Sleep has been a subject of enquiry for millennia, since the earliest Greek philosphers and scientists. But early writings on the subject amounted to little more than mere conjecture, and in terms of scientific understanding, the mechanisms of sleep remained largely mysterious until the advances of the mid-20th Century. Since then, our understanding has progressed in leaps and bounds, although there still remains some work to be done.

Below is a chronological timeline of the major landmarks in sleep research. It is necessarily a subjective list, and it owes a debt of gratitude to the Harvard Medical School Division of Sleep Medicine’s interactive website: “Sleep: A Historical Perspective” (

c. 450 BC
Ancient Greek physician Alcmaeon produces the earliest documented theory of sleep, describing it as a loss of consciousness as blood retreats from the surface of the body
c. 400 BC
Hippocrates states his own theory of sleep as being due to blood retreating to the inner regions of the body
c. 350 BC
Aristotle states another theory of sleep, which he saw as caused by warm vapours rising from the heart during digestion, and concludes that sleep is a time of physical renewal
c. 162 BC
Greek physician Galen establishes that consciousness resides in the brain, not the heart
English physician Thomas Cogan’s The Haven of Health rejuvenates Aristotle’s views on sleep
Englishman Thomas Willis publishes The Anatomy of the Brain, the first major work on the brain, and begins to ascribe specific mental functions to different parts of the brain
French scientist Jean-Jacques d’Ortous de Mairan discovers endogenous circadian rhythms in plants
Robert McNish publishes The Philosophy of Sleep, in which he concludes that sleep is “temporary metaphysical death”
English physician John Davy observes in On the Temperature of Man that body temperature and sleep patterns are not dependent of the external environment
Italian scientist Sante de Sanctis concludes that animals dream just like people, and opens the way for a more objective model of dream research
Englishman William Ogle observes that changes in body temperature are internal (endogenous), cyclical and unrelated to external environmental factors
Frenchman Marquis d’Hervey de Saint Denys publishes a book on dream interpretation and lucid dreaming
German psychiatrist Wilhelm Griesinger notes that eyelids flutter during dreaming, suggesting that sleep is a more active process than previously thought
English scientist Richard Caton detects electrical waves on the surface of the brain using a voltmeter, more than 50 years before the invention of the electroencephalograph
Thomas Edison invents the electric light bulb, which would have a huge impact on sleep patterns throughout the world
French physician Jean-Baptiste-Édouard Gélineau identifies and names the condition of narcolepsy
Spanish physician Ramon y Cajal identifies neurons as discrete entities and, in the process, helps found the modern study of neuroscience
Austrian psychiatrist Sigmund Freud publishes The Interpretation of Dreams, concluding that dreams can reveal helpful psychological insights about repressed memories and past traumas, thus founding the field of psychoanalysis
German scientists synthesize barbital, the first commercial sedative sleeping pill, later much abused
Scottish physiologists Sutherland Simpson and J.J. Galbraith identify a 24-hour cyclical temperature pattern in monkeys, regardless of environmental factors
French scientists René Legendre and Henri Pieron identify, from experiments with dogs, a hypnogenic (sleep-inducing) molecule that accumulates in the body during waking hours, which they call “hypnotoxin”
Swiss psychiatrist Carl Jung splits with Sigmund Freud and begins to develop his own “transparency” theory of dreams
French psychologist Henri Pieron publishes Le Problème Physiologique du Sommeil, one of the first books to address the physiological characteristics of sleep
Romanian neurologist Constantin von Economo pinpoints the origin of sleep and wake signals in the hypothalamus area of the brain (although he did not publish his findings until 1930)
German psychiatrist Hans Berger records the first human electroencephalogram as a graphical representation of brain waves, and would later go onto to create a machine to produce such traces
Russian-born American Nathaniel Kleitman opens the world’s first sleep laboratory at the University of Chicago, where he would go on to research circadian rhythms, sleep and wakefulness regulation and sleep deprivation
Hans Berger develops an electroencephalograph device to record brain waves, and notes differences in brain activity during sleep and wakefulness
Physicians begin treating narcolepsy with stimulants like ephedrine and amphetamines
Alfred Loomis, E. Newton Harvey and Garret Hobart identify five distinct stages of sleep, using EEG traces
Nathaniel Kleitman publishes Sleep and Wakefulness, which would become a standard reference book for years to come
Restless legs syndrome is described by Swedish physician Karl-Axel Ekborn
Nathaniel Kleitman and Eugene Aserinsky discover and name rapid eye movement (REM) sleep, distinguishing it from non-REM sleep
William C. Dement shows that sleep consists of cycles of different stages of sleep, repeated four or five times a night
American professor Charles Sidney Burwell describes and classifies obstructive sleep apnea
Melatonin, later found to have profound implications for sleep medicine, discovered by American dermatologist Aaron Lerner
French physician Michel Jouvet asserts that REM sleep is a totally distinct phase of sleep, in which brain wave activity is similar to that during wakefulness, despite the paralysis of the body (what he called “paradoxical sleep”)
Romanian-born scientist Franz Halberg (the “father of chronobiology”) researches circadian rhythms and first uses the term “circadian”
Michel Jouvet identifies the pons region of the brainstem as the area controlling REM sleep
German physicians Jurgen Aschoff and Rutger Wever find that unrestricted “free-runningcircadian clocks have a circadian period of around 25 hours, not 24 (later determined to be actually 24.2 hours on average, not 25)
French neurologist Henri Gastaut produces polysomnographic studies of sleep apnea, establishing in the process a definitive approach for the burgeoning field of sleep medicine
American physiologist John Pappenheimer isolates a sleep-inducing substance he calls Factor S, encouraging future research into the sleep effects of infection-produced cytokines
Italian Pier Luigi Parmeggiani researches the ways in which physiological homeostasis and changes in temperature, blood flow, etc, are associated with the sleep-wake cycle
Allan Rechtschaffen and Anthony Kales devise a standardized terminology and scoring system to classify the different sleep stages (which remains in force until minor changes in 2007)
William C. Dement establishes the first dedicated sleep centre to focus on sleep disorders at Stanford University
American geneticists Ron Konopka and Seymour Benzer identify the PER or period gene, which they associate with the functioning of the internal circadian clock
The body’s circadian clock is definitively located in the suprachiasmatic nucleus of the hypothalamus, by two different pairs of researchers
Long-term studies on French caver Michel Siffre support earlier (also erroneous) claims that the natural free-running circadian period of humans is 25 hours
The Association of Sleep Disorders Centers (later, in 1999, to become the American Academy of Sleep Medicine) is established, with a brief of bringing sleep medicine into the scientific mainstream
William C. Dement and Mary Carskadon develop the multiple sleep latency test (MSLT), an important diagnostic tool in identifying sleep disorders
J. Allan Hobson and Robert McCarley propose the reciprocal-interaction model of sleep regulation (based on neural circuits with both positive and negative feedback loops) to explain the switching between non-REM and REM sleep stages
Hobson and McCarley propose their activation-synthesis model of dreaming, which describes dreams as a result of random brain activity during REM sleep which is then interpreted or synthesized by the forebrain
The peer-reviewed medical journal SLEEP is established as a clearing house for clinical articles on sleep research
American sleep researcher Alfred Lewy demonstrates that exposure to artificial light can suppress melatonin secretion in humans
American sleep researcher Charles Czeisler definitively demonstrates that the duration and organization of sleep is regulated by a person’s internal circadian clock and not just the length of prior wakefulness
Colin Sullivan’s non-invasive continuous positive airway pressure (CPAP) system becomes the treatment of choice for sleep apnea
Hungarian-Swiss researcher Alexander Borbély proposes the now widely accepted two-process model of sleep-wake regulation, combining homeostatic sleep drive with the sleep-independent circadian drive for arousal
Research on rats by Carlyle Smith demonstrates how REM sleep is involved in learning and memory acquisition
Controlled experiments on rats by Allan Rechtschaffen conclusively demonstrate that sleep is a physiological necessity, and that rats totally deprived of sleep suffer severe health consequences and ultimately die
American sleep researcher Misha Radulovacki demonstrates the role of the hormone adenosine in the regulation of sleep and wakefulness
Charles Czeisler demonstrates that bright light treatment can effectively shift and reset the internal circadian clock
REM behaviour disorder is described and defined by Carlos Schenck, Mark Mahowald and others
Meir H. Kryger, Thomas Roth and William C. Dement publish Principles and Practice of Sleep Medicine, an authoritative text in the field for years to come
The Epworth Sleepiness Scale is designed by Murray Johns as a quick and cheap diagnostic tool for sleep disorders
The Journal of Sleep Research is established as a European counterpart to the American journal SLEEP
The neuronal group theory of sleep is proposed by the American James M. Krueger and the Hungarian Ferenc Obál to explain how the sleep state occurs in individual groups of neurons before the organism as a whole
American sleep researchers Craig Heller and Joel Benington propose the theory that the main role of sleep is to restore the brain’s energy as built-up adenosine is replaced by glycogen during sleep
American sleep researcher Clifford Saper identifies the “sleep switch” in the ventrolateral preoptic (VLPO) area of the hypothalamus
Japanese-American neurobiologist Joseph Takahashi identifies and clones the CLOCK circadian gene
Researchers at Stanford University and the University of Texas demonstrate the role of the neurotransmitter orexin/hypocretin in narcolepsy, which may point the way to potential therapies for the disorder in the future
Joshua Gooley and colleagues show how special ganglion cells in the retina of the eye signal light-dark cycle information to the brain’s circadian clock
Christopher Earley demonstrates a strong link between iron deficiency and restless legs syndrome, and identifies specific areas of the brain with low iron levels in RLS patients
American researcher Eugen Tarnow develops his long-term memory excitation theory of dreaming, arguing that dreams are just the brain’s internal excitations of already existing long-term memories
Charles Czeisler and the Harvard Medical School publish some alarming statistics in the New England Journal of Medicine on the detrimental effects of sleep deprivation on doctors and medical interns.
Canadian Jie Zhang proposes the continual-activation theory of dreams, which posits that dreaming and REM sleep are controlled by different parts of the brain, and that dreams are merely an ancillary part of the brain’s consolidation of working memories into long-term memories
A gene specifically linked to restless legs syndrome (RLS) is identified by the American David Rye and the Icelander Hreinn Stefansson
The influential American Academy of Sleep Medicine reclassifies the model of sleep stages for the first time since 1968, adopting among other things a three-stage classification of non-REM sleep, rather than four