emotional strain, so that only the memory of past trauma remains (but not the attached emotions). This does not happen easily for PTSD patients with much intense stress, hence them going through traumatic flashbacks. Dreams also fine-tune the ability of brain to detect even the most subtle emotions, which is important to smoothly navigate our social and often chaotic lives. And finally, dreams also foster creativity by making random connections between new and past information, leading to creative insights. (There are many stories of artists and scientists using sleep and dreams for inspiration. Google Salvador Dali using sleep techniques for inspiration!)

Part IV: Sleep and Our Modern Culture

More than two-thirds of adults in developed countries get inadequate sleep, largely due to modern culture and lifestyle. Factors include early school start times, work cultures that glorify overwork, and gadget use exposing us to LED light even in bed. Medical professionals, in particular (and sadly, ironically), suffer from sleep deprivation due to long hours and night shifts.

The author recommends several strategies for improving sleep, such as sleeping in a dark, cool room, taking hot showers before bed, and avoiding LED exposure before sleep. He envisions a society that prioritises sleep awareness at personal, educational, policy, and societal levels, recognising its importance for physical and mental well-being.

Conclusion

For a long time, the evolutionary purpose of sleep remained a mystery. Now, we understand that sleep is a multifaceted gift from evolution, crucial for our well-being. Walker emphasises that sleep is the most important of the three pillars of health, the other two being balanced diet and exercise. Ironically, for so much to their intelligence, humans are the only species that deliberately deprive themselves of sleep.

My Takeaway

This book has been enlightening on so many grounds, from viewing sleep through an evolutionary lens to understanding my own behaviors, such as increased irritability when sleep-deprived. Also, knowing individual sleep pattern is genetic encourages us to find what works best for each of us (rather than enforcing uniformity.)

It has prompted me to tweak some lifestyle changes, like avoiding caffeine later in the day. And above all, it has reinforced the need to prioritise 7-8 hours of sleep as the best investment in my daily routine.

PS: This aligns with my personal life style goal of living without an alarm clock (which sadly I still need to at times).