Book Review -Why We Sleep by Matthew Walker

by Angela Jackson

“The shorter you sleep, the shorter your life span.”

Matthew Walker in his book “Why We Sleep” suggests that routinely sleeping less than seven to eight hours a night triggers the sympathetic nervous system (fight or flight response) and disrupts hormonal balances. He moves through the journey of the sleep cycles showing the need to make sure that you get a full night’s sleep to benefit from all the functions that sleep brings.

He lists the following health risks as possible impacts of sleep deprivation:

• Lower immunity against viruses and the ability to build antibodies after vaccination of Hepatitis A and B.
• Increased risk of cancer by 40% in people who routinely slept 6 hours v 7 hours a night
• Memory loss including Alzheimer’s disease
• Disrupts blood sugar levels and craving for sweeter food
• Contributor in depression, anxiety, and psychosis
• Increased blood pressure
• 200% increased risk of cardiac event aged over 45 regularly getting < 6 hours sleep
• Infertility or sub-fertility – 20% drop in a follicular releasing hormone associated with the ability to conceive and a greater risk of miscarriage in the first trimester.
• Lower levels of testosterone – this can be associated with fatigue, lack of concentration, and loss of bone density. In athletes trying to build strength, lack of testosterone limits the building of muscle mass.
• More emotional and less rationale

The quality of the research is not stated, and it is not clear whether he has used a robust criterion in assessing the research, however, the information is presented in an easy manner to read and highlights possible impacts of erratic or reduced sleep on health. Whether this book might cause more anxiety for those who do not sleep well is going to be based on the individual but certainly raises concern about our modern-day sleep patterns and the lack of a lunchtime siesta in many countries compared to our ancestors.

What is the purpose of sleep?

When we are awake, we are continually experiencing and learning, however, Walker suggests that without sleep both before and after learning our ability to process and store those memories is inhibited. Different parts of our sleep cycle appear to be responsible for different aspects of memory.

The Human Sleep Cycle

Sleep is made up of several repetitions of the sleep cycle lasting around 90 minutes but having a different emphasis on different stages throughout the night. The sleep cycle is individual from person to person. Each cycle is composed of four individual stages, and each has a different role. Your brain switches between two types of sleep: one phase is rapid eye movement (REM) and three phases of non-REM (NREM) sleep.

Your brain stores different memories in different locations within the brain. Before sleep, we seek out these newly learned facts from the hippocampus region of the brain (short term memory), but after sleep, we retrieve these facts from the neocortex (long-term memory). The hippocampus has a limited storage capacity so like a computer hard drive, once full, you must overwrite or delete some information and move important memories to storage areas to make more space for new ones. Once full this may be why we forget things when overloaded with information.

The lighter phase of sleep, the slow-wave, pulsating NREM sleep (specifically based on the number of sleep spindles created) moves facts from the hippocampus to the long-term storage area in the cortex. Sleep, therefore, acts like a courier service transporting new memories to safer longer-term areas of the brain.

Motor skill enhancement

“Practice does not make perfect. It is the practice followed by a night of sleep that makes perfection” Matthew Walker

“If you don’t snooze you lose!

Sleep appears to identify aspects of the motor memory that were difficult to grasp today and appears to smooth them out overnight making them easier to grasp the next time you try them. Rather than being stored in the neocortex, these motor memories seem to be shifted to areas of the brain below the subconscious level where they become automated and habitual rather than requiring great effort so that when you perform, they become effortless. The increases in speed and accuracy and automation of the motor skills were associated with the highest number of sleep spindle activity in the motor cortex during the stage 2 NREM, particularly the last two hours of an eight-hour night of sleep. Consider that if you are an athlete that has early morning training sessions that might curtail your sleep and ability to create lasting motor memories.

Deep NREM sleep is obtained more in the earlier part of the night. This phase of sleep is important for learning textbook-style facts such as lists and storing and strengthening the raw data you encountered throughout the day. It is not just ensuring you have sleep after learning that aids memory but sleep before too. Sleep before learning prepares your brain for making new memories, and after learning, it cements those memories and prevents forgetting.

Modifying the depth of sleep during the NREM phase of sleep blunted the ability of the hippocampus to retain facts. A 90-minute nap provided a 20% increase in memory but shorter naps for even 20 minutes were stated to be effective in aiding the movement of memories to the cortex for longer-term storage.

Not all memories are correctly formed or filed, and sleep appears to offer a “recovery” type service to correct these so on waking you can remember better.

REM sleep is characterised by faster, frenetic brain waves. During the second phase, REM sleep helps to strengthen and smooth out our new memories, integrating and connecting new and existing memories to give us insight and the ability to gain perspective and problem solve. During REM sleep, your sense of time is expanded, your senses are heightened, and your muscles go limp to stop you from acting out your dreams. REM sleep adjusts the emotional circuits of the human brain.

If we wake during our REM sleep, we don’t start the normal sleep cycle progression again when we fall back to sleep, instead we resume REM sleep until we have had enough of it.

How does the body know when to sleep?

There are two mechanisms that regulate sleep:

1. The circadian rhythm
2. Adenosine

An internal clock sits within the suprachiasmatic nucleus within the brain. This lies just above the crossing point of the optic nerves and receives key information about light from the eyes and determines when we want to be awake and when we want to be asleep. In addition, it controls other systems such as temperature control, urine production, mood, and eating times. It is termed the circadian rhythm and lasts around 24 hours, occurring whether we sleep or not. Light is the most obvious trigger for the circadian rhythm, but temperature changes and food also play a part. Our temperature drops as we approach night-time and peaks mid-afternoon, a process that still occurs whether we sleep or not. The circadian rhythm is the same length in all individuals, but the peaks and troughs may differ, and Walker suggests 40% of the population are deemed “morning larks” and 30% “night owls” determined by our DNA. Night owls and teenagers struggle to conform as often opt for later bedtimes and must set alarms to make sure they wake up in time for work and school, slowly becoming more and more sleep-deprived. To these night owls and specifically, teenagers whose circadian rhythm often runs 2-3 hours ahead of adults, waking them at 7 am feels to them like 4 am and the author suggests it is not surprising therefore they are grumpy!

The suprachiasmatic nucleus uses the hormone, melatonin as a messenger. It rises in the early evening as darkness approaches signaling other systems to begin the sleep process. It does not however actually create sleep. When the eyes recognise sunlight, levels of melatonin are shut down and wakefulness follows.

Sleep pressure is caused by a build-up of the chemical adenosine in your brain. The longer you have been awake the greater the build-up in pressure. and it is this pressure that produces an increased desire to sleep. It rises consistently throughout the day lasting on average of 12-16 hours since your last sleep ended. Sleep naturally happens when your adenosine is at its highest and your circadian “wake drive” is at its lowest. Adenosine levels drop after around 8 hours of good sleep. Your wake drive starts up again and you feel awake because you’ve reduced the adenosine-circadian gap.

The circadian rhythm and the sleep pressure system are not linked and therefore if you miss a night’s sleep it is still possible to be alert the following day thanks to the circadian rhythm.

Do we need less sleep as we age?

Walker says no. The quantity and quality of deep NREM sleep lessen with age from the late twenties onwards until by 70 we are only getting 80-90% we were as a teenager. In addition, older adults sleep less efficiently often having to get up several times a night to empty their bladder. Fragmented sleep has been associated with a higher risk of falls, poor memory, and other health disorders.

In adults aged 60-80, a 40% reduction in the ability to create sleep spindles was noted which has been linked to a reduced capacity to remember things. NREM sleep spindles are generated to a greater degree in the latter phases of sleep so those who sleep less than 6 hours are limiting the brain’s ability to facilitate the transfer of information from the hippocampus to the cortex.

The other less known fact is that the circadian rhythm in older adults’ regresses so that older people want to fall asleep earlier, often falling asleep watching TV. Their melatonin levels rise earlier causing them to nod off. Unfortunately, this mid evening nap lowers the sleep pressure drive and then these adults find it hard to fall asleep again when they go to bed.

What factors affect natural sleep:

Constant electric light as well as LED light – Reading a book on an iPad with blue light unfiltered can delay the rise in melatonin by up to 3 hours and suppress it’s release by 50% compared to reading a printed book. Limiting exposure to bright light, especially blue light before bed is key. Applying a blue light filter on phones or tablets can help as and can dimming lights at night.

Once in bed, total darkness helps sleep using blackout curtains, no glow from plugs or clocks.

Temperature – to initiate the sleep state, core body temperature needs to drop by 2-3‘F or 1’C. It is this drop in nighttime temperature that stimulates the release of melatonin. Melatonin levels are controlled therefore not just by a reduction in light but also temperature. A bedroom temperature of 65’F (18.3’C) is around ideal. We can lose our ability to control temperature during REM sleep so abnormally hot or cold room temperature can disrupt this phase of sleep.

Caffeine – Walker explains that caffeine works by blocking the receptors that adenosine affects (after about 30 minutes) and tricks you into thinking you are more awake. It takes the body 5-7 hours to remove half of the caffeine in the body so he suggests that drinking coffee after midday (if you want to drink it at all) might be a factor in poor sleep patterns. Caffeine does not stop the build-up in adenosine so once the liver has broken down the caffeine in the body, you experience a crash because you feel the effects of all that built-up adenosine at once.

Alcohol sedates you but does not produce natural sleep. Alcohol fragments your sleep not allowing you to achieve deeper sleep and suppresses REM sleep, so you do not gain the benefits of the restorative phases of sleep. REM sleep helps integrate complex memory and thus drinking even small amounts of alcohol affects our ability to process information and remember it. It means lighter sleep.

Shift work, jet lag, and night-time working have all been linked to health issues related to irregular sleep patterns, and sleep length.

Sleeping pills do not provide the same restorative immune benefits as natural sleep. Effectively, sleeping pills sedate you, which helps you fall asleep faster, but the sleep you get is poor quality.

Walker suggests there are many things we can do to encourage healthy sleep:

Tips for healthy sleep

• Go to sleep and wake up at the same time every day.
• Take 30 minutes of exercise a day, preferably outside in sunlight but not in the 2 hours before bedtime
• Avoid caffeine, nicotine
• Avoid alcohol before bedtime
• Avoid large meals at night-time
• Avoid medications with stimulants in
• Don’t take a nap after 3 pm
• Take a hot bath before bed
• Have a dark, cool, gadget-free bedroom
• If you can’t sleep – after 20 minutes get up and relax rather than worrying about lack of sleep and don’t clock watch if you do wake up

These are the individual impressions of physio, Angela Jackson from reading this book and not a scientific observation. The sources have not been verified.