Good sleep, good learning, good life

New article by Dr. Piotr Wozniak: The true history of spaced repetition

The good educator insists on exercise, play, and plentiful sleep: "the great cordial of nature."

John Locke (1632-1704)

I have for years been interested in sleep research due to my professional involvement in memory and learning. This article attempts to produce a synthesis of what is known about sleep with a view to practical applications, esp. in people who need top-quality sleep for their learning or creative achievements. Neurophysiology of sleep is an explosively growing branch of science. Many theories that are currently contested will soon be forgotten as a result of new findings. Consequently, this text is likely to grow old very quickly (compare the old version from the year 2000 here). Still, some basic truths about sleep are well-established, and practical conclusions can be drawn with the benefit to human creativity and intellectual accomplishment. In this text, I provide some links to research papers and popular-scientific articles that advocate disparate and contradictory theories. Please consult other sources to be certain you do not to get a one-sided view! This article includes some indications on how to use free running sleep in the treatment of insomnia, advanced and delayed phase shift syndromes, and some other sleep disorders. If your own experience can contribute to the ideas presented herein, I will gladly hear from you (esp. in the context of learning and creativity).



It is everyone's dream to wake up fresh, happy, and ready for action on a daily basis. Sadly, in the modern world, only a small minority lives that dream. Yet the dream is within reach for most healthy people given:

  1. a bit of knowledge, and
  2. a readiness to make some lifestyle sacrifice.

I hope that this article compiles all the basic ingredients of knowledge that are helpful in accomplishing refreshing sleep. As for the sacrifice, it is important to begin with the understanding that one cannot eat one's cake and have it too. Healthy sleep may be incompatible with some modern habits, some cravings, or some lifestyle choices. At worst, refreshing sleep may be incompatible with one's job or even long-term goals. Due to the latter fact, this article cannot provide a solution for everyone. Moreover, having a happy and fresh mind on a daily basis is a difficult thing to accomplish even with an arsenal of knowledge and full focus on good sleep. However, let me state it emphatically, good sleep on most nights is feasible for most people!

This article was originally written a decade ago. I have always been interested in memory, learning, and sleep. In addition, in my job, sleep is as important as oxygen. As we all move deeper into the Information Age and Knowledge Economy, the issues discussed herein will become more and more important for each of us. After writing the original article, I had the great pleasure of getting in touch with hundreds of people experiencing various sleep problems. I came to see first hand how knowledge of sleep helps solve their problems. I could also see how the industrialized age lays obstacles in one's quest for good sleep and high productivity. I have witnessed a true epidemic of sleep phase disorders, an explosion of interest in polyphasic sleep, and an exponential increase in interest in the matters of sleep in general. Despite my pleas, many people just cannot avoid using an alarm clock, running all-nighters before exams, waking their kids cranky for school, popping pills before sleep, leaving babies in their cots to cry it out for sleep, etc. The picture would be pretty sad and alarming were it not for the fact that there is hope in knowledge. With a degree of determination, everyone can improve his, her, or their kids' sleep.

This article is a compilation of the most important and the most interesting things about the biology of sleep. It is supposed to help you gain knowledge needed to achieve high quality refreshing sleep that will boost your mental powers. The article explains why sleep is vitally important for health and for the brain. It argues that sleep deserves highest respect, and that most people could get excellent sleep if they only followed the prescribed rules.

Since writing the original Good sleep, good learning, good life, tremendous progress has been made in the science of sleep. My own work with tools such as SleepChart and SuperMemo has shed some interesting light on the connection between sleep and learning. As I kept addressing the progress in sleep science in minor articles and FAQs, some visitors to complained that valuable nuggets of information are dispersed throughout the site instead of being organized in a more encyclopedic manner in a single article. Here then comes a comprehensive compilation, in which I would like to retain the focus on practical knowledge that is helpful in achieving good sleep. However, I would still like to smuggle in some lesser known research findings that might be inspiring for an average reader and/or a scientist working in the fields of sleep, memory, and learning. If you believe I left out anything important that others should know, please let me know.

As the article grew to be insanely long, you may wish to begin with the summary at the bottom of the article. And if even that is too long, here are the highlights:

  • respect sleep as your tool for high IQ and good learning
  • free running sleep can help you resolve many sleep problems
  • biphasic sleep schedule is probably the healthiest schedule for creative people
  • do not wake up kids for school; if they cannot wake up in time, let them skip a class or two, or consider homeschooling
  • let babies and young children sleep on demand, co-sleeping is a great idea (even if many pediatricians will tell you otherwise)
  • exercise, learning, and sleep are your best tools for brain growth!
  • avoid regulating sleep and alertness with substances, esp. sleeping pills, alcohol, illegal drugs, nicotine, and caffeine


Incremental writing: Due to the size of the material, this article was written using a technique called incremental writing. Incremental writing is helpful in organizing a large body of earlier writings into a single linear piece. The main advantage of incremental writing is a reasonable degree of coherence despite speedy processing of materials taken from disparate sources. Texts produced with incremental writing are particularly suitable for learning with the help of incremental reading as they produce small independent Wikipedia-style sub-articles. For a linear reader, however, this may mean a degree of bloatedness and an annoying repetitiveness of the main themes for which I apologize. If the size of the article is intimidating, you could try reading it incrementally (e.g. with SuperMemo 2004 Freeware)?

References: Due to the volume of the material, I was not able to provide references for all statements included in the text. Some of these are common sense, some are common knowledge, others I took from memory or from SuperMemo without digging deep to the direct source. If you cannot find a reference for a particular claim, please let me know

Importance of sleep

Why understanding sleep is important?

Too few people realize how important sleep is! The alarm clock is an often-used fixture in an overwhelming majority of households of the modern world. By using electric lighting, alarm clocks, sleeping pills, and shift-work, we have wreaked havoc on the process of sleep.

Four examples of sleep logs that illustrate that modern human sleep patterns are as varried as snowflakes

Four examples of sleep logs that illustrate that modern human sleep patterns are as varied as snowflakes.

Over the last hundred years of the twentieth century, we have intruded upon a delicate and finely regulated process that was perfected by several hundred million years of evolution. Yet only recently have we truly become aware that this intrusion may belong to the most important preventable factors that are slowing societal growth in industrial nations! In a couple of years from now, we may look at alarm clocks and "sleep regulation" in the same way that we look today at other "great" human inventions in the league of cigarettes, asbestos materials, or radioactive cosmetics.

Check this list below and see which applies to you:

  • I often have problems with falling asleep at the right time
  • I often find it painful to get up in the morning due to sleepiness
  • I am often awfully drowsy at school or at work
  • I regularly cut my sleep by 2-3 hours as compared with what my body seems to need
  • I use the alarm clock and truly hate it
  • I drink buckets of coffee or coke
  • I often take 2-4 hour naps in the evening
  • for me, at least one of the above is a source of regular stress or reduced productivity

I bet that chances are around 90% you could subscribe to one of the above. Perhaps this is why you are reading this article. It is also highly likely you have already learned to accept the status quo, and you do not believe you can do much about it. This article may hint at some remedies. However, the bad news is that for a real solution you will probably need to change your family life, your work, your boss, or some social rules!

Sleep isn't just a form of rest! Sleep plays a critical physiological function, and is indispensable for your intellectual development! Those who do not respect their sleep are not likely to live to their full mental potential!

Modern society has developed a set of well-entrenched rules that keep sleep in utmost disregard. This has been driven to pathological levels in American society. Here are some bad rules that hurt sleep:

  • it is ok to use an alarm clock to cut sleep short
  • it is ok to work in shifts
  • it is ok to travel people around the world without much attention to the jet lag problem
  • it is ok to save time by sleeping less and working more
  • it is ok to pull kids out of bed in time for school
  • it is ok to skip nights before important exams, etc.

Cutting down on sleep does not make people die (at least not immediately). It does make them feel miserable, but the ease with which we recover by getting just one good night of sleep seems to make sleep look cheap. Even the reports from the Guinness World Record attempt at sleeplessness (Randy Gardner's awakathon in 1964 lasted 11 days) trivialized the effects of sleeplessness. Many books on psychiatry and psychology still state that there aren't any significant side effects to prolonged sleeplessness! This is false! The Guinness Book of Records has since withdrawn its sleep deprivation category due to the involved health risks.

In 1992, when Bill Clinton was running for president, he proudly admitted that he went 48 hours without sleep because he really wanted to become the next president. Former Senator Bob Dole "improved" the record in 1996 presidential campaign: We have been going 78 hours. We've got to go 96. We have been going around the clock for America. Dole's feat was matched by Vice President Albert Gore Jr., who kept campaigning for three days before the election day of November 7, 2000. After the election, Gore still kept on his feet by going into extra hours of the concede-retract cycle of his cliffhanger contest against Governor George W. Bush of Texas. When Barack Obama was asked about his most desired Christmas gift after over a year of campaigning for president, he answered without hesitation: 8 hours of sleep.

The bad example of disrespect for sleep comes from the most important people in the nation!

Yet some dramatic facts related to sleep deprivation have slowly come into light. Each year sleep disorders add $16 billion to national health-care costs (e.g. by contributing to high blood pressure and heart disease). That does not include accidents and lost productivity at work. For this, the National Commission on Sleep Disorders estimates that sleep deprivation costs $150 billion a year in higher stress and reduced workplace productivity[1]. 40% of truck accidents are attributable to fatigue and drowsiness, and there is an 800% increase in single vehicle commercial truck accidents between midnight and 8 am. Major industrial disasters have been attributed to sleep deprivation (Mitler et al. 1988[2])(incl. Three Mile Island, Chernobyl, the gas leak at Bhopal, Zeebrugge disaster, and the Exxon Valdez oil spill).

It has been known since the 1920s that sleep improves recall in learning. However, only at the turn of the millennium, research by Dr Robert Stickgold, Associate Professor of Psychiatry at Harvard Medical School, has made international headlines. Dr Stickgold's research proves a fact that has long been known yet little appreciated: sleep is necessary for learning (Stickgold 2005[3])! With less sleep, we reduce the recall of facts we learned before or after a shortened night. Studying nights before an exam may be sufficient for passing the exam, yet it will leave few useful traces in long-term memory. The exam on its own replaces knowledge as the main purpose of studying!

By cutting down on sleep, we learn less, we develop less, we are less bright, we make worse decisions, we accomplish less, we are less productive, we are more prone to errors, and we undermine our true intellectual potential!

A change in societal sleep habits can spell a social revolution in learning, health, and productivity on a scale that few imagine! "Judging from history, it would seem that fundamental changes in the way we think about sleep will be required for policy changes that would protect society from sleepy people who make catastrophic errors in industry and transportation" (Merrill Mitler, PhD)

I have studied student personalities among users of SuperMemo for over twenty years now. There are a couple of determinants that make a good, efficient and persistent student. Here are some characteristics of a person who is likely to be successful in learning:

  • highly optimistic
  • sleeps well
  • knowledge hungry
  • stress-tolerant
  • energetic, but able to slow down at the time of learning

Here are some unfortunate characteristics that do not correlate well with the ability to study effectively:

  • prone to depression or mood swings
  • problems with sleep (esp. insomnia)
  • high levels of stress
  • hyperactive and unfocused
  • low stress tolerance (smokers, abusers of mood altering substances, drinkers, etc.)
Sleeping well appears to be one of the most important factors underlying success in learning!

Why do we sleep?

For many years, the physiological function of sleep has not been clear. In most people's mind, sleep is associated with rest and time for mental regeneration. Restorative, protective and energy-conserving theories of sleep have been quite popular until quite recently, when it has become apparent that one long-lasting sleep episode with suppression of consciousness does not seem to be the right way for evolution to tackle depleted resources, toxic wastes, or energy conservation. For example, muscles do not need to shut off completely to get rest. The critical function of sleep is dramatically illustrated in experiments in which rats chronically deprived of sleep eventually die usually within 2.5 weeks (for more see: If you do not sleep, you die!).

In evolutionary terms, sleep is a very old phenomenon and it clearly must play a role that is critical to survival. Only quite recently, it has been proven beyond doubt that the function of sleep is related to learning (not all scientists agree)!

Researchers have long known about the importance of the hippocampus, a small brain organ, for memory formation. Yet it has always been difficult to find out what is special about the hippocampus that distinguishes it from other areas of the cerebral cortex that also show synaptic plasticity, i.e. the ability to store memories.

A collective effort of a number of researchers resulted in the proposition of the concept of neural optimization in sleep (see the next section for a metaphorical explanation: Disk and RAM metaphor). Ground-breaking theories of Dr György Buzsáki and his two-stage model of memory trace formation have shed new light on what might actually be happening during sleep (Buzsáki 1989[4])(important: do not confuse this two-stage model with the two-component model of memory (Wozniak et al 1995[5]) or with the two-component model of sleep regulation (Borbely 1982[6]) below). Using his knowledge of neural networks, ingenious experiments on neuronal firing, and sophisticated mathematical analysis of spatiotemporal neuronal firing patterns, Buzsáki provided a good model explaining how the two components of sleep, REM and NREM sleep, work together to optimize memories. The hippocampus acts as the central switchboard for the brain that can easily store short-term memory patterns. However, these patterns have to be encoded in the neocortex to provide space for coding new short-term memories. This complex process of rebuilding the neural network of the brain takes place during sleep. Unlike rest or conservation of energy, this highest feat of evolutionary neural mathematics requires the brain to be shut off entirely from environmental input (in most animals)! This automatic rewiring is the main reason for which we sleep and why there is no conscious processing involved! During sleep, the brain works as hard as during SAT or GRE exams. It rewires its circuits to make sure that all newly gained knowledge is optimally stored for future use.

We sleep so that the brain can integrate new knowledge and form new associations. As we must sleep for our brain to continue its function, our body attached dozens of important processes to run in sleep as well. In simplest terms, in waking we use and burn, while in sleep we restore and synthetize. Sleep affects the function and health of the entire body.

For more see:

Disk and RAM metaphor

A metaphor can help understand the role of sleep and why alarm clocks are bad. We can compare the brain and its NREM-REM sleep cycles to an ordinary PC. During the day, while learning and experiencing new things, you store your new data in RAM memory. During the night, while first in NREM, you write the data down to the hard disk. During REM, which follows NREM in the night, you do the disk defragmentation, i.e. you organize data, sort them, build new connections, etc. Overnight, you repeat the write-and-defragment cycle until all RAM data is neatly written to the disk (for long-term use), and your RAM is clear and ready for a new day of learning. Upon waking up, you reboot the computer. If you reboot early with the use of an alarm clock, you often leave your disk fragmented. Your data access is slow, and your thinking is confused. Even worse, some of the data may not even get written to the disk. It is as if you have never stored it in RAM in the first place. In conclusion, if you use an alarm clock, you endanger your data. If you do not care about your intellectual performance, you may want to know that there are many other biological reasons for which using alarm clocks is unhealthy. Many people use alarm clocks and live. Yet this is not much different from smoking, abusing drugs, or indulging in fat-dripping pork. You may abuse your brain with alcohol for years, and still become president. Many of mankind's achievements required interrupted sleep. Many inventions were produced by sleepy brains. But nothing is able to change the future as much as a brain refreshed with a healthy dose of restful sleep.

Bad sleep kills and costs billions

Sleep deprivation is a killer! It kills precious life via airplane crashes, nuclear power station failures, car crashes, oil spills, etc. Sleep deprivation can change the course of history. Charles Lindbergh would have been just a footnote in history if he had failed to recover the Spirit of St. Louis from a dive caused by microsleep. Sleep deprivation has changed the future of nuclear fission and the future of oil exploration. Poor sleep kills as many people on the roads as alcohol. 1550 annual fatalities in the US can be attributed to drowsy driving. That's nearly an equivalent of six WTC collapse tragedies in a decade! Amazingly, as the pain and suffering is diluted in the population, drowsy driving does not nearly make as many headlines as a terrorist attack. At least a third of Americans have fallen asleep behind the wheel at least once! During the shift to DST in spring, car accidents increase by 9%. Sleep deprivation carries an astronomical cost to industrialized societies. There are zillions of hours wasted on unproductive learning in schools, and zillions of man-hours wasted on futile tossing and turning in bed. There is also a cost to grumpy behaviors and snappy outbursts. The quest for better sleep provokes desperate solutions such as the Uberman polyphasic sleep, "safe alarm" contraptions, hundreds of books and thousands of blogs with good advice on falling asleep fast, getting up early, or sleeping little. At the same time real solutions are simple and obvious! Read portions of this article and try free running sleep for at least a month to quadruple your knowledge about sleep and its potential to change your life for the better. We need to respect sleep, let kids sleep, design smarter night-shift schedules, and minimize sleep deprivation in jobs that weigh on life and death (e.g. the medical profession).

In a comment to the conclusion of a sleep deprivation debate organized by the Economist, Karen M. wrote: "We don't get enough sleep, and we are not going to "change our ways" because there are already too few hours in most people's days to do things they enjoy. Call it a sad fact of life because that's what it is". Even though Karen attempted to represent the entire population saying "we", many readers of this article will disagree and do their best to get as much sleep as physiologically necessary. Otherwise my writing effort would not be needed. Good sleep makes us nicer, smarter, and saves lives!

See: 10 Things to Hate About Sleep Loss from WebMD.

If you do not sleep, you die!

Nearly everyone has pulled an all nighter once upon a time. Even if this is often an unpleasant experience, it nearly always ends up with a 100% recovery after a single night of solid sleep. It is therefore a bit surprising to know that that a week or two of sleep deprivation can result in death! Sleep researchers constructed a cruel contraption that would wake up rats as soon as they fell asleep. This contraptions showed that it takes an average of 3 weeks to kill a rat by sleep deprivation (or some 5 months by REM sleep deprivation alone)(Rechtschaffen 1998[7]). Dr Siegel demonstrated brain damage in sleep-deprived rats (Siegel 2003[8]). Due to an increase in the level of glucocorticoids, neurogenesis in some portions of the brain is inhibited by lack of sleep[9]. In short, sleep deprivation is very bad for the health of the brain.

Sleep deprivation is a well-known form of torture. Yet, for ethical reasons, the rat experiment could not be reproduced in humans (to its ultimate end). However, we have a rough idea as to the degree of human durability in sleep deprived state due to fact that we can study the effects of sleep disorders. One of them is fatal familial insomnia, in which a mutation causes the affected people to suffer from a progressively worsening insomnia that ends in death within a few months. Another example is the Morvan's syndrome in which an autoimmune disease destroys neuronal potassium channels that lead to severe insomnia and death (unless the disease progresses into remission).

You may have heard of reports of people who do not sleep at all. These are certainly inaccurate or false. Those who report never sleeping are either boasting or experiencing a sleep state misperception that leaves them with an illusion that they do not sleep when resting in bed.

Brain's garbage collection

Why is sleep deprivation fatal? Death of sleep deprivation is like death of an old age in general. Very often, multiple causes conspire to produce the final inevitable outcome. Probably nobody knows the exact answer to this mystery. However, research into the role of sleep gives us pretty strong hints. One of the most important functions of sleep is the re-organization of neural networks in the brain. During the day, we learn new things, memorize, acquire skills, figure things out, set new memories through creative associations, etc. After a long day of waking, the brain is full of disorganized pieces of information that need to be integrated with things we have learned earlier in life. Without this re-organization, the brain would harbor chaos, and would quickly run out of space to store new memories. This neural role of sleep is so fundamental that sleep deprivation affects nearly all functions of the body that are governed by the nervous system. Without a regular garbage collection, individual networks begin to malfunction. These initially minor malfunctions can add up to a serious problem for the entire organism. Most prominent effects of sleep deprivation are problems with thermoregulation, decline in immune function, hormonal changes (e.g. increase in glucocorticoids and catecholamines), metabolic changes[link: Sleep and Glucose metabolism], malnutrition, hallucinations, autonomic system malfunction, changes in cell adhesion, increase in inflammatory factors (e.g. IL-6, TNF, C-reactive protein, etc.), skin lesions, oxidative stress, DNA damage, etc. Those problems become serious enough to kill. Metaphorically speaking, if we compared a less developed organism to a WW1 bomber, we could imagine that the process of evolving into a human being is like acquiring the software needed to fly a B-2 bomber. Even though B-2 is ages ahead of a plane constructed during the life of Orville Wright, it is enough to plant a bug in its software to make it fall out of the sky. Human body in sleep deprivation is like a B-2 with a progressive software malfunction. It may be technologically advanced, it may be smart, and yet it is very vulnerable. The reliance on advanced software or neural function is always dangerous! Luckily, all we need to eliminate the danger is to just go to sleep every day. For more see: Neural optimization in sleep.

Sleep protection

There is a second layer of trouble in sleep deprivation. Due to the importance of sleep, all advanced organisms implement a sleep protection program. This program ensures that sleep deprivation results in unpleasant symptoms. It also produces a remarkably powerful sleep drive that is very hard to overcome. Staying awake becomes unbearable. Closing one's eyes becomes one of the most soothing things in the universe. Are these symptoms a result of network malfunction? Definitely not. If they were, the drive to sleep might malfunction as well. Moreover, recovery from sleep deprivation would not be as fast, as easy, and as complete! Sleep protection program is there, and it can make the effects of sleep deprivation worse. Like a cytokine storm in an overzealous immune system, sleep protection program can potentially add to the damage caused by the network malfunction in sleep deprivation.

Anabolic sleep

Last but not least, sleep has evolved to become a chief anabolic state of the organism. Without it, the body keeps using itself up, without much time to rebuild. Turning on anabolic state does not require turning off the consciousness, however, the time of night rest seems to be the best time for the body to do all the rebuilding. As we must sleep anyway, that anabolic functions became consolidated with other functions of sleep, and now may be indispensable. The anabolic state, and the nighttime increase in GH or testosterone, also affects the neural networks and the status of our "mind software". Hormonal changes stimulate and/or inhibit neural growth. Dr Michael Stryker, best known for demonstrating the role of sleep in brain development (Stryker et al. 2001[10]), says that nighttime hormonal changes may "play a crucial role in consolidating and enhancing waking experience"[11]. One of the leading causes of death in sleep deprivation seems to have been opportunistic bacterial infections caused by a decline in the immune function (e.g. no febrile response). That decline could be caused equally well by (a) poor neural control of the immune function or (b) straight effect of hypercatabolism. Whatever the cause, scientists have quickly figured out that application of antibiotics did not help much in preventing death from those infections. Sleep deprived rats would die anyway. The infection might speed up death that was otherwise inevitable.

Why do we die without sleep?

It is impossible to quantify the contribution of those three factors to the fatal outcome of prolonged sleep deprivation:

  1. network malfunction, or
  2. secondary effects of sleep protection program, or
  3. continuous catabolic state.

Even though the latter two could possibly be remedied pharmacologically, there is no way around network remolding in sleep. Researchers who hope to find a remedy against sleep are plodding a blind path. Without some serious nanotechnology bordering on science fiction, sleep is here to stay with human race for many years to come. Even though, sleep deprivation could kill, sleep is good news. It makes us smarter! We should all embrace the blessings of healthy unrestrained sleep. After all, there are few better things in life than a good night sleep after a well-spent day. Sleep should be listed among basic human rights!

Two components of sleep

Electric lighting and stress are the two chief culprits that have converted the natural process of sleep into a daily struggle for millions. In the new millennium, we can rarely hope to get a good night sleep without understanding the science and the art of sleep. Currently, the societal understanding of sleep and its functions is as dismal as the understanding of the health risks of cigarettes in the 1920s. A majority of the population inflict pain, misery and mental torture on themselves and their children by trying to regulate their sleep with alarm clocks, irrational shift-work patterns, sleeping pills, alcohol, caffeine, etc.

For a chance to break out from unhealthy sleep habits, you need to understand the two-component model of sleep regulation.

There are two components of sleepiness that drive you to bed:

  • circadian component - sleepiness comes back to us in cycles which are usually about one day long
  • homeostatic component - sleepiness increases with the length of time we stay awake

Only a combination of these two components determines the optimum time for sleep. Most importantly, you should remember that even strong sleepiness resulting from the homeostatic component may not be sufficient to get good sleep if the timing goes against the greatest sleep propensity determined by the circadian component.

Circadian component

There are around hundred known body functions that oscillate between maximum and minimum values in a day-long cycle. Because these functions take about a day's time to complete, the term circadian rhythm was coined by Dr Franz Halberg of Germany in 1959 (in Latin circadian means about a day). The overall tendency to maintain sleep is also subject to such a circadian rhythm. In an average case, the maximum sleepiness comes in the middle of the night, reaches the minimum at awakening, and again increases slightly at siesta time in the afternoon. However, the circadian sleepiness is often shifted in phase as compared with your desired sleep time. Consequently, if your maximum sleepiness comes in the morning, you may find it difficult to fall asleep late in the evening, even if you missed a lot of sleep on the preceding day. In other words, the optimum timing of your sleep should take into consideration your circadian rhythm.

Homeostatic component

Homeostasis is the term that refers to maintaining equilibrium or balance in physiological and metabolic functions. If you drink liquids containing lots of calcium, homeostatic mechanisms will make sure that you excrete calcium with urine or deposit it in the bones. This is used to make sure your blood levels of calcium remain the same. Similar mechanisms are used to regulate overall sleepiness and its multiple subcomponents. The longer you stay awake, the more you learn, the more you think, the higher your tendency to fall asleep. On the other hand, caffeine, stress, exercise and other factors may temporarily reduce your homeostatic sleepiness. The homeostatic mechanism prepares you for sleep after a long day of intellectual work. At the same time it prevents you from falling asleep in emergencies.

Clock and Hourglass metaphor

A metaphor is useful in explaining the two components of sleep (for a more scientific explanation see: Borbely model). Deep in the brain, your body clock is running a 24 hours cycle of activity. Every 24 hours, metaphorically, the clock releases a sleepy potion that puts you to sleep (for details see: Why we fall asleep). If you try to sleep at wrong hours, without the sleepy potion, you may find it very hard to fall asleep. All insomniacs suffer from the lack of sleepy potion. If they go to sleep too early, before they get their fix of sleepy potion, they will toss and turn. Often for hours. You need to listen to your body clock to know the right moment to go to sleep.

It is important to know that sleepy potion produced by the body clock is not enough to put you to sleep. The brain also uses the hourglass of mental energy that gives you some time every day that you can devote to intellectual work. When you wake up, the hourglass is full and starts being emptied. With every waking moment, with everything your brain absorbs, with every mental effort, the hourglass is less and less full. Only when the hourglass of mental energy is empty will you able to quickly fall asleep.

To get a good night sleep, you need to combine two factors:

  • your body clock must be saying "time to sleep" (circadian component of sleep)
  • your hourglass of power must be saying "no more mental work" (homeostatic component of sleep)

If your sleepy potion tries to put you to sleep but your hourglass of mental energy is full, you will be very groggy, tired, but you will not fall asleep. If, on the other hand, you try to sleep without the sleepy potion while the hourglass of power is empty, you may succeed, but you will wake up very fast with your hourglass full again. That will make sleeping again nearly impossible. Insomniacs go to sleep before the body clock releases the sleepy potion. When you wake up early with an alarm clock, you can hardly get to your feet because your body is full of sleepy potion, which begs you to go back to sleep. When you are drowsy in the afternoon, your hourglass of mental power might be almost empty. A quick nap will then help you fill it up again and be very productive in the evening. If you drink coffee in the morning, it helps you charge the hourglass and add some extra mental energy. But coffee combined with the sleepy potion produces a poisonous mix that engulfs your brain in sickly miasma. If you try to drink coffee to stay up in the night, you will feel like a horse kicked you in the stomach. That's the acme of a criminal attack on your brain's health.

The fundamental theorem of good sleep

Let us now formulate the fundamental theorem of good sleep:

To get high quality night sleep that maximizes your learning effects your sleep start time should meet these two criteria:
  • strong homeostatic sleepiness: this usually means going to sleep not earlier than 15-19 hours after awakening from the previous night sleep
  • ascending circadian sleepiness: this means going to sleep at a time of day when you usually experience a rapid increase in drowsiness. Not earlier and not later! Knowing the timing of your circadian rhythm is critical for good night sleep

You should be aware that using the circadian component will only work when all its physiological subcomponents run in sync (as it is the case in free running sleep). People with irregular sleep hours and highly stressful lives may simply be unable to locate the point of ascending circadian sleepiness as this point may not exist! For a visual illustration of circadian and homeostatic components, see section Two-component sleep model in SuperMemo. For more on the two components of sleep see: Borbely model.

When good sleep might not come?

You may be surprised to find out that your internal circadian oscillation is based on a period that is closer to 25 hours than to 24 hours! To be exact, it varies between individuals, seasons, and other daily factors such as stress, timing of sleep, timing of the light period, intensity of light, exercise, and many more. Usually it falls into the range from 24.5 hours to 25.5 hours.

Most of us are able to entrain this 25 circadian rhythm into a 24-hour cycle by using factors that reset the oscillation. These factors include intense morning light, work, exercise, etc. German scientists have named these factors zeitgebers (i.e. factors that give time). As a result of the influence of zeitgebers, in a well-adjusted individual, the cycle can be set back by 30-60 minutes each day. However, the entrainment to the 24-hour cycle may come with difficulty to many individuals due to factors such as:

  • blindness (i.e. the inability to use the main zeitgeber: light)
  • short-sightedness (i.e. reduced sensitivity to light zeitgeber)
  • increased demand for sleep (e.g. as a result of intense learning, highly creative job position, exercise, etc.)
  • stress
  • endocrine disorders
  • sleep disorders
  • adolescence

A great deal of sleep disorders can be explained by entrainment failure (i.e. the failure to reset the 25-hour circadian rhythm to the 24-hour daylight cycle). In other words, in the interdependence between sleep disorders and entrainment failure, the cause-effect relationship will often be reversed! Due to the physiological function of sleep, which is the rewiring of the neural networks of the brain, we can naturally expect that the demand for sleep be associated with the amount of learning on the preceding days. This link may also explain a decreased demand for sleep in retirement due to a decrease in intellectual activity. This age-related drop in the demand for sleep is less likely to be observed in highly active individuals. For similar reasons, the entrainment failure can often be found among students during exams. It is not clear how much of this failure can be attributed to stress, or to the desire to do more on a given day, or to the actual increase in the demand for sleep.

Formula for good sleep

There is a little-publicized formula that acts as a perfect cure for people who experience continual or seasonal problems with sleep entrainment[glossary]. This formula is free running sleep!

Free running sleep is defined by the abstinence from all forms of sleep control such as alarm clocks, sleeping pills, alcohol, caffeine, etc. Free running sleep is a sleep that comes naturally at the time when it is internally triggered by the combination of your homeostatic and circadian components. In other words, free running sleep occurs when you go to sleep only then when you are truly sleepy (independent of the relationship of this moment to the actual time of day). Night sleep on a free running schedule lasts as long as the body needs, and ends in natural awakening. No form of sleep disruption is allowed. In particular, any use of an alarm clock is the cardinal violation of the free running sleep principle.

The greatest shortcoming of free running sleep is that it will often result in cycles longer than 24 hours. This eliminates free running sleep from a wider use in society. However, if you would like to try free running sleep, you could hopefully do it on vacation. You may need a vacation that lasts longer than two weeks before you understand your circadian cycle. Even if you cannot afford free running sleep in non-vacation setting, trying it once will greatly increase your knowledge about natural sleep cycles and your own cycle in particular. You should also know that it is possible to entrain one's sleep to a desired sleep bracket (e.g. early rising). However, the entrainment requires iron self-discipline and the religious adherence to the entrainment rules.

Free running sleep

Free running sleep is sleep that is not artificially controlled to match our schedules and desires. It is a sleep without alarm clocks and sleeping pills. Mankind has practised free running sleep for as long as it existed. Our ancestors were gently encouraged to retire to bedtime at sunset, and would wake up naturally, probably after having spent no less than 8-10 hours in bed (see also Segmented sleep). All departures from that healthy practise were an imposition of culture, habit, religion, and/or tradition. Despite our ancestors' lives being fraught with danger, superstition, wars and disease, we should pause and ponder the marvellous impact of this naturally undisturbed sleep on their health. The arrival of fire and candlelight did not provide much incentive to stay up except for those few that have always had much to do in the evening: the first bookworms and artists. Only the genius of Edison and the like brought in the true sleep scourge: the electricity. With the wide dissemination of printed matter and electric lighting, millions would find their evening book far more interesting than sleep. Enter the web. In 2012 AD, we have an endless spectrum of entertainments and distractions that lure everyone away from bed and healthy slumber. More and more, we want to squeeze sleep into designer brackets. We wish to fall asleep at a specific time, and wake up at a specific time. Amazingly, a big chunk of the population does not realize that this is not possible without a detriment to health! Luckily, nearly everyone has the intuition that sleep is vital for healthy living. Those who would want to dispense with sleep altogether form a tiny minority. Nearly all creative people would wish to wake up fresh and ready for action. Preferably at a specified time. The same people wish to be less tired in the evening before sleep, and fall asleep instantly. Preferrably at a specified time. Let me then state it in bold print:

If we exclude unhealthy techniques:

  1. It is not possible to fall asleep whenever we wish.
  2. It is not possible to wake up whenever we wish.
  3. It is not possible to eliminate evening sleepiness.

However disappointing this might be, everyone would do better in life if those truths were assimilated. If we agree to wake up naturally at one's body's preferred time, it should be possible to be fresh and dandy from the waking moment. However, a decline in mental capacity over the waking day is inevitable. It is natural. Midday dip in alertness is also inevitable. And the optimum bedtime is hardly movable. If you try to advance it, you will likely experience insomnia. If you try to delay it, you will cut down on sleep and possibly wake up unrefreshed. If you try to wake up earlier than your natural hour, e.g. by employing an alarm clock, you will wake up with a degree of sleep deprivation that will affect the value of sleep for your learning and creativity. Don't be fooled by the illusive boost in alertness caused by the alarm clock. Yes. This happens to some people, some of the time. This perpetuates the myth that it is possible to wake up fresher with the ring of the alarm.

There is only one formula for healthy and refreshing sleep: Go to sleep only when you are very tired. Not earlier. Not later. Wake up naturally without an alarm clock.

This simple formula is called free running sleep. For many people, after years of sleep abuse, even free running sleep can be tricky. It will take a while to discover one's own body's rules and to accept them. You will know that you execute your free running sleep correctly if it takes no more than 5 min. to fall asleep (without medication, alcohol or other intervention), and if you wake up pretty abruptly with the sense of refreshment. Being refreshed in the morning cannot be taken for granted. Even minor misalignment of sleep and the circadian phase will take the refreshed feeling away. After months or weeks of messy sleep, some circadian variables might be running in different cycles and