Causes of sleep problems


And extract from: Taylor-Byrne, R.E. (2021) How to Stop Wrecking Your Mental, Physical and Emotional Health: Answers from the science of sleep.  Hebden Bridge: The Institute for E-CENT Publications.

Copyright (c) 2021, Renata Taylor-Byrne


Chapter 6: The Major Sleep Destroyers

This chapter explores five of the most important destroyers of sleep, which are: stress; alcohol; caffeine; electric light; and nicotine.  Of course there are several other important sleep destroyers, including the following:

– Noisy neighbours, or noisy households;

– Exercising close to bedtime;

– Sleeping tablets (which are mentioned later in this book), and some recreational drugs;

– And some physical diseases.

However, in this chapter, I want to mainly focus in on the negative effects of alcohol, caffeine, electric light and nicotine.

But because stress is such an important factor in causing insomnia, I want to begin by repeating part of what I wrote about stress and insomnia in an earlier chapter:

6.1: Stress as a cause of insomnia

Sleep & Health book, Front coverPenn State University, the Autonomous University of Madrid, and the University of Athens conducted a joint study of people who suffered from chronic insomnia[i]. What the researchers found of significance was the way in which the insomnia sufferers handled their day to day stress. They tended to handle their stressors, including disagreements they experienced during the day, by suppressing their emotions – keeping them inside and not talking about them or writing about them. This strategy led to emotional over-stimulation:

“At bedtime they were characteristically tense, anxious and ruminative about issues associated with health, work, personal affairs, death, etc.”

Because they are emotionally upset, at bedtime, their whole body becomes stirred up and they have difficulty getting to sleep, or returning to sleep if they wake up in the night. The result is that they develop a dread of not being able to get to sleep, and this makes them even more wound up, which keeps them awake, and it becomes a vicious circle.


For more on this link between stress and insomnia, please see, or revisit, section 5.1(c) above. See also section 9.1 below, in which Dr Robert Sapolsky (2004) states that roughly 75% of insomnia is caused by stress. Also section 10.2 on the effects of stress on sleep.  Plus 10.3 on handling worry and rumination.

6.2: Alcohol

“Alcohol is hell on your sleep.”  

Winter, 2017


Sleep & Health book, Front coverYou may find it difficult to believe, but alcohol doesn’t help you to sleep, after a night out. That is because of the way it works: it’s a sedative, and it works by stopping part of the brain from functioning – the pre-frontal cortex. This part of the brain is the headquarters of your brain’s ability to reason, think and solve problems and it’s also the part where your social control resides, and your ability to control anti-social behaviour.

The result is that people become less inhibited and more confident, and if they keep drinking, then the drug deactivates other parts of the brain and reduces their awareness.

Alcohol destroys sleep

There are two ways in which it actually destroys sleep:

Firstly, one of the by-products of alcohol is aldehyde (formed by the oxidation of alcohol) and what this does is blocks a very important part of sleep – rapid eye movement sleep (REM) sleep. This means that no dream sleep (which is very nourishing for the brain) will take place and problems of poor concentration and fatigue will happen the following day.

Secondly, alcohol actually breaks up sleep, and people wake up frequently, at very short intervals, but because of the brevity of those awakenings, they aren’t remembered by the person. But they are picked up by brain scans in sleep labs! And because the sleep is broken up, it’s not beneficial in the way that normal sleep is; and the full recharging of people’s energy doesn’t take place if their sleep is constantly interrupted.

Alcohol also creates further problems because of the suppression of rapid eye movement (REM) sleep, which is essential for processing the emotions of the day.  Without sufficient REM sleep you will enter a new day carrying much of the stress and unresolved emotional problems from yesterday.

Furthermore, in the case of students and others who are trying to learn new material, REM sleep helps the brain connect different ideas with each other, and make sense of new information.

Research evidence

To assess the effect of the use of alcohol on human memory, via the level of rapid eye movement (REM) sleep, a big group of students were assembled for a research project lasting seven days[ii].

On day 1 of the experiment, all the students learned a new type of grammar, which was fabricated and a bit like learning a new type of computer coding language. Everyone who took part in the learning exercise learned it well and to a 90% standard of competence. The researchers then split the large group into three smaller groups, and created three different sorts of experimental conditions to do with sleep and alcohol:

Group 1. In the first group, the ‘control condition’, the participants were able to have uninterrupted sleep for the six nights.

Group 2. In the second group, at the end of the first night following the learning exercise in which they had taken part, and just before going to bed, these participants were encouraged to, and indulged in, a modest amount of alcohol which made them a bit intoxicated. (They each got two to three vodka drinks combined with orange juice). The amount of alcohol given took into account the body weight and gender of the Group 2 members.

–  Group 3. The members of the third group were permitted normal sleeping on the first and the second day of the experiment. But on the third night, they were also given several shots of vodka combined with orange juice directly before going to bed.

The three different groups all learned the material on day 1 of the experiment and on day 7 they were tested on their memory of what they had learned in the grammar learning program.

This strategy of providing set amounts of alcohol was cleverly designed by the research team, so that it would be obvious whether variations in the amounts of information remembered by the three groups would be related to alcohol consumption or sleep disturbance.

The results showed that:

Group 1, the control group, who had had a good night’s sleep for six nights, had excellent recall of the material they had been given, and demonstrated an increased level of understanding of the material as well.

Group 2, who had taken alcohol on the first night after their learning, forgot more than 50% of the material they had learned.

– And Group 3, in spite of the fact that they had got two night’s sleep before having alcohol on the third night, had a similar result to Group 2: They could only recall 40% of the material that they had worked at learning at the start of the experiment.

These results show how rapid eye movement sleep (REM) plays a crucial role in the processing of information and memorising, and how the alcohol stopped this process.

Even a glass of wine or two can interfere with the processing of new information. The brain is still processing information and encoding memories after the first night’s sleep, so alcohol can subvert this fragile process by the way it plays havoc with rapid eye movement (REM) sleep, blocking the consolidation of new memory traces.

In short, alcohol makes people forget things; makes the quality of their sleep deteriorate; and (the research further showed) if you have breathing difficulties, then alcohol will worsen them.

Alcohol and pregnant mothers – a warning

In relation to pregnant mothers, alcohol affects the sleep of unborn babies and new born infants:

Pregnant mothers who have alcoholic drinks in excess may be unaware of how alcohol affects the sleep of their unborn babies in the womb. Alcohol goes from the circulation of the mother to the foetal blood of the unborn baby, according to research that has been undertaken by sleep scientists. They undertook research with women who were either alcoholics or heavy drinkers.[iii]

Havlicek and colleagues (1977) investigated the brain waves of babies, using electrodes on their heads, as they slept, soon after they had been born.

Their research results showed that, by contrast with new-born babies of non-drinking mothers, these babies had a lot less rapid eye movement (REM) sleep.

There was also a change in the form of the electrical signals that came from the babies who had heavy–drinking mothers. There was much less electrical activity when these babies were in rapid eye movement (REM) sleep, a reduction described by the researchers as being about half of the highly active brainwaves of healthy babies.

Later on in this book I’ll describe research on the impact of alcohol and lack of sleep combined.

But as you can see, from the research findings above, the power of alcohol to disrupt our sleep and reduce our vital rapid eye movement (REM) sleep is beyond question.

6.3: Caffeine

“Caffeine has an average half-life[iv] of five to seven hours.”

Walker (2017)


Sleep & Health book, Front coverCaffeine is a very popular drug, present in tea, coffee, soda, Cola, ice-cream, energy drinks, chocolate, pain-killers and weight reduction tablets. And decaffeinated drinks still have caffeine in them. (A ‘decaffeinated’ coffee drink has roughly 15% to 30% of the amount of a regular cup of coffee, which is definitely not free from caffeine).

Caffeine sticks around for a long time in the body

First, it’s important to know the length of time that caffeine affects our body, which helps us understand how caffeine steals our sleep. Scientists describe the ‘half-life’ of a drug, as the amount of time needed to get 50% of the drug out of your body.

A single dose of caffeine takes between five and seven hours to reduce by half, and this means that if someone drank a caffeine-containing drink with an evening meal, then the caffeine would still be affecting their brain (at half its original strength) in the early hours of the morning, with a potentially negative effect upon their attempt to sleep. And a further five to seven hours later, the caffeine would be reduced to 23% of the original strength. And during this process sleep could be disrupted, if the original dose of caffeine was strong enough.

How caffeine disrupts our sleep

How caffeine disrupts our sleep is as follows:

We have a chemical in our bodies called adenosine – which is a bit like a ‘sleep drug’ – which builds up in us the longer we are awake. The length of time we’ve been awake is registered by our bodies, and as the day advances, so does the adenosine. We then get a very strong desire to sleep, and this occurs usually if someone has been awake for between twelve and sixteen hours.

This is a natural, sleep-inducing process and what caffeine does is interfere with the proper functioning of adenosine. Caffeine stops adenosine operating normally in the brain, by fixing onto the adenosine receptors there, and thus it conceals the messages of tiredness sent to the brain by the adenosine. Caffeine creates an artificial wakefulness. But all the while the adenosine is accumulating in the brain.

Later, when the liver has finished processing the caffeine, then the body experiences two types of sleep debt:

Firstly, the tiredness felt before taking the caffeine drink is registered by the body; and:

Secondly, all the adenosine which has been building up in the brain, whilst the experience created by the caffeine was going on, kicks in.

In other words, when the caffeine leaves the adenosine receptors, as it is processed and metabolized, the adenosine returns and envelops those receptors, and then there’s a big increase in the urge to sleep, which was there before the caffeine was taken, plus the accumulated adenosine held back by the caffeine. Walker (2017) describes this as a ‘caffeine crash’, which means that you experience a really strong desire to sleep. Your energy level drops very low, as your body craves the sleep it needs.

What happens as we get older, from our forties onwards, is that the enzyme in our livers which processes caffeine becomes less efficient, and we become more sensitive to the effects of caffeine in our bodies and it takes longer for the caffeine to be removed from our bodies.

People can vary in their ability to tolerate caffeine, but if you stop taking caffeine, then your immunity to its full impact on the nervous system goes. And after several weeks without it, you become as susceptible to its stimulation as you did when you first started taking it!

A fascinating piece of research was conducted in the 1980’s by NASA – (which is the US National Aeronautics and Space Administration) – which was an investigation into the toxicity of stimulant drugs such as caffeine, LSD, amphetamine (speed) and marijuana. The results showed what happened to spiders’ attempts to make a web, when injected with a size-appropriate amount of each of those drugs mentioned. The shocking effect of the caffeine injection was apparent in the picture of the webs constructed under the influence of this drug.[v]

Unlike the webs created by the spiders under the influence of the other stimulant drugs, the webs created by the caffeine-drugged spiders were almost totally disorganised and chaotic. This research finding is of value when we consider the foods that could be consumed by growing children, whose brains are undeveloped.

As part of their school lunch, children can be offered protein bars: some can contain 50 mg. of caffeine (more than a can of Coke). And in a report by the Centre for Science in Public Interest, they found that one scoop of ice-cream which contained coffee, (from the most popular brands), contained 60 mg. of caffeine (approximately the equivalent of two cans of Coke).

A tall, hot, chocolate drink from Starbucks contains 20 mg of caffeine; and the Mocha, (a drink of the same size, containing coffee and chocolate syrup), contains 95 mg – a very powerful dose of caffeine[vi].

The following experiment shows how powerful this stimulant drug is: Christopher Drake, PhD (Associate Professor of Psychiatry and Behavioural Neurosciences at Wayne State University of Medicine in Detroit), ran an experiment into the effects of coffee.[vii]

The participants were given caffeine at three different times of the day:

– Six hours before bedtime,

– Three hours before bedtime, and:

– Immediately before bedtime.

All of them showed noticeable effects of the caffeine on their sleep. So even if you have caffeine six hours before bedtime, it will still have a measurably negative impact on your sleep.

The way the impact of the caffeine was measured is revealing in terms of our own perception of the quality of our sleep: it was measured in two ways.

Firstly, the subjective experience of those taking part and the felt effects on their sleep was written about in their diaries. And

Secondly, the objective, scientific analysis of their sleep levels were measured on sleep monitors at the participants’ homes.

The findings were as follows:

  1. Objective measurement: When the participants took caffeine six hours before bedtime, the sleep monitor showed that they had lost one hour of sleep.
  2. Subjective measurement: But when the participants wrote about their experience of sleep, in the same period, in their journals, it showed that they weren’t conscious of this loss of one hour’s sleep. To them, they were fast asleep, even though their normal sleep patterns of rapid eye movement sleep (REM sleep) and deep sleep (Non-rapid eye movement sleep) had been badly affected.

The results of this experiment are very important, because they show that people don’t realise they are experiencing negative effects upon their sleep, from the caffeine they drank before bedtime.

They also don’t take into account the negative effects of caffeine taken earlier in the day.

But because they haven’t had enough nourishing, deep sleep, they feel tired. This makes them reach for a coffee, to ‘wake them up’. And this coffee leads to poorer sleep on the following night. And so the sleep debt builds up. Shawn Stevenson (2016) describes this process as: “The vicious cycle of sleep deprivation.”

Caffeine affects our nervous system. But it also affects our endocrine system, which consists of glands that create and release hormones. Hormones are chemical substances, produced in the body, that regulate the activity of cells or organs.

Two of the hormones which are released under caffeine’s influence are adrenaline and cortisol, common stress hormones, which also play a part in affecting people’s sleep: the adrenaline switches on our ’fight or flight’ response, which is a very powerful protective response of our bodies to the threat of attack. It causes an increased heart rate, high blood pressure and rapid, shallow respiration, and our body temperature increases and causes sweating.

And if there is too much cortisol (a steroid hormone which is released in response to stress) in someone’s body, then this affects their ability to sleep at night, and affects the stomach, which is very sensitive to stress hormones. It can cause anxious feelings and a nervous stomach, and feelings of paranoia and panic.

We should also distinguish between ‘naturally occurring caffeine’ and ‘synthetic caffeine’.

Naturally occurring caffeine

Reducing naturally occurring caffeine, which is found in tea and coffee – or avoiding it altogether – will have a really beneficial effect on a person’s sleep, by not blocking the powerful, biological processes which have been created by evolution to prepare us for restful sleep each night. Consuming small amounts of caffeine in coffee and tea, early in the day, is less disruptive of sleep; as well as providing a cognitive boost; and contributing antioxidants to the diet. A reasonable guideline would be one cup of freshly ground coffee per day, or two cups of tea.

Synthetic caffeine

Synthetic caffeine, unlike the caffeine in tea and coffee, is a different substance altogether. It is found in energy drinks, and it comes mostly from large factories in China. The Nazis created it in 1942, and then subsequently it was manufactured by Monsanto. It is a combination of different types of chemicals. Here is how Dr Mercola[viii] describes its effects:

“Just one-fourth of a teaspoon can make your heart race and bring on anxiety. Drink enough energy drinks, one after another quickly enough, and you could find yourself in serious trouble… One tablespoon of synthetic caffeine is equivalent to drinking 50 cups of coffee.”

Imagine the effect of 50 cups of coffee on your attempt to find sleep.  And imagine how many multiples of 5-7 hours it would take to get that over-stimulant out of your system. Energy drinks are clearly very bad for the body, on the basis of this research finding by Dr Mercola.

6.4: Electric light

Sleep & Health book, Front coverNew electronic gadgets can be really appealing and very attractive and efficient, and many people can be influenced into buying them, without being fully aware that the price tag for the gadget might be more than financial. There can be a severe physical cost due to the disruptive effects on sleep.

The evidence for this is shown in the details of an experiment into the effects of iPads and other electronic equipment that uses LED lights, (such as mobile phones, and lap top computers) on human beings.

What is LED light?

LED stands for Light Emitting Diodes.

A light-emitting diode is a special kind of diode that glows when electricity passes through it. They are commonly used to illuminate computer screens, iPads, iPhones, etc.)

“Evening blue LED lights have twice the harmful impact on night-time melatonin suppression than the warm, yellow light from old incandescent bulbs, even when their lux intensities are matched.” (Walker, 2017)

Making sure we get a decent night’s sleep is an essential part of preparing for work, school, childcare, college, or any other activity that requires physical and mental energy. And it’s essential for physical and mental health. But the invention of LED lights has made it more difficult to get adequate sleep, for many users of LED based technologies.

LED’s began their popular life in 2014.  In that year, a Nobel Prize in Physics was awarded to the three creators of LEDs: Shuji Nakamura; Isamu Akasaki; and Hiroshi Amano. They got this award because they were responsible for the creation of blue, light-emitting diodes which were described as: “Monumental energy-saving lighting technology”.

These LED lights use less energy than normal light bulbs and therefore they reduce running costs, and have a longer life span. “But they may be inadvertently shortening our own (life)”, is the opinion of Matthew Walker (2017).[ix]

The blue light, which is emitted by the LED lights, is very powerful (twice as powerful as an ordinary light bulb).  But it is also twice as powerful at inhibiting the release of melatonin in our bodies/brains in the evening – (and melatonin, you will recall, is essential for the onset and maintenance of sleep).

Most people may not be aware that they are making it difficult to sleep by using these devices at night.

Why is melatonin so important?

Let’s look at melatonin: I want to explain why melatonin is very important to us: it’s described as the ‘hormone of darkness’, and it increases in volume at dusk or during the early evening.

Its release within our bodies is governed by a part of our brain called the ‘suprachiasmatic nucleus’ which simply means ‘the 24 hour clock within our brain’; and under the orders from this nucleus, melatonin is released into the bloodstream via the pineal gland, which is located deep in the brain.

Melatonin gives a very powerful chemical message to the body-brain that sleep will be coming soon. Melatonin in itself doesn’t bring about sleep, but it signals to the brain regions that do generate sleep, that sleep must be started.

Then, when sleep is underway, melatonin slowly reduces in strength during the night and into the early morning.  Melatonin release is finally stopped when the pineal gland, which had been releasing it, switches the hormone off as the dawn lightens the sky.

Human sensitivity to blue LED light

As human beings, we have a sensitivity to blue LED lights.  This sensitivity within us causes a health problem. Here is an example:

Four researchers, Anne-Marie Chang, Daniel Aeschbach, Jeanne F. Duffy and Charles A. Czeisler, decided to find out what the effect of our sensitivity to LED light has on the ability of people to get a good night’s sleep, if any.[x]

The research experiment was conducted in 2014. The four researchers, mentioned above, got together a group of adults who didn’t have any health problems. Those adults took part in two different processes in which the experimental conditions were different.

Each of the participants experienced both processes, as follows:

Process 1. This involved five nights of reading a book on an iPad for two hours before bed (and they weren’t allowed any other activities like going on the internet and/or checking their emails).

Process 2. This process consisted of five nights reading a paper-based book for several hours before bed.

And to increase the validity of the experiment, some people experienced book reading (Process 2) first; while the remainder started by reading the iPad (Process 1) first.

The experiment lasted for two weeks in a tightly-controlled laboratory experiment. What the researchers found was that the result of reading on an iPad before going to sleep, as compared to reading a paper-based book, held back the release of melatonin by over 50%.

This meant that there was a delayed release of melatonin – (the hormone that signals the brain-body to sleep) – by up to three hours. Consequently, their melatonin didn’t arrive until early in the morning, instead of late at night.

In other words, when they read an iPad, it took much longer to get to sleep than when they read a paper-based book.

And also it was discovered that there were three specific differences in the quality of the sleep of the research participants, when the results of the two processes were compared.

  1. Firstly, because of the delayed onset of sleep as a result of reading the iPad, the participants felt sleepier and had less energy the following day.
  2. Secondly, the iPad reading had the effect – for a few days after the experiment ended – of delaying the release of melatonin[1] by 90 minutes, so that the research participants took longer to fall asleep for those subsequent nights.
  3. Thirdly, the iPad use affected their sleep and reduced their rapid eye movement sleep (REM) sleep, which is vital for brain repair and regeneration.

Blue LED light causes sleep problems

The researchers came to the following conclusion:

“Overall, we found that the use of portable light-emitting devices immediately before bedtime has biological effects that may perpetuate sleep deficiency and disrupt circadian rhythms, both of which can have adverse impacts on performance, health, and safety.”

The evidence from the research study, which I have described above, has shown the full, negative impact of blue LED light when the research participants’ eyes were exposed to it at night-time.  From this evidence, we can infer that children who take their mobile phones and other LED-using devices to bed, at night, will be negatively affected, resulting in a relative ability of the children to get a decent night’s sleep.

The popular, public reaction to the creation of LED light was very enthusiastic. It was described as: “A monumental energy-saving lighting technology”.  And the creators obtained a Nobel Prize in Physics. But the evidence is rapidly building up about the damage that it does in disrupting sleep patterns and the need for very careful management of these electronic devices.

6.5: Nicotine

Sleep & Health book, Front coverThe use of nicotine, found in cigarettes, cigars and pipe tobacco, has several negative effects on sleep, because it speeds up our central nervous system.  Nicotine is a stimulant, which means that it acts as a stressor on the body. The effect of nicotine on the body is stimulating for the smoker, and that’s because the heart starts beating faster, and adrenaline is released into the bloodstream.

Nicotine activates the stress response

What has been activated, by ingesting nicotine, is the ‘fight or flight’ response, and this response makes us tense up our bodies, ready for action. But if we don’t balance this bodily response with periods of rest, then the body will become drained of energy.

Cigarettes, and other sources of nicotine, may seem to give us energy; but in fact they are taking it out of our bodies. And with each drag or inhalation nicotine, carbon monoxide – at 600 times the safety level set by industry (Collings, 1993)[xi] – is entering the bloodstream.

This is why smoking before bedtime will make it difficult to get to sleep: because the nicotine activates the stress response.

Nicotine breaks the ‘sleep architecture’

Furthermore, smoking before sleeping will result in a change in the sleep architecture, (which is the pre-ordained, biological sequencing of distinct stages of sleep that we experience during the sleeping process – including shallow sleep, REM sleep, and NREM sleep). Nicotine causes fractures in the sleeping process.  Consequently, it takes you longer to get to sleep if you smoke. And when you do get to sleep, you have less slow-wave sleep, (called deep sleep) which is an important loss, because slow wave sleep is nourishing for the body and brain[2].


For more information about this book, please click this link: How to Stop Wrecking Your Mental, Physical and Emotional Health: Answers from the science of sleep. 



[1] This delay in the release of melatonin showed that the blue LED light had the power to delay the onset of our normal sleep rhythms.  (These rhythms of sleep and wakefulness, which are known as circadian rhythms, are very powerful, internal, biological regulators of our body temperature, sleep-wake cycle, hormone release, eating habits and other body functions. And they are known to be affected by light and darkness).

[2] Apparently some smokers enjoy smoking in bed, which is highly dangerous. Winter (2017) cites a research study which was carried out in 2005 in Canada, and before the study had started, 24% of the smokers described themselves as having fallen asleep whilst smoking.

[i] Basta, M., Chrousos, G., Vela-Bueno, A. and Vygontzas, A. (2007):  As above.

[ii] Walker, M. (2017) Why We Sleep. London: Allen Lane.

[iii] Havlicek, V., Childiaeva, R. and Chernick, V. (1977) EEG frequency spectrum characteristics of sleep states in infants of alcoholic mothers. Neuropadiatrie 8, no. 4 (1977): Pages: 360-73.

[iv] A ‘half life’ in biology refers to the time it takes for a quantity of a substance, taken into a body, to be reduced to half its initial value or level.  So, in the case of caffeine in our example, if we call your consumption of caffeine ’20 units’, it will take 5 to 7 hours for that value to be biologically reduced (or metabolized) to 10 units; and a further 5 to 7 hours for that level to fall to 5 units; and so on.  The take home message is that caffeine stays in our system for many hours, over-stimulating our central nervous system.

[v] Noever, R., Cronise, J. and Relwani, R.A. (1995) Using spider web patterns to determine toxicity. NASA Tech Briefs 19, No.4 82. And:

Witt, P.N. and Rovner, J.S. (1982) Spider communication: Mechanisms and ecological Significance.  New Jersey: Princeton University Press.

[vi] Smith, C. (2016) Surprising Foods That Have Caffeine and Make You Gain Weight Fast. Online Blog:  Accessed: 18th April 2018.

[vii] Drake, C., Roehrs, T., Shambroom, J. and Roth, T. (2013). Caffeine Effects on Sleep Taken: 0, 3, or 6 Hours before Going to Bed. Journal of Clinical Sleep Medicine. November 15, 2013. Volume 9 (11) Pages: 1195-1200; published online November 15, 2013. Available online:

[viii] Mercola, J. (2019) Pump up Your Performance With Artery-Loving Nitric Oxide and Help Fight Free Radical Damage With Molecular Hydrogen in Just Minutes With One Super fuel Drink’ email&utmcontent=dpe&utm_campaign=20190320Z1_NB_UCM&et_cid=DM275071&et_rid=572156922 © 1997-2019 Dr. Joseph Mercola. All Rights Reserved.

[ix] Walker, M. (2017) Why We Sleep. London: Allen Lane.

[x] Chang, A., Aeschbach, D., Duffy, J.A. and Czeisler, C. A.  (2015). Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proceedings of the National Academy of Sciences of the United States of America. January 27, 2015. Volume 112 (4) Pages: 1232-1237; published ahead of print December 22, 2014. Available online: 10.1073/ pnas.1418490112.

[xi] Collings, J. (1993) The Ordinary Person’s Guide to Extraordinary Health. London: Arum Press.