SCIENCE23 - The Science of Sleep Part 1: Timeline of our Understanding of Sleep

As regular readers of this blog know, I get ideas for subjects from many sources.  My primary requirement is that the subject is something I know little about, and want to learn more.  Believe it or not, I found the subject for this blog on an online list of possible blog topics.

 

The Science of Sleep is a big subject, very complex, and not well understood yet, despite centuries of observations and decades of scientific research.  I decided to try to cover the topic in two parts.  This blog, Part 1, is an historic listing of opinions, observations, and selected significant scientific research milestones, as we tried to understand this phenomenon.  My objective is to show how our understanding of sleep (what is it, how it works, and why do we need it) has expanded over the years.  For convenience, I will discuss this in chronologically-ordered time periods of history, from ancient times through today.  This review of progress in our increasing understanding of sleep will necessarily be incomplete; the major goal is to give the reader an appreciation for some of the key building blocks in our building the science of sleep.

Part 2 will discuss in more detail what we know about sleep today - a description of the current science of sleep.

I will list my principal sources at the end.

 

Introduction

I include this short introduction as a “snapshot” of our current understanding of some of the key elements of sleep so that the reader can appreciate the gradual build-up our historical understanding of sleep.

Sleep is a state of reduced mental and physical activity in which consciousness is altered and certain sensory activity is inhibited.  During sleep, there is a marked decrease in muscle activity and interactions with the surrounding environment.  While sleep differs from wakefulness in terms of the ability to react to stimuli, it still involves active brain patterns, making it more reactive than a coma or disorders of consciousness.

Sleep occurs in repeating periods, during which the body alternates between two distinct modes: rapid eye movement sleep (REM) and non-REM sleep.  REM sleep is characterized by rapid eye movements, increased brain activity, and the occurrence of dreams.  Dreams are a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep.  REM sleep typically accounts for about 20-25% of a night's sleep in adults, and plays a crucial role in memory consolidation, learning, and emotional processing.  Non-REM sleep is characterized by the absence of rapid eye movements and a slower pace of brain activity, breathing, and heart rate.  It's the more restful and restorative phase of sleep, playing a crucial role in tissue repair, building bone and muscle, and strengthening the immune system.  An internal clock regulates sleep-wake cycles and other body functions, influenced by light and darkness. 

Everyone needs sleep.  Sleep affects almost every type of tissue and system in the body - from the brain, heart, and lungs to metabolism, immune function, mood, and disease resistance.  A chronic lack of sleep, or getting poor quality sleep, increases the risk of health problems like high blood pressure, cardiovascular disease, diabetes, depression, and obesity.  Without sleep, we can’t form or maintain the brain pathways that let us learn and create new memories.  Lack of sleep makes it harder to concentrate and respond quickly.

Sleep is a critical part of our daily routine - we spend about one-third of our lives doing it. 

 

Ancient Civilizations

Ideas about sleep clearly occupied the conscious thoughts of ancient people.  The Rishis of India, who created some of the earliest philosophies in human history, describe states of waking consciousness and dreaming.  The ancient Egyptians built temples to the goddess Isis, where devotees met with priests to engage in an early form of hypnosis and dream interpretation.  The Greeks and Romans had powerful sleep deities, such as Hypnos, Somnus, and Morpheus, who were thought to be the source of any human element occurring in dreams.  And sleep was often compared with death, as in this ancient Chinese proverb attributed to Lao Tzu: "He holds nothing back from life; Therefore, he is ready for death, As a man is ready for sleep, After a good day's work."

It was also well known that many ancient people experienced sleep in two shifts, the first period starting around 8-9 pm, lasting 3-3 ½ hours, followed by a period of wakefulness lasting about an hour or two.  This time was used for various activities, such as prayer, socializing, or even intimacy.  After this interval, people would return to bed for their "second sleep," which would last until dawn. 

Around 450 BC, a Greek physician named Alcmaeon postulated that sleep was a spell of unconsciousness brought on by the lack of circulation to the brain because of blood draining from the body surface.  Similarly, around 400 BC, it was thought that the drop in the surface temperature of a sleeping person was the cause of sleep.

About 50 years later, Aristotle, ancient Greek polymath and philosopher, commented that sleep was an arrest of consciousness in the heart, which, at the time, was thought to be the seat of sense and sensibility.  He also connected the digestive process with the onset of sleep.

It wasn’t until AD 162 that Galen of Pergamum, Greek physician and philosopher, identified the brain, rather than the heart, as the seat of consciousness. 

Greek physician and philosopher, Galen, viewed sleep as a natural process involving the brain as the “master sense organ.”

 

Over the next 1600 years, little progress was made in understanding the nature of sleep.  Sleep was thought of as a detoxifying process, one which shut down some bodily mechanisms, or as being brought on by the lack of blood.

 

1400-1599 Renaissance

While some philosophers/scientists embraced the notion that sleep is triggered by a lack of blood, or perhaps oxygen, to the brain, others thought that toxins accumulate in the body during the day, and slowly drain away during sleep.  Still others believed that sleep is brought on by an "inhibitory reflex" that causes a mechanism in the body to be shut off.  These notions would not be disproved for several centuries, not until scientists began to understand the brain’s role in controlling sleep and wakefulness.

 

1600-1799 Enlightenment

A shift in dream interpretation began.  Previously people considered their dreams to be primarily of sacred origin and left the interpretation of dreams to priests and holy people.  In the early 1600’s, scientists like French scholars Marquis d'Hervey de Saint-Denis and Alfred Maury began to keep a daily record of their own dreams, which they began to interpret themselves without the help of priestly guidance.  Their actions began a trend of self-observation of dreams among the intellectual class.

For others, sleep was often less an intellectual exercise and more an exercise in futility.  Obstacles to sleep were abundant.  The poor and laboring classes especially suffered chronic sleep deprivation.  Weary servants rose frequently during the night to meet their employers' demands.  On top of that, drafty dwellings, noxious chamber pots, the proliferation of lice, bedbugs, fleas, and vermin made sleep a challenge for everyone, including those of higher social classes.

 

1800-1899 Industrial Age

While it was once acceptable to receive visitors in bed, as King Louis XIV of France had done, some cultures - in Victorian England, for example - began to view the bedroom as strictly private and no longer a showpiece of the home.  In addition, sleeping more than seven or eight hours per night became widely viewed as lazy and slothful.

 

1900-1949 Role of the Brain, Structure of Sleep

New technology, instruments, and clinical procedures enhanced the scientific approach to sleep research.  Scientists began to develop theories based on neurological research that definitively refuted previously held explanations that suggested sleep was brought on by gases produced in the intestines during digestion, or by a decrease in stimuli once the Sun went down.  Brain studies began to show the role the brain plays in controlling both sleep and wakefulness.

A growing concern about a lack of sleep in schoolchildren raised public awareness about the need for sleep.  At the beginning of the century, two doctors at Stanford University evaluated the sleep habits of 2692 children and adolescents.  Performing a comparison study with similar research done in England and Germany, Lewis Terman and Adeline Hocking discovered that the American youngsters got noticeably more sleep, often one and a half hours more per day, than their European counterparts.  Terman and Hocking explained this revelation as being a consequence of differences among the countries in the start time of schools, the climate, and overall lifestyle.

Significant milestones in the first half of 20^th century included:

·     1903:  the first sleeping pill, barbital.  Within 30 years, it had become one of the most abused drugs in the United States.

·      1913: French psychologist Henri Piéron, published the first book to attempt to deal with the physiology of sleep.

·  1917: Constantin von Economo, an Australian psychologist and neurologist, studied patients with encephalitis who displayed sleep abnormalities, and identified the region of the brain called the hypothalamus as the center of sleep. 

·   1924: the electroencephalogram (EEG) was invented; however, the article on this discovery wasn’t published until 5 years after its original development.  The differing brain electrical waves during sleep and wakefulness were discovered during this process. 

·   1935: German researcher Irwin Bunning recognized the existence of an internal biological clock, and found that it is inherited in each species.  

·   1937: the American research team of Alfred Loomis and coworkers classified sleep into five stages based on EEG characteristics: drowsy sleep, sleep onset, light sleep,, intermediary sleep, and deep sleep.  The structure of sleep as we know it today was unveiled during this time.

·   1939: Nathaniel Kleitman, American physiologist and sleep researcher, who eventually became one of the most distinguished sleep scientists in the field, published the seminal book Sleep and Wakefulness.  The book covered many years of sleep research, sleep disorders, temperature changes during sleep, and sleep-wake cycles.  Further work established the role of the brainstem in skeletal muscle relaxation during sleep, which advanced the understanding of the neurophysiology of sleep.

Nathaniel Kleitman, American physiologist and sleep researcher, is recognized as the father of modern sleep research.

1950-1959 Understanding REM Sleep

Artificial lights were introduced to northern latitude native Intuit people, enabling them to adapt to a sleep pattern of eight to nine hours per night throughout the year.  Before this time, their sleep patterns were highly influenced by changes in light depending on the Arctic season.  They endured the long, dark winter days by sleeping 14 hours per day, and then switched to sleeping only six hours per day during the constant light of summer.

The existence of REM sleep was first documented by American researchers Eugene Aserinsky and Nathaniel Kleitman in 1953.  Their research established the link between rapid eye movements during sleep and dreaming.  While others had previously noted eye movements during sleep, Aserinsky and Kleitman were the first to systematically study these movements and connect them to dream states.  The next year, it was found that night-time sleep consists of several repeating cycles.  The natural brain hormone melatonin was discovered in 1958, and proved to be key in regulating sleep cycles.

In 1959, a crucial distinction was made between REM and non-REM sleep by French neuroscientist and medical researcher, Michel Jouvet.  Jouvet found that during REM sleep, skeletal muscles are actively inhibited, experiencing a state of paralysis, except for a few key muscles like those in the diaphragm, respiratory system, eyes, and inner ears. This inhibition prevents the body from acting out dreams, a protective mechanism against potential physical harm.  Comparatively, in non-REM sleep, this inhibition is not seen and brain activity is low. 

That same year, the internal biological clock that regulates sleep cycles was named circadian rhythms by Romanian scientist Franz Halberg, who identified and studied them in humans for the first time, and went on to discover other biological rhythms in the human body, thus becoming the father of chronobiology. 

Romanian scientist Franz Halberg was the first to study circadian rhythms.

1960-1969 Sleep Disorders

Sleep research characterized the physiological changes that result from obstructive sleep apnea (OSA). a common sleep disorder where breathing pauses or significantly decreases during sleep due to a blockage in the upper airway.  This systematic description of OSA led to a new way of approaching sleep disorders in general, and ultimately give rise to the discipline of sleep medicine, including a systematic study of temperature, circulatory, and breathing changes during sleep.

Parasomnias (undesirable behaviors or experiences that occur during sleep or during the transition between sleep and wakefulness - sleepwalking, sleep terrors, nightmares, and sleep paralysis - skeletal muscle inhibition as identified above) were studied.  Parasomnias and bed-wetting were identified to be products of confusion awakening from slow-wave sleep, rather than REM sleep, in a 1968 paper by Canadian researcher Roger Broughton.

 

1970-1979 Sleep Research Centers

With the fields of sleep research and sleep medicine growing quickly, several centers and associations were established to expand the reach of sleep scientists and clinicians. The first sleep center was established at Stanford University in 1970.  The Association of Sleep Disorders Center, a professional organization, was created in 1975, and the first issue of the important peer-reviewed scientific journal SLEEP was published in 1978.  

In 1979, an objective measure for quantifying “sleepiness.” the Multiple Sleep Latency Test, was devised by American sleep researchers Mary Carskadon and William Dement.  The test measures the time it takes for a person to fall asleep in a controlled sleep environment, and is still in use today. 

 

1980-1989 Circadian Rhythms, Duration of Sleep

In 1980, the connection between the circadian rhythm, the body's internal clock, and the duration of sleep was determined, with the circadian rhythm primarily governing the timing and duration of sleep.  It was also found that the biological urge to sleep that accumulates as you stay awake. builds up due to the accumulation of adenosine (chemical found in all living cells), and decreases during sleep. 

The most authoritative book on sleep research to date, Principles and Practice of Sleep Medicine, was published in 1989 by Meir H. Kryger, Thomas Roth, and William C. Dement, researchers at the Yale School of Medicine.

 1990-1999 Theories of Sleep

Sleep research subjects in this decade included: narcolepsy; sleep and cardio-respiratory issues, and studies of pain and sleep, circadian rhythms, shift work and its effects on sleep, sleep deprivation, sleep and aging, and infant sleep.  There were over 200 accredited sleep disorders centers and laboratories in the United States alone - designed to recognize and treat all disorders of sleep.  The decade saw the emergence of numerous working theories of sleep.

In 1998, the orexin receptor, a molecule whose absence produces narcolepsy, was discovered.  This discovery revolutionized our understanding of sleep and wakefulness, revealing the crucial role of orexin in regulating these states. 

In 1999, American researcher Eve van Cauter and her group started to explore the effects of sleep deprivation on carbohydrate metabolism.  They found that even young adults experienced impaired carbohydrate metabolism and endocrine function due to sleep debt (cumulative hours of sleep lost). 

In 1998 and 1999, the role of light and melanopsin, a retinal pigment, in setting the biological clock was uncovered, thereby shedding light upon the process of sleep regulation.  Specifically, melanopsin-containing retinal cells in the eye detect light and send signals to the brain's central clock, to regulate various physiological processes like sleep, wakefulness, and melatonin secretion. 

 

2000-2009 Regulating Brain Internal Communication, Sleep Deprivation

Research and advice from physicians regarding the importance of sleep began to seep into the public conscience.  However, changing work and personal schedules to accommodate more sleep continued to be a struggle for many people.   A 2002 poll conducted by the National Sleep Foundation suggested that as many as 47 million adults in the U.S. may be putting themselves at risk for health and behavior problems or injury because they failed to meet their minimum sleep requirements.

Sleep and memory consolidation, as well as sleep deprivation and the risk of poor judgment and motor errors, were studied and data published.

In 2003, American researchers, Giulio Tononi and Chiara Cirelli proposed the theory, that sleep is primarily a process for regulating and maintaining the strength of electrical communication paths in the brain (synapses).  The theory explained that during wakefulness, synaptic connections in various neural circuits strengthen due to learning and experience.  This process leads to a net increase in overall synaptic strength.  Sleep serves to downscale or reduce the strength of these synapses, essentially resetting them to a baseline level. 

American researchers, Giulio Tononi and Chiara Cirelli, proposed that sleep is primarily a process for regulating and maintaining the strength of brain synapses. 

2010-Present Novel Therapies for Sleep Disorders, Cleaning Out the Brain

Novel therapies in the treatment of sleep disorders were developed, including targeted medications, light therapy, and brain stimulation techniques.  However, because of work and social obligations, many people still struggled to find time to accommodate enough sleep for optimal health. 

In 2013, Danish neuroscientist Maiken Nedergaard and her team discovered the glymphatic system, a network in the brain that clears waste products during sleep. 

The National Sleep Foundation's 2020 Sleep in America poll found that 72% of Americans felt sleepy at least two days per week.  The COVID-19 pandemic in 2020-2023 resulted in additional barriers for obtaining sufficient quality sleep.

Since 2020, sleep research has seen advancements in understanding the impact of pandemic lockdowns on sleep, and the role of the intestines, (populated by trillions of microscopic organisms) in sleep regulation; and the potential of ultrashort light flash therapy for circadian rhythm adjustment.  Researchers have also explored the relationship between sleep and cognitive function, including the use of brain stimulation to mimic some benefits of sleep. 

 

 

 

Conclusion

The search for a single unified theory of the cause and effects of sleep in human life goes on, with hundreds of sleep centers currently operating in the United States alone.  While we understand much about sleep's benefits for memory, mood, and brain health, there are still key unknowns about why we sleep and how it affects our bodies.  We don't fully understand the exact mechanisms by which sleep facilitates memory processing or the brain's "rinse cycle".  Additionally, while we know sleep is crucial for overall health, the precise reasons for needing seven to nine hours of sleep each night remain unclear.  Current studies on sleep are looking to establish the genetic, environmental, and psychosocial aspects of sleep and sleep disorders.

 

“The amount of sleep required by the average person is five minutes more.” - Wilson Mizener

 

or alternatively

 

“What a topic; it almost put me to sleep.” - Bob Ring

 

 

 

Sources

My principal sources include “Sleep,” en.wikipedia.org; “Why Sleep Matters: Historical and Cultural Perspectives of Sleep,” and “Science of Sleep: What is Sleep?,” sleep.hms.harvard.edu; “History of Sleep,” news-medical.net; “The Science of Sleep:  Understanding What Happens When You Sleep,” hopkinsmedicine.org; “Brain Basics: Understanding Sleep,” ninds.nih.gov; “How Sleep Works: Understanding the Science of Sleep,” sleepfoundation.org, plus, numerous other online sources.  I particularly want to recognize the use of online artificial intelligence ChatGPT summaries on many subjects related to this blog.