Sleep is universal in animals (even in insects and worms). These deep biological roots suggest a vital function and that it isn’t simply a vestigial byproduct of evolution.
Humans in today’s nutrient-rich environment need 8 hours of sleep to function optimally.
Sleep has two general types - NREM and REM.
**Sleep deprivation shows consistently bad...
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(Chapter 1 is just an introduction, so we’re skipping it.)
Sleep is universal in animals, even in insects and worms, despite its apparent drawbacks (vulnerability to predators, loss of time for productivity). When a biological feature is preserved deep in evolutionary history, it is usually a critical function. This must mean sleep is a critical function, and it’s important to understand why it’s important.
Sleep is regulated by two mechanisms:
These two mechanisms interact as shown in this...
Now you understand how your sleep rhythm gives a regular schedule of sleep from night to night. Next, we’ll look into how, within a single night of sleep, your brain cycles between different types of phases of sleep. This is important to understanding the function of sleep for your brain.
In summary, your brain switches between two types of sleep - REM (rapid eye movement) and non-REM (NREM) sleep. The two types of sleep have different functions - simplistically:
In total for a single night, there’s about an 80/20 split between NREM/REM sleep.
When you sleep, your brain goes through sleep cycles that each last about 90 minutes. Each sleep cycle generally begins with NREM sleep, then ends with REM sleep. As one cycle ends, the next begins. You can see this in a sleep graph here:
A deeper line means going deeper into NREM sleep.
Sleep is present in all animal species, even invertebrates. And bacteria that survive for longer than 24 hours have circadian-like rhythms. As we’ve said before, this suggests that it’s universally critical for survival.
We ask all the time why we sleep. One researcher posed an interesting inversion of the question - if wakefulness is damaging to the body and sleep recovers it, why did life ever bother to wake up? (Shortform note: of course, you can’t be productive and reproduce when sleeping, so sleeping too much would be evolutionarily disadvantageous.)
The amount of sleep per day varies from 4 hours in elephants to 19 hours in bats. There are no strong correlations between animal characteristics and amount of sleep, though within animals of a similar size, a more complex brain increases sleep.
Among animals, REM sleep occurs only in birds and mammals. Because REM evolved independently in these two distant evolutionary trees, REM likely performs a critical function that NREM cannot accomplish, or that REM is more efficient at accomplishing.
Interesting animal sleep patterns
Fetuses spend almost all of the time in a sleep-like state. It doesn’t yet have the part of the brain that causes muscle-atonia during sleep, thus explaining why babies in the womb kick and punch.
During the last 2 weeks of pregnancy, REM sleep in fetuses ramps up to 12 hours a day. This causes rapid synaptogenesis and building of neural pathways throughout the brain. In experiments with rat fetuses, disturbing REM sleep stalls construction of the cerebral cortex.
Alcohol impedes REM sleep in fetuses and babies, causing abnormal synaptogenesis. Once disrupted, a fetal brain may never fully regain normal function.
Because REM sleep is involved in emotional recognition and social interaction, disrupting REM sleep in utero might contribute to the autism spectrum.
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Getting good sleep improves your brain in these ways:
1) Sleep improves long-term factual recall
Your brain stories different memories in different places. The hippocampus stores short-term memory with a limited capacity; the cortex stores long-term memory in a large storage bank.
The slow-wave, pulsating NREM sleep moves facts from the hippocampus to the cortex. This has two positive effects: 1) it secure memories for the long term, and 2) it clears out short-term memory to make room for new information and improves future learning.
Have you ever woken up recalling facts that you couldn’t have recalled before sleeping? Sleep may make corrupted memories accessible again.
While good sleep improves memory, sleep deprivation can prevent new memories from being formed. In part, this might be because the hippocampus becomes less functional with less sleep, partially because lack of NREM sleep prevents solidifying of new memories.
Unfortunately, making up a sleep deficit later doesn’t help recover a previous days’...
While getting great sleep is good for the brain, sleep deprivation is unambiguously harmful to the brain. We’ll show damage in three ways: to attention, to emotion control, and for Alzheimer’s Disease.
Sleep deficits are very bad for attention and concentration. This is especially harmful during high-risk activities, like driving.
Here are some ways to put the risk into perspective:
Sleep deficits add up over time, and performance progressively worsens with greater sleep deficit. Having 10 nights of 6-hour sleep is equal in damage to one all-nighter, as is 6 nights of 4-hour sleep.
Why does sleep cause more accidents? Part of it is delayed reaction time. Another part is a “microsleep,” where your eyelids shut for just a few seconds and you go unconscious and lose motor control. If you’re in...
In addition to the damage it causes to the brain, sleep deprivation disrupts the normal function of many physiological processes, likely contributing to chronic diseases. In this chapter, we’ll cover a bevy of health issues associated with sleep deprivation.
At a high level, sleep deprivation of even just 1-2 hours triggers the sympathetic nervous system (fight or flight response) and disrupts hormonal balances. This also implies that sleep is necessary for the normal maintenance of physiology.
Many of the population studies cited in Why We Sleep are correlational - e.g. people who sleep less are more likely to have heart disease, after controlling for many other factors. But the causation is unclear - some other factors that predispose people to get heart disease (like a high baseline level of stress) could also reduce sleep.
To address this, the experimental studies cited attempt to link lack of sleep to a middle physiological state, which itself is causative for the disease. For instance, lack of sleep increases blood pressure, which the medical consensus believes is causative for heart disease.
Ideally, the “smoking gun”...
Dreaming is a bizarre sensation. You’re unconscious, but you perceive intense vivid sensations and hallucinate things that aren’t there. You feel like you’re moving in the world, but your muscles have been forced to be limp. You remember faces and memories that you haven’t thought about for years, maybe decades. You had no control over your emotions, swinging from intense rage and jealousy to exuberance. Finally, when you woke up, you promptly forgot everything. If you experienced all of this while awake, you’d think you had a psychosis episode!
It’s not surprising then that dreaming has had a complicated history. In the ancient past, Egyptians and Greeks wondered if dreams were divine gifts from gods.
Freud helped dispel this, firmly centering it within the human brain. He considered dreams as expressions of repressed desires, and he built a psychological movement around interpreting dreams as such.
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We’ve talked before about how sleep deprivation causes disease. Now we’ll discuss sleep disorders, or primary issues with abnormal sleep, and their consequences.
Sleepwalking is the act of walking and performing other behaviors while asleep. Automatic, nonconscious routines are executed, like brushing teeth or opening the refrigerator.
Sleepwalking happens during NREM sleep, and not REM dreaming sleep (like some think). Neurologically, sleepwalking is accompanied by an unexpected spike in nervous system activity, causing the person to be stuck somewhere between sleep and wakefulness.
Sleepwalking is more common in children than adults, for unknown reasons - possibly because kids spend more time in NREM sleep than adults do.
In one of the most extreme cases, a sleepwalker drove 14 miles to an in-laws’ home, stabbed the mother-in-law to death, and strangled the father-in-law (who survived). This person was later acquitted as not being control of his actions. This defense has been tried in later cases (most unsuccessfully).
Insomnia is defined as making enough time for sleeping, but having insufficient sleep...
Five influences have drastically changed how we sleep: caffeine, light, temperature, alcohol, and alarms.
This was already discussed in chapter 2. The tips, for good measure:
Light is a signal for the suprachiasmatic nucleus to regulate the circadian rhythm (by signaling to the pineal gland to secrete melatonin). In the natural world, when the sun goes down, there’s little light. But nowadays, artificial light bathes our homes and disrupts our circadian rhythm.
Any light is disruptive to the circadian rhythm. Electric light delays your 24-hour circadian rhythm by 2-3 hours each evening.
In addition to avoiding all the problems from the last chapter (eg caffeine, alarms), here are more tips:
Sleeping pills are typically sedatives that put the body into a state that doesn’t fully...
Sleep deprivation goes far beyond our individual sleep practices. Our society has structurally locked in sleep deprivation in 4 ways.
The ethos at many companies sees sleep as an indulgence for the weak. They lionize the road warrior who fearlessly crosses time zones on tiny amounts of sleep and answers emails at 1AM. In their minds, more hours worked = more productivity.
This is short-sighted. The effects of sleep deprivation are costly to employers:
We’ve seen chronic sleep deprivation caused by a variety of factors, from the individual scaling up to the societal. The book ends with the author’s musings on how to improve sleep quality systemically: