99 Fun Facts About Sleep

Dolphins sleep with one eye open; what is this slow-wave sleep pattern called?

Unihemispheric slow-wave sleep lets one half of the brain rest while the other stays alert; the eye opposite the sleeping half shuts while the other stays open to keep watch or keep surfacing to breathe.

What is the term for the temporary muscle paralysis that normally occurs during REM sleep?

Atonia is the REM-sleep paralysis produced by brainstem inhibition of motor neurons; when it fails, sleepers act out dreams, the hallmark of REM sleep behavior disorder.

Roughly how long did 17-year-old Randy Gardner stay awake for his famous 1964 record?

11 days: Gardner stayed awake about 264 hours ending 8 January 1964, observed by Stanford sleep researcher William Dement, and recovered after a few long sleeps.

Roughly how long does one complete adult sleep cycle, through NREM and REM, last?

90-110 minutes covers it: the standard figure for an adult sleep cycle is 90 minutes, with the normal range stretching to about 110.

What is the body's internal process that regulates the sleep-wake cycle roughly every 24 hours called?

The circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and other bodily functions over an approximately 24-hour period.

What is the term for the drowsy transitional state while falling asleep, often with vivid imagery?

Hypnagogia is the drowsy state while falling asleep; its mirror image on waking is hypnopompia, and both can bring fleeting, harmless hallucinations.

Historian Roger Ekirch argued pre-industrial Western Europeans commonly slept in what pattern?

The 'first sleep' and 'second sleep' were separated by an hour or so around midnight that people used for prayer, chores, sex or conversation.

Which NREM Stage 2 EEG waveform, a sharp negative then slow positive wave, may follow a sudden noise?

K-complexes are large, distinct waveforms primarily seen during NREM Stage 2 sleep, which can occur spontaneously or in response to external stimuli, and are thought to help protect sleep and aid memory.

Which part of the night's sleep is the deepest and most restorative for tissue repair and immunity?

NREM Stage 3, often called deep sleep or slow-wave sleep, is essential for the body to repair and regrow tissues, build bone and muscle, and strengthen the immune system.

Which rare neurological disorder, mostly affecting adolescent males, causes recurring episodes of sleep lasting days or weeks?

Kleine–Levin syndrome brings persistent episodic hypersomnia with cognitive and behavioural changes; it was added to the International Classification of Sleep Disorders in 1990.

Non-24-hour sleep-wake disorder, where the body clock drifts free of the day, is especially common in people with what?

Without light reaching the retina, the brain's master clock is never reset to the 24-hour day, so bedtime drifts later and later.

What term describes a sudden, brief loss of muscle tone triggered by strong emotion, such as laughter?

Cataplexy, a hallmark of type 1 narcolepsy, leaves the person fully conscious during an attack, which can range from a drooping jaw to complete collapse.

Where did body-clock researcher Michel Siffre spend six months of 1972 in isolation?

A Texas cave: in 1972 Siffre spent 205 days in Midnight Cave, Texas, with no clocks or daylight, a decade after his two-month 1962 stay underground in the Alps.

REM sleep behavior disorder, acting out dreams, is a strong early predictor of which condition?

Parkinson's disease: RBD can begin years or decades before tremor and stiffness, and a majority of people with RBD go on to develop Parkinson's or a related synucleinopathy.

Which hormone, secreted in the brain at night, helps regulate the sleep-wake cycle?

Melatonin is primarily produced and released by the pineal gland in response to darkness, signaling to the body that it is nighttime and promoting sleep.

Fatal familial insomnia, an inherited disease that destroys the ability to sleep, belongs to which class of disorders?

Prion diseases: FFI is caused by a D178N mutation in the prion protein gene PRNP on chromosome 20, the same gene behind Creutzfeldt-Jakob disease.

What disorder involves recurrent upper airway obstruction while a person is asleep?

Sleep apnea is a common sleep disorder where breathing repeatedly stops and starts during sleep, often due to obstruction of the upper airway.

Which chronic neurological condition causes overwhelming daytime sleepiness and sleep attacks?

Narcolepsy: type 1 involves the loss of roughly 70,000 orexin (hypocretin) neurons in the hypothalamus, which is why low orexin in spinal fluid confirms the diagnosis.

What is the most common sleep disorder, defined by persistent difficulty falling or staying asleep?

Insomnia is the most prevalent sleep disorder, characterized by difficulty falling asleep, staying asleep, or experiencing non-restorative sleep, leading to daytime impairment.

Which chronic neurological disorder is also known as Willis–Ekbom disease?

Restless Legs Syndrome, or Willis–Ekbom disease, is usually chronic and brings an uncontrollable urge to move the legs, typically from uncomfortable sensations at night or at rest.

What is the medical term for sleepwalking?

Somnambulism, also called noctambulism, is a NREM parasomnia in which the sleeper walks in a blend of sleep and wakefulness, usually during deep sleep early in the night.

Which theory says dreams are the brain making sense of random neural activity in REM sleep?

Activation-synthesis theory, proposed by Harvard psychiatrists J. Allan Hobson and Robert McCarley in 1977, treats dreams as the brain's story-making response to brainstem activation during REM sleep.

Which parasomnia, often with a felt presence or chest pressure, strikes while falling asleep or waking?

Sleep paralysis is a temporary state of being unable to move or speak that occurs when a person is waking up or falling asleep, often accompanied by vivid hallucinations.

Chronic sleep deprivation significantly increases the risk of which of the following health conditions?

Type 2 diabetes: people on restricted sleep clear glucose more slowly and show reduced insulin sensitivity, and habitual short sleep is also tied to weight gain and heart disease.

In 1953, graduate student Eugene Aserinsky discovered REM sleep while working at which institution?

University of Chicago: Aserinsky, working under Nathaniel Kleitman, spotted the rapid eye movements of sleeping subjects there; their 1953 Science paper founded modern sleep research.

What is the recommended sleep duration for adults aged 18 to 64 per night?

7-9 hours is the National Sleep Foundation's 2015 recommendation for adults 18-64; newborns need 14-17 hours, and habitual sleep under seven hours is linked to coronary heart disease.

Which brain structure is the master pacemaker of the mammalian circadian clock?

The suprachiasmatic nucleus (SCN) of the hypothalamus acts as the master circadian pacemaker in mammals, receiving light input from the retina and regulating daily rhythms.

What other name for REM sleep did French researcher Michel Jouvet coin in 1959?

Paradoxical sleep: the EEG looks almost like waking, rapid and low-voltage, while the voluntary muscles are switched off; Michel Jouvet coined the term in 1959.

What is the sudden, involuntary muscle twitch that can occur just as a person is falling asleep?

A hypnic jerk, also known as a sleep start, is a sudden, brief, and involuntary contraction of muscles that occurs as a person is falling asleep.

Which of the following is considered a good sleep hygiene practice?

A consistent bedtime and wake-up time, even on weekends, is one of the most reliable ways to steady the body clock; caffeine, irregular hours and late intense exercise all disrupt sleep.

What sleep study records brain waves, eye movements, muscle activity and breathing overnight?

Polysomnography (PSG) is a multi-parametric test used to record various physiological parameters, such as brain activity (EEG), eye movements (EOG), muscle activity (EMG), heart rhythm (ECG), and breathing, during sleep.

Which neurotransmitter is primarily associated with wakefulness and alertness?

Histamine, produced in the hypothalamus, plays a crucial role in promoting wakefulness and alertness. Antihistamines often cause drowsiness due to their effect on these histamine receptors.

Which colour of evening screen glow does most to suppress melatonin?

Blue light: wavelengths around 460-480 nm, sensed by melanopsin cells in the retina, suppress melatonin most, and the effect grows with brightness and duration of exposure.

Which substance's secretion peaks most predictably about an hour after falling asleep?

The surge coincides with the first plunge into deep sleep, which is one reason chronic short sleep is thought to hamper tissue repair in adults and growth in children.

What is the term for a dream in which the dreamer knows they are dreaming and may steer it?

Lucid dreaming is a dream in which the sleeper knows they are dreaming, and in some cases can steer its story or setting; the field that studies it is oneirology.

What are brief, unintended episodes of sleep that occur during a person's waking hours?

Microsleep refers to very brief, involuntary periods of sleep that can last from a fraction of a second to several seconds, often occurring when a person is sleep-deprived and trying to stay awake.

What is the name of the graph produced during a sleep study that plots the stages of sleep against time?

A hypnogram plots sleep stages against time; a typical night shows four to six cycles, with deep sleep concentrated early and REM periods lengthening toward morning.

What are the brief high-frequency brain-wave bursts of NREM Stage 2 called?

Sleep spindles fire at about 12 to 14 Hz for half a second or more, generated by the thalamus, and are linked to memory consolidation and to screening out noise during sleep.

What is the cumulative effect of repeatedly getting too little rest over an extended period?

Sleep debt, or sleep deficit, is the cumulative effect of not getting enough sleep over time. It can lead to fatigue, reduced cognitive function, and other health issues.

Which small brain gland is nicknamed the 'third eye'?

The pineal gland, named for its pine-cone shape, secretes melatonin at night; it gets light information indirectly via a pathway that starts with photosensitive cells in the retina.

What system, active during sleep, is responsible for clearing metabolic waste products from the brain?

The glymphatic system is a waste clearance system in the brain that becomes significantly more active during sleep, effectively flushing out metabolic byproducts that accumulate during wakefulness.

Which parasomnia makes a person hear an imagined gunshot or cymbal crash while drifting off?

Exploding Head Syndrome is a parasomnia where a person hears a loud, imaginary noise, like an explosion or crash, as they are falling asleep or waking up.

What is the term for being a 'lark' or an 'owl', your natural timing for sleep?

Chronotype is your built-in propensity to sleep at a particular point in the day; 'larks' and 'owls' mark its extremes, and it tends to shift later in adolescence.

What is the physiological term for the amount of time it takes to fall asleep once in bed?

Sleep latency is the time taken to fall asleep; a healthy adult typically takes 10 to 20 minutes, and the Multiple Sleep Latency Test uses it to diagnose narcolepsy.

What term describes external cues like daylight that reset the body clock?

Zeitgebers (German for 'time-givers') are external cues, most notably light, that reset and synchronize the body's internal circadian clock to the 24-hour day-night cycle.

What is the grogginess and disorientation felt immediately after waking up called?

Sleep inertia is the temporary feeling of grogginess, disorientation, and impaired cognitive and motor performance that occurs immediately after waking up, especially from deep sleep.

What is the increase in dreaming sleep on recovery nights after being deprived of it called?

REM rebound: on recovery nights people enter REM sooner and spend longer in it, and also drop into deep stage 3 sleep faster, though the extra REM rarely equals what was missed.

What is the standard first-line mask treatment for moderate-to-severe obstructive sleep apnea?

A CPAP machine holds the airway open with mild continuous positive airway pressure, and it only keeps working if used for every sleep, naps included.

Which rare disorder causes excessive daytime sleepiness and long, unrefreshing naps despite adequate night-time sleep?

Idiopathic hypersomnia is a rare chronic neurological disorder characterized by excessive daytime sleepiness despite adequate nighttime sleep, without an identifiable cause.

Roughly how long after falling asleep does a person's first REM episode typically begin?

About 70 minutes: the first REM period usually arrives 70 to 90 minutes after sleep onset and lasts only a few minutes, with each later episode running longer.

Which slow EEG waves, below 4 Hz, define stage N3 sleep?

Delta waves, 0.5 to 4 Hz and high in amplitude, give stage N3 its name of slow-wave sleep; the AASM scores N3 when such waves fill at least 20% of a 30-second epoch.

Which EEG rhythm, at 8 to 12 Hz, dominates in relaxed wakefulness with the eyes closed?

Alpha waves at 8 to 12 Hz appear over the occipital cortex in relaxed wakefulness with eyes closed; their breakup into slower theta activity marks the onset of stage N1 sleep.

What does the Epworth scale, scored from 0 to 24, measure?

Daytime sleepiness: the Epworth Sleepiness Scale, devised by Murray Johns in Melbourne in 1991, asks how likely you are to doze in eight everyday situations, each scored 0 to 3.

Roughly what share of an adult's total sleep is REM, a stage newborns spend far more of their sleep in?

REM takes 20–25% of adult sleep; newborns spend far more of theirs in REM, one reason researchers think it helps the developing brain.

Hans Berger's 1929 paper introduced which recording technique, later the basis of sleep staging?

Electroencephalography (EEG): German psychiatrist Hans Berger published the first human EEG recordings in 1929 after experiments begun in 1924, and its rhythms became the basis of sleep staging.

Which brain region largely switches off during REM sleep, one reason dreams go unquestioned?

The dorsolateral prefrontal cortex, seat of working memory and self-monitoring, shows markedly reduced blood flow in PET scans during REM sleep, while the amygdala and visual areas light up.

Which of these mammals sleeps the least, under five hours a day?

At the other extreme, some bats sleep 18 to 20 hours; giraffes lie down for REM sleep only a few minutes at a time.

Wild sloths fitted with miniature recorders in 2008 turned out to sleep roughly how long a day?

9.6 hours: Rattenborg's team recorded wild three-toed sloths in Panama with miniature EEG loggers, far below the nearly 16 hours a 1983 study had found in captive animals.

Which insects have been shown to use sleep to store long-term memories?

Honeybees: a 2012 Berlin study found bees deprived of sleep after learning a new route failed to retain it, evidence that sleep consolidates their navigation memory.

A 2017 study found which mammal sleeps only about two hours a day in the wild?

African elephant: two wild matriarchs in Botswana tracked with trunk-mounted activity meters in a 2017 study averaged about two hours of sleep a day and once went 46 hours without any.

Which mammal shows the largest amount of REM sleep known in any animal?

REM activity in the platypus never reaches the forebrain, so the egg-laying mammal probably does not dream.

Killer whales and some other dolphins go entirely without sleep during which period?

Their first month of life: a 2005 Nature study found orca and bottlenose calves and their mothers stayed continuously active for weeks after birth, the calves swimming in the mother's slipstream.

Which ducks in a resting flock do the most one-hemisphere sleeping?

The ones on the outer edge: in a 1999 Nature study, mallards at the ends of a row slept with one hemisphere far more often, the open eye facing outward to watch for threats.

An octopus's 'active sleep' stage, full of eye and body movement, lasts roughly how long?

About 1 minute: in a 2023 Nature study, Octopus laqueus cycled roughly every hour from pale-skinned 'quiet sleep' into a minute-long burst of colour-flashing, eye-moving 'active sleep'.

In 2016, REM- and NREM-like sleep stages were reported in which reptile?

Bearded dragon: Gilles Laurent's Frankfurt lab reported in Science that Pogona vitticeps cycles between slow-wave and REM-like states about every 80 seconds; two-stage sleep had been thought unique to birds and mammals.

The word 'siesta' comes from a Latin phrase naming which hour, counted from dawn?

Sixth: siesta comes from Latin hora sexta, the sixth hour after dawn, about noon, when Romans paused for the heat of the day.

NASA found a nap of what length boosted pilot alertness by 54%?

26 minutes: Mark Rosekind's 1990s NASA study of long-haul pilots found a planned cockpit nap averaging 26 minutes improved alertness by 54% and performance by 34%.

In Greek myth, Hypnos, the personification of sleep, is the twin of which figure?

Thanatos, the personification of death, is Hypnos's twin; Hesiod makes both sons of Nyx (Night), and Hypnos's name gave English the word hypnosis.

In Ovid, which son of Sleep specialised in appearing in dreams in human form?

Morpheus: in Ovid's Metamorphoses he is the son of Sleep who takes human form in dreams, while Phobetor becomes beasts and Phantasos inanimate things; morphine is named after him.

In the Iliad, Hera wins Hypnos's help by promising him marriage to which Grace?

Pasithea: in Iliad Book 14 Hera swears by the Styx to give Hypnos the Grace Pasithea; he had hidden with his mother Nyx after an earlier favour enraged Zeus.

In Andersen's 1841 tale, the Sandman is called Ole Lukøje, a surname meaning what?

Shut-eye: Lukøje is Danish for 'close-eye', so Andersen's Sandman is Ole Shut-Eye; he carries two umbrellas, a painted one for good children and a blank one for the naughty.

The phrase 'jet lag' first appeared in print in 1966 in which newspaper?

Writer Horace Sutton called it 'a debility not unakin to a hangover' that comes from jets leaving your rhythms behind.

On average, how long does the body need to reset for each hour of time-zone change?

One day: the rule of thumb is about a day of adjustment per time zone crossed; morning light advances the clock for eastward trips, evening light delays it for westward ones.

Most travellers find jet lag harder to shake after flying in which direction?

East: flying east forces a phase advance, and the body clock, which naturally runs slightly longer than 24 hours, adjusts more easily to delays than to advances.

Which drink's maker sponsored Nathaniel Kleitman's early sleep research?

Ovaltine: the Wander Company, maker of the malted bedtime drink, funded Kleitman's Chicago work in the 1930s, hoping it could be sold as a remedy for sleeplessness.

At which Kentucky landmark did Kleitman run a 32-day sleep experiment in 1938?

Mammoth Cave: in 1938 Kleitman and student Bruce Richardson lived 32 days underground there trying to adopt a 28-hour day; he later studied sleepers in submarines and above the Arctic Circle.

The wake-promoting neuropeptide orexin takes its name from the Greek for what?

Appetite: Sakurai's team coined orexin from Greek orexis in 1998 because it stimulated feeding in rats; a rival team found the same molecule and called it hypocretin.

An orexin receptor mutation causes an inherited sleep-attack disorder in which dog breed?

Doberman Pinscher: Emmanuel Mignot's Stanford colony of narcoleptic Dobermans led to the 1999 discovery of a hypocretin (orexin) receptor-2 mutation, showing how central orexin is to staying awake.

The 2017 Nobel Prize for circadian-clock mechanisms honoured work on which organism?

Fruit fly: Jeffrey Hall, Michael Rosbash and Michael Young traced the daily rise and fall of the period gene's protein in Drosophila, the fruit fly, sharing the 2017 Nobel Prize in Physiology or Medicine.

Caffeine keeps you awake by blocking the receptors of which drowsiness-building molecule?

Adenosine builds up during waking hours and promotes drowsiness; caffeine is shaped alike enough to sit in adenosine receptors without switching them on.

The Z-drug zolpidem, approved in the US in 1992, is sold under which brand name?

Ambien: zolpidem won US approval in December 1992; it trims time to fall asleep by only about fifteen minutes, and guidelines rank it behind cognitive behavioural therapy for insomnia.

Which country, with Austria-Hungary, first adopted nationwide daylight saving in 1916?

Germany: on 30 April 1916 Germany and Austria-Hungary became the first nations to adopt daylight saving, to save wartime coal; Britain followed three weeks later and the United States in 1918.

Astronauts on the International Space Station average roughly how much sleep a day?

6 hours: a 2014 Lancet Neurology study of ISS crews found they averaged about six hours a night, well short of NASA's 8.5-hour allotment, in sleeping bags strapped to the wall of phone-booth-sized cabins.

Which nation's top court acquitted sleepwalker Parks of killing his mother-in-law in 1992?

Canada: the Supreme Court of Canada in R v Parks (1992) upheld Kenneth Parks's acquittal on the ground of non-insane automatism; sleepwalking defences go back at least to Albert Tirrell's 1846 Boston case.

The 'mare' in the word nightmare originally referred to what creature from European folklore?

A chest-sitting demon, the folkloric mare, sat on sleepers and brought frightening dreams; French cauchemar and Spanish pesadilla likewise come from words for pressing and weight.

Aaron Lerner isolated the night hormone in 1958 after cow-gland extract did what to frogs?

Lightened their skin: dermatologist Aaron Lerner, hunting skin treatments, found cow pineal extract blanched frog skin by contracting pigment cells; he named the factor melatonin after melanin and serotonin.

A 2000 study likened driving after 17–19 hours awake to which blood-alcohol level?

0.05%: Williamson and Feyer's 2000 study found 17 to 19 hours awake impaired performance as much as a 0.05% blood-alcohol level, with longer wakefulness matching about 0.1%.

Maureen Weston's 1977 record of 449 hours awake was set in what kind of marathon?

Rocking-chair: Weston stayed awake 449 hours, 18 days and 17 hours, in a 1977 rocking-chair marathon in Peterborough, England, far beyond Randy Gardner's 264 scientifically monitored hours.

Which migrating bird's ability to go weeks on little sleep inspired DARPA's sleepless-soldier research?

White-crowned sparrow: Niels Rattenborg's 2004 study found captive sparrows in migratory restlessness cut sleep by about two-thirds for weeks with no loss of accuracy, which drew DARPA's interest.

In Greek legend, Epimenides of Crete fell asleep in a cave and woke after how many years?

57 years: Diogenes Laërtius says Epimenides dozed off in a Cretan cave and woke 57 years later to find the farm under new owners and his younger brother grown old, one of the earliest long-sleeper tales.

Washington Irving wrote 'Rip Van Winkle' in 1818 while living in which English city?

Birmingham: Irving wrote the tale in June 1818 at the home of his brother-in-law Henry Van Wart in Birmingham, reportedly in a single night, before its 1819 publication in The Sketch Book.

A 77,000-year-old bed of sedges and insect-repellent leaves was found at Sibudu Cave in which country?

South Africa: the bedding at Sibudu Cave in KwaZulu-Natal, reported in Science in 2011, layered sedges with Cryptocarya woodii leaves, a natural insecticide that probably kept mosquitoes off the sleepers.

The best-known classical lullaby, 'Wiegenlied' of 1868, is by which composer?

Brahms: his Wiegenlied, Op. 49 No. 4, was written in 1868 for the birth of Bertha Faber's son; some researchers suspect the heavy-set, snoring composer had obstructive sleep apnea.

Which scholar wrote in 1508 that one man's yawning makes another yawn?

Erasmus: the 1508 Venice edition of his Adagia records that one man's yawning makes another yawn; a later French proverb claimed one good gaper makes seven others gape.

Mask-pressure therapy for obstructed breathing in sleep was first described in which city?

Sydney: Colin Sullivan at Royal Prince Alfred Hospital described nasal CPAP in The Lancet in 1981; the first machines were bulky, noisy vacuum-cleaner motors, but within a decade the design had spread worldwide.

Excessive teeth grinding or jaw clenching, often during sleep, is known medically as what?

Bruxism, from Greek brychein 'to gnash', affects many sleepers who have no idea they do it; the first clue is often a partner's complaint or a dentist spotting worn enamel.

The first primate known to hibernate, a fat-tailed dwarf lemur, lives on which island?

Madagascar: Cheirogaleus medius spends up to seven months a year torpid in tree holes in the island's western dry forests, even though daytime temperatures can top 30 °C, so cold is not the trigger.

In captivity, which of these mammals sleeps least, about 2 hours a day?

Horse: captive horses sleep about two to three hours a day, mostly standing, with only brief lying-down bouts of REM; at the other end, little brown bats manage nearly 20 hours.

Which hormone typically rises through the night to peak around waking time, then falls during the day?

Cortisol climbs through the second half of the night and peaks within an hour of waking, then falls through the day; melatonin runs the opposite course, rising after dark.