Health · Learning

Why we sleep by Matthew Walker

This book was recommended by so many podcasts and work colleagues and I finally got around to reading it. It’s an excellent read I would thoroughly recommend it as one of the most potentially life changing reads you could complete. As a society we don’t understand sleep, and don’t value it. This book will change your attitudes towards sleep completely.

Sleep is one of the oldest evolutionary functions present in all known animals even down to the most basic like worms (and sharks and fish in case you wondered do sleep even though they keep moving, just like your heart keeps pumping). So it must serve some very useful functions! But the nature of the sleep does differ significantly between species

The basic structure of sleep

NonREM sleep – slow wave sleep with regular pulses of activity from the prefrontal lobe to the back of the brain. These are accompanied by occasional “spindles” of electrical activity that appears to be associated with the transfer of memories. This deep sleep facilitates the “file transfer” or consolidation of information recently learnt from the hippocampus used for short term memory to the neo-cortex for longer term storage, freeing up space to absorb new information. It is also used to pair down unnecessary connections and undo information that we know to be incorrect. Ie it is selective and intelligent about what memories and facts are stored or forgotten depending on how they are labelled! This seems to happen because of cycles of the spindles between the frontal lobe which directs intention and action, and the hippocampus. This storage function also applies to “motor memory” (particularly in hours 7 and 8 of sleep) and in fact can even enhance performance of repetitive tasks relative to what we were able to do pre sleep! We actually get better at tasks because of sleep!

REM sleep – rapid eye sleep, when we dream, the brain switches off our voluntary motor control systems so that we are paralysed and don’t act out our dreams. In this stage it seems new data is integrated into our brains forming and strengthening new connections between neurons allowing association between very different historic experiences to develop, and creativity to flourish. Our prefrontal lobe which regulates emotional control when we are awake is more disabled during this period allowing emotional processing to occur, almost like having your own therapist. (Disorders like PTSD could be due to an inability to process emotionally traumatic events during sleep). Without this sort of processing during sleep our ability to respond appropriately to emotional cues decreases substantially the next day.

We cycle between NREM and REM every c 90 minutes throughout the night, typically for about 5 cycles in a night. We get more NREM earlier in the night and more REM later in the night. The mix and length of sleeping also varies dramatically over our lifetimes. Babies have massive REM as the brain forms, in the late teenager years we need more NREM in order to prune down the connections and develop more emotional control.

The most important point is that your really really need both types of sleep to function well.

What drives sleep?

Melatonin production is triggered by darkness in the early evening leading to the onset of sleep a few hours later, but it’s not what makes us specifically go to sleep. Melatonin production is substantially decreased by blue light especially LED light from our devices.

Our circadian rhythm is our “wake up drive”. It is controlled by our bodies internal clock driven by a group of neurons in the brain called the suprachiasmatic nucleus that sits just above the crossing point of the optical nerves. This naturally keeps track of an approximately 24 hour cycle but takes signals from senses such as our eyes and eating cycles to keep the clock synchronised to 24 hours.

The circadian rhythm drives many functions across the body including in part our sleeping rhythms but also core body temperature which peaks in the evening and then drops as we go to sleep until the early morning when it starts increasing again.

Adenosine is a “sleep pressure hormone” that creates our “go to sleep drive”. It increases steadily from morning until late evening. Once we fall asleep it decreases until we wake up again in the morning.

It’s the combination of the circadian rhythm and Adenosine that makes us go to sleep or wake up. The difference between our circadian rhythm and adenosine peaks in the late evening when we feel tired, and troughs in the early morning as the Adenosine is at its lowest and the circadian rhythm begins to pick up.

Fun facts and hypotheses

The patterns of sleep differ substantially between species. Most fish, insects, amphibians seem only to have NREM sleep. Most birds and all mammals with a few exceptions seem to have REM sleep (birds and mammals evolved separately so REM sleep seems to have evolved twice independently). The exceptions are aquatic mammals like dolphins and killer whales because they can’t afford to be paralysed in the water (they may experience REM sleep but for very brief periods of time). Seals have REM sleep on land but only NREM sleep if in the water. Some mammals and birds can sleep one half of their brain while the other half remains active! Some birds can literally have one eye open and vigilant for danger, the other closed and sleeping, but they can’t sleep for long periods while flying (on long migrations some do have short periods of sleep in the air).

Humans have more REM sleep than other species. Unlike all other apes we have much more REM sleep. Probably a function of not sleeping in trees: during REM sleep muscles are paralysed which is a bad idea if you are sleeping in a tree. Apes sleep in trees to avoid predators and ground bugs. It’s possible the development of REM came about as a result of the mastery of fire which allowed human ancestors to start sleeping on the ground with smoke and fire scare off predators and bugs. Hence enabling greater periods of REM sleep and perhaps leading to our greater creative intellect.

Babies in the womb have huge amounts of REM sleep as the brain develops, in the second and third trimesters, peaking at up to 12 hours a day just before birth. However that’s not when baby is kicking you because the REM sleep paralyses the muscles!

Infants have 50/50 % NREM/REM decreasing to 70/30 % by age 5 and 80/20 by the late teens. NREM is also when the brains usage is “pruned” aiding brain development and maturation as a teenager refining cognitive skills, reasoning and critical critical thinking.

Teenagers also have a shift in their circadian rhythm towards waking up later. There are compelling studies to show that allowing teenagers to sleep in rather than waking too early for school dramatically improves their mental and learning abilities. There is good evidence from many studies globally that starting school later in the morning eg. A 9am or even 9:30 start leads to better outcomes particularly in teenagers.

In mid to old age, quality of sleep deteriorates. It’s not that older people need less sleep, it’s that they fail to get good quality sleep. NREM sleep quality is typically what deteriorates into late 20s and 30s. By your 40s your quality of NREM sleep has typically decreased by 60 to 70% compared to your teenage years. Sleep becomes more fragmented. Starting off with having children. Then later we wake up more often at night, for many reasons but including because of a weakening bladder, needing to head to the loo more often. One challenge is that we are really poor at self assessing the quality of our sleep so we may not notice this, or make the association between poor sleep and other health conditions we might be facing.

The human population consist about a 1/3rd Larks, 1/3rd Late Night owls and 1/3rd in between. These are genetic drivers linked to the circadian rhythm. Evolutionarily this is valuable because it gives members of a group ability to overlap their wakeful hours reducing the total time the whole group are asleep and might be exposed to danger because of sleep.

Health and lifestyle impacts

Matt Walker links a lack of sleep to a whole host of health problems. Good sleep is linked to a whole host of lifestyle benefits. At first you think these will just be associative links but in case after case he points to very tangible scientific evidence of the linkage to sleep with credible biological mechanisms. These are the things that really drove me to seriously consider changing my sleeping habits. Whether they actually achieve that shift is another thing… check in with me in a year!

Humans do naturally have biphasic sleep. We tend to want a post lunch afternoon nap of 30 to 60 minutes, preferably before 3pm so as not to interfere with our night time sleep. And then 7 to 9 hours of night time sleep. There seems to be good evidence of increased risk of 37 % of death due to heart disease in societies forced to move from a siesta to a continuous working culture eg. A study done in Greece.

To learn best we need to have had good sleep before we start learning. This resets the hippocampus, transferring information to longer term memory allowing us to store new information. Naps during the day will allow us to reset and store more information.

A good sleep the night after and for the next several nights then seals this storage of information (via NREM sleep, particularly later NREM closer to the morning even though sleep is the night more dominated by REM) and makes new creative associations (via REM sleep). Good sleep can even allow us to repair and remember past memories we might have been unable to recall. And you can even be intentional about which memories you wish to store by intentionally recollecting those specific thoughts prior to going to sleep. The brain will actively filter those for storage and discard any data you are intentional about wanting to forget! And the brain can even improve on what you learnt during the day, working on problems that you could not master during the day but having slept on them, you can find they come naturally the next day.

REM sleep drives creativity by creating links between distant ideas stored in our brains, particularly as the executive function control and logic is suspended allowing us to make unusual leaps.

Conversely studies show that poor sleep makes us much more likely to forget things. Not only because the “file transfer” doesn’t happen front he hippocampus but also because the hippocampus is less able to absorb new information.

Poor sleep makes our emotional control less good the next day and we are less able to assess social cues. This is driven both by more activation of our amygdala (which controls flight or fight responses) and by our striatum (which is assosciated with reward responses) and deactivation of our prefrontal cortex which gives us executive function and control. It can cause us to swing both positive and negative exacerbating conditions like depression, aggression, overeating, and substance abuse.

Poor sleep is a massive contributor to motor accidents. People who have been sleep deprived are not only less alert, they can experience “micro sleeps” when they loose consciousness for a few seconds often leading to dangerous motor accidents. You are 11 times more likely to be involved in a car accident if you have had less than 4 hour sleeps, and twice as likely if you have had between 5 and 6 hours sleep compared to over 8 hours.

Caffeine has a half life on 5 to 7 hours, ie 50pct is still active after 5 to 7 hours. Caffeine bonds to receptors of all of the cells in our body preventing the normal sleep hormone Adenosine, binding keeping us awake. Once the caffeine wears off the built up Adenosine hits and you crash Ie. Don’t have caffeine after mid day if you want a good nights sleep. Caffeine disrupts NREM sleep.

Alcohol is one of the most powerful suppressors of REM sleep. Alcohol is a sedative. But sedation is not sleep. It has more in common with anaesthesia than natural sleep. Alcohol results in very fragmented sleep, though we may not be aware of this fragmentation afterwards. It is a significant suppressor of REM sleep, due to aldehydes that are metabolised from the alcohol by the body. Memory retention is far worse in experiments for those having alcohol compared to those don’t who did not.

Athletes perform more poorly and get more injuries on less sleep. On less than six hours, physical endurance can decrease by 30 %.

A lack of good quality REM sleep could be correlated with conditions like autism, as REM sleep is thought to be important in wiring the brain, and there seems to be some correlation between atypical sleep patterns (30 to 50 pct less REM sleep than a normal child normal ) and autism. Causality is unclear.

Other psychiatric disorders like schizophrenia, ADHD, bipolar, depression all tend to appear first in teenage years and their may also be linkages to sleep quality, here particularly to do with NREM sleep. ADHD in particular symptomatically is easily confused with poor sleep, and unfortunately the treatment with amphetamines like Ritalin causes those children to be very much awake.

In older age poor sleep can also contribute to and is Assosciates with diabetes, depression, stroke, cardiovascular disease and Alzheimer’s!

Stroke recovery is aided by good sleep, particularly REM sleep as the brain rewires itself to bypass injured parts of the brain.

In older age, some symptoms that might be attributed to dementia might in fact be due to poor sleep quality eg. Forgetting things. There is a link between deterioration in brain health and deep sleep in that brain deterioration often begins in the middle frontal lobe that generates NREM deep sleep waves. This may explain the forgetting since it’s not the memory part of the brain but this “file transfer function” that is most affected.

NREM sleep already deteriorates with age, but in Alzheimer’s patients that is much more pronounced. Alzheimer’s is assosciated with a build up of a protein called beta-amyloid, particularly in the frontal lobe, responsible for generating NREM deep sleep. Rather than affecting the memory parts of the brain (the hippocampus and neocortex) the protein buildup is seen more in the frontal lobe. Hence the poor memory may be a function of the failure of the file transfer part of the brain because of the failure of the NREM sleep. A system in the brain called the glymphatic system, is responsible for “clearing the brain” of all the waste products produced by metabolism in the brain throughout the day, including the beta-amyloid and tau proteins associated with Alzheimer’s. This systems functioning increases 20 fold during NREM sleep, as glial cells shrink between neurons allowing a deep “wash” of cerebrospinal fluid cleaning out the brain. Without proper NREM deep sleep this cleansing system may not be working properly allowing further buildup of the harmful proteins and hence further deterioration in NREM in a self reinforcing cycle that makes the disease worsen.

In support of these hypotheses, there is now increasing evidence that getting too little sleep through the adult life may heighten the risks of Alzheimer’s disease, explained by the mechanisms above. Conversely it may be possible to delay or minimise the chances of getting Alzheimers by getting good sleep.

Poor sleep is also directly associated with heart disease and heart attacks. Studies show this even after correcting for other influences such as smoking diet etc. A short nights sleep increases blood pressure and heart rate measurably, people is less than 6 hours sleep are 2 to 3 times more likely to have calcification of arteries which can lead to heart-attacks. The bodies sympathetic nervous system controls the fight or flight response we have when we come under stress. A lack of sleep puts this nervous system into a continual state of activation, causing higher heart rates, construction of blood vessels through, higher cortisol release (a stress hormone), and a reduction in Growth hormone – a repair hormone that is released at night that repairs arteries and other parts of the body. NREM sleep is usually associated with a slower heart rate, and a calming effect in the cardiovascular system so you need it! Every year when we switch to daylight savings time with one hour less sleep, that day correlates with a spike in heart attacks relative to base rate, and when we get an extra hour sleep it’s associated with a drop!

A lack of sleep is also assosciated with weight gain and increased chances of type 2 diabetes. Like heart diseases the effect is found in studies even after allowing for other common correlates such as body weight, alcohol consumption, smoking etc. After a week of sleeping only 4 hours a night, participants in a study were 40pct less able to absorb a dose of glucose than normal. This was due to both less insulin release and cells becoming less sensitive to insulin. Also affected are hormones grehlin which triggers the sensation of feeling hungry and leptin which signals feeling full. With short sleep, grehlin increases and leptin decreases causing us to eat more and put on weight. In well controlled trials people on short sleep ate significantly more calories and tended to gravitate more towards unhealthy snacks. This may be explained by sleep depravation decreasing activity in the prefrontal cortex where we exercise judgement and increased activity in parts of the primal brain like the striatum that drive desire. On top of that when sleep deprived we tend to feel more lethargic and be less prone to exercise. Further more if you are sleep deprived and trying to diet, more weight loss comes from muscles, whereas if well slept more comes from fat. When sleep deprived the body tries to hang on to its fat reserves.

Very interestingly, some argue that we need more calories when we are sleep deprived because we will burn more as we are awake longer. it turns out this is not true. A person who is awake for 24 hours burns only 147 more calories than someone who has 8 hours sleep! Sleep is actually quite a metabolically active state for the brain and the body.

Ample sleep can really help achieve the opposite in many of these cases. It may also affect your microbiome, again through less activation of the sympathetic nervous system which cultivated different bacteria in our guts compared to when less cortisol is present.

Sleep seems essential for a good immune system response to infection. Hence why we tend to sleep when we get sick. Those with better sleep have better initial immunity to avoid catching a cold. You are much more likely to build much higher level antibodies if you get good sleep prior to a flu vaccine than not (and this must therefore be similar to getting sick since a vaccine is using a dead virus to activate the immune system to get antibodies).

Disruption of sleep and circadian rhythm is assosciated with increased risk of various cancers shown by studies on shift workers. The WHO classifies night time shift work as a possible carcinogen. A european study showed a 40 % increase in cancer risk for those sleeping 6 hours or less. Again the sympathetic nervous system seems to be implicated here, putting the body into a state of more sustained inflammation when we get less sleep. Some cancers use the bodies inflammatory response to start growing, and may aggravate metastasis (spreading to other parts of the body). In mice models sleep deprived mice develop cancer tumours at 3 times the rate of non deprived mice. It looks like a lack of sleep may also diminish cancer fighting M1 macrophage immune cells and increase cancer inducing M2 macrophage cells in the body.

Finally it seems that poor sleep can actually damage cells directly though shifting the expression of different genes, (increasing harmful, decreasing helpful expressions by as much as 200%) and even damage the DNA in our cells through shortening the telomere tips that protect chromosomes in our DNA from damage. This is a very similar effect seen in natural aging a telomeres get shortened… ie poor sleep makes you age faster literally.

Poor sleep in the health professional, especially by doctors where long shifts are seen as a badge of honour could be contributing to massively to incidence of mistakes, so our life might depend on other people’s lack of sleep

A final finding: there is no good evidence that too much sleep is bad for you. We naturally seem to need somewhere around 8 hours in 24. If you are sleeping for significantly longer on a continual there might be something else going on but sleep itself does not seem to be harmful. Ideally you sleep until you wake up naturally rather than having to have an alarm clock.

What’s going on when we dream?

One function of Dreaming is to be our own inbuilt therapist to help us process emotional experiences. We dream during REM sleep. During REM sleep a key stress hormone Noradrenaline is completely shut off within our brain (the brain equivalent of the bodies adrenaline). Key emotion and memory related structures are activated during REM sleep. It’s possible to therefore reprocess emotional states in a “safe” dream state. This may help us remember helpful details and forget some of the more painful parts as we experience the experience without the same stress.

PTSD sufferers suffer from disrupted REM sleep. PTSD might well be contributed to by a breakdown in this emotional trauma processing function in REM sleep, particularly because noradrenaline is not shut off. A drug that reduces noradrenaline has now been approved for the treatment of PTSD.

REM sleep generally seems to retune our emotional calibration and function the next day, making us more able to understand the emotions playing across other peoples faces, a key skill in navigating every day life as a human.

Dreaming also seems to be the creative centre of our brain. Many great breakthroughs seem to have come to people after a night of dreaming including the construction of the periodic table by Mendeleev, some of the great songs by the Beatles etc. Experiments on associations formed in wakeful periods versus just after REM sleep show very different associative patterns, with dream associations being far less obvious and connecting more distant concepts compared to the waking associations which tend to be far more logical.

In studies participants are far more able to spot short cuts and innovative solutions after sleeping on a problem, 20 percent when presented with the problem and asked for a solution 8 hour later compared to 60 % spotting it if they had slept overnight!

It does seem that the content of the dream matters. If you are dreaming about something related to the problem you are working on you may fix it, if it’s something else it might not be very effective.

Some people including Thomas Edison develop habits of waking themselves up from a dream to then write down the creative ideas they had.

Some people seem to experience “lucid dreams” where either they are aware that they are dreaming or in some cases they seem to be able to direct what the dream is about. Experiments have shown that lucid dreamers are able to communicate to researchers through their eye movements while in REM sleep.

Recommendations for good sleep

Matt has a whole section on sleep disorders which I will not recount, if you suffer from any then please read the book. His biggest recommendation is to be very wary of sleeping pills, most of which do not result in a proper natural restful sleep but more like a coma induced state without the benefits of either REM or NREM sleep.

He also makes the point that the issue is really as a society we need to change our attitude towards sleep: to move away from wearing short sleep as a badge of honour and undervaluing it and towards understanding that getting proper sleep is one of the most powerful life and health enhancing habits we develop.

His strongest recommendation To get good sleep: go to sleep at a consistent time every night (including weekends) and wake up at a consistent time every morning. This stops unnatural shocking of the circadian rhythm on a regular basis.

Don’t look at blue light late at night, this suppresses melatonin production by only 50 %. Make sure your device tones down blue light after a certain time. Ideally just ban devices from the bedroom.

Maintain darkness for your sleep, blackout curtains, no flicking lights from devices.

Keep the temperature cool. To initiate sleep your body’s core temperature needs to drop by 1 degree Celsius. Hence it’s easier to fall asleep in a cool room. A drop of temperature also indices melatonin production. Your hands feet and head help radiate away heat from the core. A room temperature as cold as 18.3degree C seems to be optimal given standard bedding. A hot ironically helps with this because it draws blood to the skins surface allowing you to cool off as you dry off and go to sleep.

Don’t doze off or nap after 3pm, that will disrupt the later circadian rhythm. Don’t doze off in the evenings infront of TV, if your are sleepy go to bed.

If you are awake in bed, struggling to sleep, don’t lie there, get up and do something and then come back to bed

Cut out alcohol and caffeine at night, perhaps even after 3pm

An alarm clock can be bad for your blood pressure. Hitting the snooze button repeats this. Try to just have a consistent time getting to sleep and waking up. But if you need to use an alarm clock this is okay if you are following a consistent sleep schedule.

Exercise is good for sleep and sleep is good for exercise but don’t exercise in the three hours before going to sleep.

Don’t eat and drink too much late at night

Do something relax before going to bed, not on a screen. Eg have a hot bath, read, listen to music

Get bright light, preferably sunlight in the morning. 30 minutes of sunlight is good.

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