About . Stuff
Why We Sleep
05 Jul 2021

Alex’s Book Notes | Why We Sleep | 2021/07/07

Table of contents:

Part 1: This Thing Called Sleep

There are two factors which determine when we sleep and when we wake.

The first is the Circadian rhythm. Humans have an internal clock which keeps a consistent cycle. This clock is about 24 hours and 15 minutes long on average, which is notably longer than the 24 hour day. The internal rhythm synchronizes with external signals such as the sun to get a precise 24 hour cycle. The Circadian rhythm is controlled by the suprachiasmatic nucleus in the brain, and it sends signals out to the rest of the body to control various physiological functions. Temperature control is an example - our body gets cooler at night. Another is sleep, which is signaled outward from the brain via melatonin. Notably, this cycle will happen regardless of whether we sleep or not; in other words, if we delay sleep, our Circadian rhythm still proceeds. This partially explains why we feel an increase in alertness in the morning when we pull an all nighter.

The second factor is sleep pressure in the form of adenosine buildup. Adenosine accumulates in our brain when we are awake, and is cleared out of our brain when we are asleep. The more adenosine that is built up in our brain, the sleepier we feel. Caffeine functions by blocking adenosine receptors - thus, when caffeine wears off, we often feel a crash due to adenosine binding to receptors en masse since it has been building up despite the caffeine.

Caffeine is worth important mention due to its ubiquity. The half life is roughly five to seven hours, which means if we have a cup at noon, then half a cup’s worth of caffeine is still circulating in our brain by 5:00 PM - and that is not a trivial amount.

These two factors - Circadian rhythm and adenosine buildup - are entirely independent. A delay in sleep causes more adenosine buildup, but your Circadian rhythm marches onward regardless. Optimal sleep requires the two to be in sync.

How do you know if you’re getting enough sleep? A good rule of thumb is that you are not if you are feeling tired by 11 AM. Another rule of thumb is that if you are using an alarm clock and would naturally sleep beyond it, you are not getting enough sleep.

What subjective experience tells ourselves that we are asleep? First, the loss of external awareness. The thalamus in our brain blocks external perception when we are asleep. Second, the feeling of time distortion is a feature that helps us know we were asleep. While we are asleep, our conscious sense of time is lost but non-consciously, we are still tracking it accurately.

Science and technology has enabled us to get objective measurements of sleep, e.g. with electrodes on the head. This had led to defining sleep as two phases with distinct characteristics - rapid eye movement (REM) sleep, and non-rapid eye movement (NREM) sleep. Furthermore, NREM sleep is divided into four phases (phase 1, phase 2, and so on), with phase 3 and phase 4 of NREM sleep being classified as deep sleep. These total five phases of sleep have different characteristics and functions. In multiple ways, they are not uniform. Throughout life, we get different amounts of different types. REM sleep is heavy during childhood, and NREM becomes more important in teenagers and young adults. In the human adult, NREM sleep dominates the early part of the night and REM sleep dominates the tail end of the sleep session. This has important consequence - if we go to sleep at 4 AM and wake up at 10AM, you’re cleaving off the early part of the night thus you are missing out on important NREM sleep. Conversely, if you sleep at 10PM and wake up at 4AM, you’re disproportionately losing out on REM sleep.

As for function, NREM is seemingly important for removing unnecessary neural connections, and REM sleep is important for creating new ones and strengthening existing connections. NREM also is critical for memory - during NREM, short term memory gets consolidated into long term storage. REM is critical for creativity and emotional health.

What is the natural sleep pattern of humans? There is significant evidence that the natural pattern is to have a long bout of sleep at night (7-8 hours), and to have a shorter period of sleep (30 to 60 minute) in the early afternoon. This has been observed in the few modern hunter gatherer tribes. When the few modern cultures such as Spain and Greece started moving away from this biphasic pattern by removing the afternoon siesta, a plethora of negative health outcomes were observed in studies. Also, the period of night sleep has been delayed so it starts later and ends later (or, for many, it ends earlier) with the advent of electricity in particular.

The young brain is particular vulnerable to disruption. Many developmental disorders like autism have been at least linked to abnormal sleep patterns. Proper sleep especially at early life is obligatory.

Part 2: Why Should You Sleep?

Sleep is critical to every aspect of health. One of the most well understood benefits from sleep is on memory.

Sleep prior to learning refreshes the ability to make new fact-based memories - specifically, during stage 2 NREM sleep (and even more specifically with sleep spindles within that stage), hightened activity around the hippocampus (the short term storage site for facts) and the cortex (the long term storage site) has been observed and improved memory acquisition - or rather, the restoration of learning ability to its peak - has been observed for those who slept better compared to those who slept worse.

Sleep after learning is also critical to fact-based memory. Memory consolidation occurs during sleep, which is effectively the process of protecting memories against forgetting. For fact-based memory, deep NREM sleep is the stage of sleep which is critical to consolidation. Deep NREM sleep has also been shown to actually recover memories which were lost prior to sleep.

Perhaps even more interesting, sleep has been shown to be able to preferentially consolidate certain memories, without enhancing others. Sleep doesn’t help preserve all information indiscriminately; it’s able to choose which memories to strengthen based on things that happen during initial learning (e.g. “I must remember this for the test”). Once again this has been shown to be associated with sleep spindles during NREM sleep.

How about non-fact based memory? There are other types of memory e.g. motor skill memory such as that used in riding a bike. The brain is responsible for what we colloquially deem “muscle memory”. Studies have shown that these motor skill routines continue to be improved (in speed and accuracy) during sleep - that is, in the absence of practice. A pianist might practice a routine, go to sleep, and upon waking be able to perform it consistently better than prior to sleep - and critically, this performance jump will be better than those that continued to practice. These benefits are due to stage 2 NREM sleep, especially the sleep in the last two hours of an eight hour night of sleep. This overall has huge ramifications on a wide swath of pursuits such as music and athletics, in both achieving mastery and preventing injury.

Sleep also aides in creativity. Connections between disparate information happens during REM sleep to foster that creativity.

We’ve covered the benefits of sleep. What are the consequences of sleep deprivation?

One of the most obvious is with increased accidents, which itself has two separate causes. Acute sleep deprivation can cause sleep at non-normal times, e.g. while driving. This has obvious ramifications. The more interesting cause of increased accidents is actually with chronic sleep deprivation (defined as getting less than 7 hours of sleep per night on a routine basis). With chronic sleep deprivation, the brain can frequently become blind to all perception (not just visual) of the outside world. The person it happens to likely doesn’t even know it and they rate their own concentration and performance as much higher than it objectively is. Recovery to baseline performance after chronic sleep deprivation such as this does not happen in a short period of time either - you can’t recover to that baseline simply with a few days of extra sleep.

Can we mitigate the effects of sleep deprivation? Studies show that naps prior to sleep deprivation can help lessen the effects to a large magnitude than naps after the sleep deprivation.

Does sleep deprivation effect everyone equally? For the most part, with one exception. Less than 1 percent of the population is genetically more resilient to the effects of chronic sleep restriction due to a particular gene.

Loss of emotional control is another significant effect of sleep deprivation. The amygdala (which is associated with fight-or-flight, and strong emotions like anger) shows amplification in activity for sleep deprived people. The prefrontal cortex (which is associated with rational, logical thought and decision-making) shows less activity, resulting in lower inhibition. Insufficient sleep actually makes the extremes of emotions larger - both in the negative and positive directions.

There is no major psychiatric condition in which sleep is normal. Sleep and its relation to psychiatric conditions is best viewed as a two way street; lack of sleep can amplify or even cause some disorders (or their symptoms), while other disorders can result in worse sleep. Improving sleep quantity, quality, and regularity has been shown to aide in treatment of numerous disorders. One of the most important techniques here is cognitive behavior therapy for insomnia (CBT-I), which has been shown to help achieve this for a broad array of disorders.

“The best bridge between despair and hope is a good night’s sleep.”

  • E. Joseph Cossman

Sleep deprivation is hugely detrimental to learning in general. Some specific points are interesting. Sleep deprivation in the first night after learning something, you lose the chance to consolidate that memory, regardless of follow up catch-up sleep.

What other disorders does sleep relate to? Alzheimer’s disease is a big one. Sleep quality especially with deep NREM sleep deteriorates with age, even for healthy adults. The disruption of deep NREM sleep is much greater for Alzheimer’s patients. Greater sleep disturbance actually precedes the onset of Alzheimer’s for years. This link may enable earlier diagnostics as medicine advances. The amount of sleep disruption increases with symptom severity as the disease progresses. Separately, over 60 percent of those with Alzheimer’s disease have at least one clinical sleep disorder. All in all, sleep and Alzheimer’s are significantly related. Alzheimer’s disease is associated with the buildup of beta-amyloid in the brain. The brain has a glymphatic system (think of it as a sewage system) which is responsible for draining metabolic contaminants from the brain - one of the components it takes care of is amyloid proteins. This glymphatic system is active during wakefulness, but much more active during deep NREM sleep, and the glial cells of the brain shrink during deep NREM which enables that glymphatic work to be more effective as well. Alzheimer’s disease and inadequate sleep form a viscious cycle. Without sufficient sleep, amyloid plaquies build up in the brain, focusing on the deep-sleep-generating regions, and attack and degrade them. This damage results in less deep NREM sleep which makes the body less able to recover from this damage. Studies have shown that adults who get sleep disorders treated slow their rate of cognitive decline significantly and delay the onset of Alzheimer’s by a decade. Sleep is not a magic bullet that eradicates dementia, and the science isn’t super mature, but prioritizing sleep is likely a significant factor for lowering Alzheimer’s risk.

Sleep deprivation has strong effects on cardiovascular health. The ways in which it does this are numerous but seemingly all centered around the sympathetic nervous system. Lack of sleep is associated with an overactive sympathetic nervous system, and being in that state chronically results in cardiovascular damage. One interesting tidbit is that heart attack rates increase significantly after daylight savings time begins in the spring (resulting in people getting less sleep) and plummet after daylight savings time rolls back in the autumn (resulting in people getting an extra hour of sleep).

Sleep deprivation is also related to weight gain and diabetes. People are more likely to have ravenous appetite after a night of less sleep, whereas the hunger levels were more under control for people with consistent good sleep. Interestingly, those who are losing weight and sleep deprived lose much more lean body mass (i.e. muscle) compared to fat, whereas those with adequate sleep lose much more fat. Short sleep will increase hunger and appetite, and will impede impulse control within the brain, increase food consumption, decrease feelings of food satisfaction, and prevent effective weight loss when dieting.

The reproductive system is also at play. Reproductive hormones, reproductive organs, and physical attraction are all degraded by short sleeping.

The immune system is another area. One interesting study showed that sleep impacted the response to a standard flu vaccine significantly. Those with adequate sleep generated a powerful antibody response to the vaccine, whereas those with sleep restriction had a much smaller immune response. Similar to memory, with the immune system catch up sleep cannot restore all the benefits of sleep to the level of having gotten consistent good sleep in the first place. Studies have also shown that cancer-fighting immune cells are much less prevalent in those with sleep deprivation. Many epidemiological studies have shown that nightime shift work increases the odds of developing some forms of cancer.

Part 3: How and Why We Dream

Dreaming occurs during REM sleep. Four main clusters of the brain spike in activity during dreaming: the visuospatial regions at the back of the brain, the motor cortex which supports movement of the body, the hippocampus and surrounding regions which support autobiographical memory, and the amygdala and cingulate cortex - the emotional centers of the brain. Some other regions of the brain such as the prefrontal cortex (which supports rational thought) actually decrease significantly in activity during dreaming.

Scientists have been able to actually decipher general content of a dream (e.g. this person was dreaming of a car) using brain scans during dreaming.

What is the purpose of dreaming? What function does it fulfill?

The first function of dreaming is to nurse our emotional and mental health. It’s been shown that those who dream about painful experiences around the time they happen actually experience more emotional recovery from those painful experiences compared to those who do not dream about those experiences. Noradrenaline in the brain blocks the ability to enter REM sleep; studies indicate that this is an underlying mechanism to PTSD; noradrenaline prevents the subject from entering REM sleep and thus dreaming, and thus dreaming cannot afford the subject some emotional calm.

A second function of dreaming is to tune the brain’s emotional instrumentation, which allows accurately perceiving emotions of others. At a high level, dreaming appears to be critical to the sociability of humans.

A third function of dreaming is to inspire creativity and promote problem solving. An example perhaps best supports this. Mendeleev thought there was an organizational logic to the known elements in the universe. He made a set of cards which each represented an element. He tried to use these to figure out that overall organizational structure. After apparently not sleeping for three days and nights, he slept and during that sleep he dreamt of the proper organization. The answer to the puzzle was reached. This was the discovery of the periodic table of elements.

Another interesting story is related. Thomas Edison would supposedly position a chair with armrests at the side of his desk. He had a metal saucepan turned upside down on the floor directly below the right armrest. He would hold a few ball bearings in his right hand, and then allow himself to sleep. At the moment he began to dream, his muscle tone would relax (this happens because the brain paralyzes voluntary muscles during REM sleep) and thus he would drop the ball bearings which would crash on the saucepan and wake him up. He would then write down any creative ideas in his head at that moment.

Lucid dreaming is another interesting area. Science has been able to recognize lucid dreaming as a real, repeatable observation because voluntary eye movement is not blocked during REM sleep. Thus lucid dreamers have been able to communicate with experimenters who were scanning their brains; the brain patterns showed the subjects were dreaming.

Part 4: From Sleeping Pills to Society Transformed

There are a number of sleep disorders that affect humans. Perhaps none is more prevalent than insomnia. One of the most common culprits of insomnia is an overactive sympathetic nervous system.

Much of our modern lifestyle has transformed our sleep, and not in a positive direction. We’ll explore that here.

Exposure to artificial light can harm our sleep quality. How do we mitigate that? First, we can create lowered, dim light in rooms where evening hours are spent (avoid powerful overhead lights, favor mood lighting). Wearing yellow-tinted glasses which filter out blue light (which suppresses melatonin release) is a more drastic option. Maintaining complete darkness throughout the night is critical as well (e.g. use blackout curtains).

Alcohol is also bad for sleep. Alcohol suppresses REM sleep and also harms learning. It’s best to actually drink in the morning so it’s out of your system by the time you sleep.

Body temperature is important for sleep. A lower body temperature aides in sleep. A bedroom temperature of around 65F/18C is a good goal. A hot bath before bed actually helps because blood rushes to the surface of the skin, and those surface blood vessels are able to radiate out inner heat faster which results in the body cooling more efficiently. Hot baths before sleep have been shown to induce 10 to 15 percent more deep NREM sleep in healthy adults.

Waking up at the same time of day, ever day is the ideal. A stable sleep schedule is one of the most consistent and effective ways of ensuring good sleep.

Sleeping pills are actually quite harmful. The book goes into a lot of detail as to why and how. The salient point is to avoid them. Alternatives such as CBT-I have been shown to be much more useful already.

Other sleep suggestions are to establish regular bedtime and wake-up time, to go to bed only when sleepy and avoid early evening sleeping on the couch, to never lie awake in bed for much time (get out of bed instead), to avoid daytime napping if having difficulty sleeping at night, to reduce anxiety-provoking thoughts by decelerating before bed, and remove visible clock faces from view in the bedroom.

This section of the book also goes into some detail about society’s view on sleep at large. This is an interesting section but I won’t recount it here.

© 2021