Glossary

Last Updated on June 17th, 2017

While I have tried to keep the language in this website as non-technical as possible, it is not possible to entirely avoid jargon and technical and scientific terms, particularly in more technical sections like How Sleep Works, for example. This section provides a quick and dirty reference for some of these terms, again without going into too much technical detail. Words in blue bold in the definitions have their own entry in this Glossary.

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acetylcholine	an important neurotransmitter in the peripheral and central nervous system, involved with the transmission of nerve impulses throughout the body
actigraph	a wrist-mounted unit which measures gross motor activity and movement, and therefore gives a rough and ready guide to sleep and wake time over an extended period (the physical output of an actigraph is called an actigram)
activation-synthesis model	a theory of dreams in which, as circuits in the brain become activated during REM sleep, so do areas of the limbic system involved in emotions, sensations and memories, and dreams are the brain’s attempts to synthesize and find meaning in these random signals
active unlearning	the theory that dreams actively help us to forget non-meaningful or unwanted information by eliminating certain synaptic connections and neural pathways, rather than storing it in long-term memory
adenosine	an inhibitory neurotransmitter and neuromodulator (as well as a major molecular component of ATP, which transports chemical energy within cells), that plays an important role in promoting sleep and suppressing arousal
adrenaline	another name for epinephrine
advanced sleep phase syndrome (ASPS)	a circadian rhythm sleep disorder where a person’s natural rhythm leads to difficulty staying awake in the evening, and concomitant difficulty staying asleep in the morning
agonist	a drug or other chemical that combines with a cell receptor to produce a physiologic reaction similar to that of a naturally occurring substance (the opposite of an antagonist)
alpha waves	brain waves with a frequency of 8-13 Hz (typical of resting wakefulness light non-REM sleep)
amygdala	a structure in the limbic system of the brain, involved with memory and emotions (particularly unpleasant emotions like anger and fear)
anabolic	a constructive metabolic process during which new molecules are constructed and built up, and more complex substances are synthesized from simpler ones, requiring energy
anesthesia	general or local insensibility to pain and other sensation, either as a result of nerve damage or the deliberate administration of drugs (spelled anaesthesia in England)
antagonist	a substance or drug that counteracts, blocks or inhibits the effects of another substance or drug (the opposite of agonist)
apnea	a pause in breathing of at least 10 seconds during sleep, which occurs as a result of sleep apnea (spelled apnoea in England)
arousal	an abrupt change from sleep to wakefulness (in which case it is also called awakening), or from a deeper stage of non-REM sleep to a lighter stage, measurable by encephalography
arousal threshold	the relative ease or difficulty with which a sleeper can be awakened at a particular time (e.g. by a noise, by shaking, etc)
atonia	the muscle paralysis, or complete absence of muscle tone and skeletal muscle activity, that accompanies REM sleep, when motor neurons are not stimulated (also called muscle atonia, muscular atonia, motor atonia or atony)
auditory sleep start	another name for exploding head syndrome
automatic behaviour	a symptom of narcolepsy, in which sufferers may continue to perform normal daily activities or tasks during daytime sleep episodes even though technically asleep, with little or no subsequent recollection of the events, or of having slept at all
awakening	the transition from sleep into wakefulness (also called arousal)
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bed-wetting	uncontrolled urination during sleep, particularly among young children (also known as enuresis)
beta waves	brain waves with a frequency of 12-30 Hz (within the normal range for the active awake state)
biphasic sleep	sleep broken up into two main chunks, separated by an hour or two of quiet restfulness and interludes of sex, common in nomadic or hunter-gatherer societies even today (also called two-phase sleep)
brainstem	the part of the brain connecting the spinal cord to the main forebrain, and responsible for the control and regulation of many vital body functions such as respiration, heart rate and blood pressure
brain waves	electrical impulses or fluctuations of voltage between parts of the cerebral cortex in the brain that are detectable with an electroencephalograph
bright light therapy	a treatment for circadian rhythm sleep disorders in which bright full-spectrum light is administered at desired wake-up times and darkness maintained at sleep times, in an attempt to reset the patient’s circadian clock
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caretaker insomnia	sleep disruption experienced by parents of newborn babies, or adults who care for family members with medical disorders, etc
catabolic	a destructive metabolic process during which molecules are broken down for subsequent re-use, and more complex substances are disassembled simpler ones, releasing energy in the process
cataplexy	a symptom of narcolepsy, in which sudden muscular weakness is experienced during periods of wakefulness, similar to the muscle atonia normally experienced during REM sleep (also called a cataplectic attack)
catathrenia	a parasomnia in which the breath is held during REM sleep, followed by a long drawn-out groaning, wheezing or squeaking sound during exhalation (also called nocturnal groaning)
cathemeral	animal behaviour characterized by sporadic, irregular and random internals of activity and sleep during the night or day
central nervous system	that part of the nervous system responsible for the coordination of motor and mental activities, consisting of the brain and the spinal cord (i.e. excluding the peripheral nervous system)
central sleep apnea	a relatively uncommon variant of sleep apnea which occurs when the brain fails to send the signal to the muscles to take a breath during sleep, and so there is no muscular effort to take a breath
cerebral cortex	the folded and furrowed sheet of neural tissues, forming the outer surface of the brain, which is associated with consciousness, awareness and higher functions such as voluntary movement, coordination of sensory information, attention, learning and memory and the expression of individuality (sometimes just cortex)
chronic insomnia	ongoing insomnia symptoms that recur at least two days a week for at least a month (also called persistent insomnia)
chronobiology	the science of timing medical attention to various organisms of the body depending on the most propitious time of day for those particular organs, according to their particular biological rhythms
chronotherapy	a treatment for circadian rhythm sleep disorders in which the patient’s circadian clock is gradually reset by the systematic manipulation of sleeping and waking times
chronotype	an assessment of what time of day a particular person’s physical functions are most active, depending on their particular circadian rhythms, e.g. whether they are “larks” (morning people), or “owls” (evening people)
circadian alerting system	an alerting pulse sent out by the suprachiasmatic nucleus throughout the day, which counteracts the increasing homeostatic sleep pressure and creates the circadian drive for arousal
circadian clock	the biochemical mechanism, located in the suprachiasmatic nucleus of the hypothalamus in the human brain, that regulates the circadian rhythm of an organism (also known as the biological clock, body clock, circadian pacemaker, circadian system, circadian oscillator, etc)
circadian drive for arousal	a tendency towards wakefulness, arousal and alertness, generated by the alerting pulses of the circadian alerting system
circadian period	the length of the circadian rhythm of an individual, which typically varies between 23.5 and 24.5 hours, with an average of around 24.2 hours
circadian rhythm	a daily cycle of biological activity (including sleep-wake regulation and the circadian alerting system, hormone secretion, body temperature, etc), based on a roughly 24-hour period, and regulated by the body’s internal circadian clock in conjunction with external stimuli such as the light-dark cycle (also called circadian cycle)
circadian rhythm sleep disorders (CRSD)	a family of related sleep disorders, all characterized by an inability to sleep and/or wake at appropriate times, due to the dictates of the individual’s circadian clock
cognitive behavioural therapy (CBT)	a psychotherapeutic approach to dealing with sleep disorders by firstly convincing a patient that their problems are manageable, and secondly establishing a good system of sleep hygiene
coma	a state of prolonged unconsciousness and unresponsiveness to external stimuli, from which arousal is not possible
conditioned insomnia	insomnia perpetuated by stress and anxiety about difficulties sleeping
confusional arousal	a parasomnia, similar to but less violent than a night terror, in which a period of movement and crying gives way to a partial and confused awakening, marked by disorientation, mental dullness, slurred speech and slowed reactions (also called sleep drunkenness)
continual-activation theory	a theory of dreams which asserts that dreams are part of the brain’s consolidation of working memories into long-term memories
continuous positive airway pressure (CPAP)	the most effective treatment for obstructive sleep apnea, in which mild air pressure from a machine helps to keep an airway open during sleep
cortisol	a hormone, produced in the adrenal gland, that helps regulate metabolism, blood sugar levels and the immune system (also called hydrocortisone, and sometimes called the stress hormone as it is also released in response to stress)
cortisol awakening response	an increase of up to 50% in levels of the hormone cortisol, which occurs 20-30 minutes after awakening in preparation for the anticipated stress of daytime activities
co-sleeping	a sleep practice where a baby or young child sleeps in the same bed as its mother, common up until the 19th Century, and outside of the industrialized West even today
crepuscular	animal behaviour characterized by activity mainly during the twilight hours of dawn and dusk
cytokines	proteins (including interleukin and interferon) produced by the immune system while fighting an infection, which also act as powerful sleep-inducing chemicals
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daydream	a level of consciousness somewhere between sleep and wakefulness, in which the mind begins to wander and the imagination may create imagined scenarios and fantasies (usually pleasant and positive)
delayed sleep phase syndrome (DSPS)	the most common circadian rhythm sleep disorder where a person naturally has a much later-than-normal timing of sleep onset, and may experience peak alertness in the middle of the night
delta waves	brain waves with a frequency of 0.5-4 Hz (typical of deep slow-wave non-REM sleep)
diurnal	animal behaviour characterized by activity during the daytime and sleep at night
dopamine	a neurotransmitter in the central nervous system that helps regulate movement and emotion, and is also involved in dreams
dream	images, ideas, emotions and sensations that occur involuntarily in the mind during sleep, mainly during periods of REM sleep
dyssomnia	another name for sleep disorder
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electroencephalography (EEG)	a technique for measuring the electrical activity of the brain using electrodes on the scalp to measure electrical potentials in the brain (an electroencephalograph is the machine that creates this record, and an electroencephalogram is the physical output of the process)
electromyography (EMG)	a technique for measuring the electrical activity of the skeletal muscles (an electromyograph is the machine that creates this record, and an electromyogram is the physical output of the process)
electrooculography (EOG)	a technique for measuring the electrical activity of the retina of the eye over a period of time (an electrooculograph is the machine that creates this record, and an electrooculogram is the physical output of the process)
encephalitis lethargica	a rare viral infection of the hypothalamus, resulting in extreme lethargy, sleep period inversion, even catatonia or an unresponsive coma-like state
endogenous	naturally produced by the body itself, i.e. originating, growing or developing from within an organism, rather than externally
epinephrine	a hormone and neurotransmitter involved in the regulation of heart rate, blood vessels, air passages, metabolic shifts and the fight-or-flight response (also called adrenaline)
episodic insomnia	insomnia symptoms that last up to three weeks (also called short-term insomnia)
Epworth sleepiness scale	a quick and cheap diagnostic tool for sleep disorders, consisting of just eight simple questions about daytime sleep propensity
estivation	the habit of some mammals, reptiles and amphibians of passing the hottest and/or driest months of the year in a dormant condition with a greatly reduced metabolism (the hot weather equivalent of hibernation) (spelled aestivation in England)
excessive daytime sleepiness (EDS)	a major symptom of narcolepsy and hypersomnia and a secondary symptom of many other sleep disorders, characterized by a difficulty in maintaining wakefulness during the day, a tendency to fall asleep when seated or resting, and a general listlessness and lack of energy (also called somnolence)
excite	to encourage or speed up a chemical reaction or process (a substance, such as a neurotransmitter, that excites a process is called excitatory)
exploding head syndrome	a parasomnia in which a person experiences a sudden load noise, sometimes accompanied by a flash of light and a sense of fear or anxiety, apparently from within their own head, just as they are about to fall asleep (also called an auditory sleep start)
extreme napping	a planned sleep pattern consisting of segmented and broken-up sleep periods, e.g. 15-minute naps every 4 hours throughout the day and night (also called polyphasic sleep or polycyclic sleep)
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fatal familial insomnia (FFI)	a very rare fatal sleep disorder in which malformed proteins called prions attack the sufferer’s thalamus
forebrain	the main part of the brain (including the cerebral cortex, limbic system, thalamus and hypothalamus, and excluding the brainstem)
free-running sleep	sleep that is not adjusted or entrained to the usual 24-hour cycle (e.g. in sleep experiments involving constant light or constant darkness)
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gamma waves	brain waves with a frequency of 25-100 Hz (within the normal range for the awake state)
ganglion cells	photoreceptive neurons found near the inner surface of the retina of the eye, which transmit visual information on the light-dark cycle to the brain
gene	a sequence of molecules in a segment of DNA that provides coded instructions leading to the expression of particular hereditary characteristics in the organism (genetics is the science of heredity)
glutamate	the most common excitatory neurotransmitter in the nervous system, with a key role in learning and memory among other things
glycogen	the main form of energy storage in animals, stored in the liver and muscles, and easily convertible into glucose
growth hormone	a hormone, essential to the repair and restoration processes of the body and mainly secreted during non-REM sleep, that stimulates growth and cell reproduction and regeneration
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hallucination	waking dreams, often as a result of a mental or physical disorder, in which vivid and substantial perceptions occur in the absence of external stimuli, but which nevertheless have a compelling sense of reality
hibernation	the habit of some mammals, reptiles and amphibians of passing the winter in a dormant condition with a greatly reduced metabolism
histamine	a neurotransmitter released by the body during allergic reactions and immune responses, as well as in sleep modulation
homeostasis	any internal biochemical system that regulates the internal environment, with a view to maintaining body properties such as temperature, acidity, etc , in a stable and relatively constant condition (e.g sleep-wake homeostasis)
homeostatic sleep drive	the pressure to sleep, generated by the process of sleep-wake homeostasis (also called homeostatic sleep pressure)
hormone	a chemical substance, secreted by the endocrine glands and transported through body fluids such as the blood, to affect the functions of specific organs or tissues in different parts of the body
hypersomnia	a sleep disorder characterized by excessive daytime sleepiness, excessive sleep periods each day or just an inability feel refreshed even after an apparently adequate sleep period (also called hypersomnolence or idiopathic hypersomnia)
hypnagogic	relating to the drowsy state just before sleep onset
hypnic jerk	a sudden twitch or start, often accompanied by a falling sensation and a short awakening, commonly experienced just as a person is starting to fall asleep (also known as a hypnagogic jerk or sleep start)
hypnogenic	sleep inducing (e.g. a substance in the body, or a drug)
hypnogram	a simplified graph representing the stages of sleep (based on electroencephalography recordings over the period of a night) as a function of time
hypnopedia	another name for sleep learning
hypnophobia	an anxiety disorder manifesting as an abnormal fear of sleeping or going to bed (also called somniphobia or negative sleep conditioning)
hypnopompic	relating to the drowsy semi-conscious state just before complete wakefulness
hypnotic trance	a semi-conscious state induced by hypnosis in which a person is apparently unaware of the environment and is highly susceptible to suggestion
hypocretin	another name for orexin
hypothalamus	a region of the brain, located between the thalamus and the mid-brain, which acts as a major control centre for the autonomous nervous system, regulating sleep cycles, body temperature, appetite, hormone production, etc
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idiopathic	arising spontaneously, not resulting from any other disease, arising from no known cause or trigger
idiopathic insomnia	persistent insomnia, usually beginning in childhood, which is unrelated to psychosocial stressors or medical disorders
immune system	the network of cells, biological structures and processes that work to protect an organism from diseases and foreign substances
inhibit	to stop, or decrease the rate of, a chemical reaction or process (a substance, such as a neurotransmitter, that inhibits a process is called inhibitory)
insomnia	a general term for any difficulty in falling asleep or staying asleep, such that the sufferer is still tired, unrefreshed and unrested on waking
irregular sleep-wake rhythm disorder (ISWRD)	a circadian rhythm sleep disorder, mainly found in sufferers from dementia, where a disrupted circadian rhythm leads to numerous naps at irregular times throughout a 24-hour period, with no main nighttime sleep episode (polyphasic sleep)
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jet lag	a temporary mismatch between the external environment and a person’s internal circadian clock that occurs when someone travels rapidly across multiple time zones (e.g. in an airplane), causing sleep patterns to be impacted (sometimes recognized as a sleep disorder, even though it is usually short term and self-imposed)
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K-complex	a short negative high voltage peak in brain wave activity, followed by a slower positive complex, and then a final negative peak, with each complex lasting 1-2 minutes, typically experienced during Stage 2 non-REM sleep
Kleine-Levin syndrome	an extreme variant of hypersomnia occurring in teenage boys, in which sufferers may sleep for several days at a time, waking ravenously hungry, irritable and sometimes hypersexual (also known as Sleeping Beauty syndrome)
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light-dark cycle	the natural daily cycle between sunlight and darkness resulting from the Earth’s turning on its axis
light sleep	the early stages of a sleep cycle, comprising stage 1 and stage 2 non-REM sleep, in which brain wave activity is still relatively fast and awakening is relatively easy
limbic system	a group of structures deep in the centre of the brain, including the hippocampus and amygdala, which are involved in emotion, behaviour, motivation, memory, olfaction, and various autonomic functions
locus coeruleus	a group of neurons in the pons region of the brainstem responsible for the blocking or inhibition of motor signals during the atonia of REM sleep (also spelled caeruleus or ceruleus)
long-term memory excitation theory	a theory of dreams which argues that dreams are just the brain’s internal excitations of already existing long-term memories (a process which is going on all the time, but the suppression of the “reality checking” of the brain’s executive function during sleep allows us to notice it)
lucid dreams	extremely realistic and vivid dreams, in which the sleeper is actually aware of dreaming, and in which it may be possible to consciously continue, or even to exert some control over, the dream
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maintenance of wakefulness test (MWT)	a method of measuring daytime sleepiness, by looking at an individual’s ability to stay awake while reclining in a quiet darkened room
melanopsin	a light-sensitive pigment (most sensitive to short wavelength “blue light”) in the ganglion cells of the retina of the eye
melatonin	a naturally occurring hormone, secreted in the pineal gland in the brain in response to darkness, that plays a key role in biological circadian rhythms such as sleep and reproductive cycles
memory bias	a deviation in judgement or perceptual distortion, either deliberate or subconscious, that enhances, impairs or changes a particular memory, making it unreliable
memory consolidation	the stabilization and strengthening of a memory trace after its initial acquisition, through the repeated reactivation of synaptic connections and pathways within a neural network
metabolic process	an organic process in a cell or organism that is necessary for life, collectively known as metabolism
metabolism	the physical and chemical processes (which may be anabolic or catabolic) by which an organism functions and operates, and by which it grows, reproduces, maintains its structures and responds to its environment
metaturnal	animal behaviour characterized by activity (and sleep) partly during the day and partly at night
micro-awakening	a very short awakening during a sleep period, of just a few seconds or even fractions of a second, often not consciously recognized or remembered
microsleep	a very short episode of sleep during a period of wakefulness, lasting from a fraction of a second up to about 30 seconds, often resulting from sleep deprivation or sleep disorders
mixed sleep apnea	a relatively rare combination of central sleep apnea and obstructive sleep apnea
monophasic sleep	sleep organized into just one long period each day (also called monocyclic sleep)
multiple sleep latency test (MSLT)	a method of measuring daytime sleepiness, by looking at the time needed to go from wakefulness to sleep onset in a series of daytime nap opportunities at two hour intervals
muscle atonia	another name for atonia (also called muscular atonia, motor atonia or atony)
muscle tone	the amount of tension, or resistance to passive stretching, in the muscles, which helps to maintain posture, and which is dramatically reduced during the atonia of REM sleep (also called tonus)
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nap	a brief period of sleep taken outside of the main sleep period, i.e. usually during the daytime (also known as a siesta)
narcolepsy	a chronic sleep disorder characterized by uncontrollable excessive daytime sleepiness, sleep attacks at inappropriate times, hallucinations, automatic behaviour, episodes of sleep paralysis and/or cataplexy, and an abnormal tendency to pass directly into REM sleep from wakefulness
negative sleep conditioning	another name for hypnophobia
nerve cell	another name for neuron
neural network	a group or circuit of neurons connected or functionally related to each other through synapses and synaptic connections
neuromodulator	a neurotransmitter (such as dopamine, serotonin, histamine, acetylcholine, etc) which diffuses through large areas of the nervous system, thereby affecting many different neurons
neuron	a specialized electrical impulse-conducting cell that processes and transmits information through the brain and nervous system by means of electrical and chemical signals (also known as a nerve cell)
neuronal group theory of sleep	the theory that individual groups of neurons in the brain enter into a state of sleep after a certain threshold of activity has been reached, and that, once enough groups of neurons are in this sleep state, the whole organism falls asleep
neurotransmitter	a chemical substance that transmit signals, or nerve impulses, between neurons across a synapse (examples include acetylcholine, adenosine, glutamate, epinephrine, norepinephrine, dopamine, serotonin, histamine, orexin, etc)
nightmare	an unpleasant dream with particularly vivid and disturbing content, usually accompanied by a strong negative emotional response (e.g. fear, horror, despair, anxiety, sadness, etc) and often resulting in awakening
night terror	a parasomnia manifesting as intense, violent and inconsolable feelings of terror or dread on waking from deep slow-wave sleep, often accompanied by screaming, thrashing, a period of confusion and almost complete amnesia (also called sleep terror or pavor nocturnus)
nocturia	an excessive need for nighttime urination, particularly common in pregnant women and the elderly (also called nycturia)
nocturnal	animal behaviour characterized by activity during the night and sleep during the daytime
nocturnal dissociative disorder	a psychiatric wakefulness disorder (as opposed to a sleep disorder), superficially similar to sleep-walking, in which sufferers wander off or act out scenarios while technically awake, but without being aware and with no subsequent memory of them, usually as a psychological response to earlier abuse or trauma
nocturnal groaning	another name for catathrenia
nocturnal myoclonus	another name for periodic limb movement disorder
non-24-hour sleep-wake syndrome (Non-24)	a circadian rhythm sleep disorder, mainly found among blind people with no light perception, in which the sleep-wake cycle and sleep patterns are totally desynchronized from the normal 24-hour day and follow a free-running cycle (also called free-running disorder)
non-REM sleep (NREM)	non-rapid eye movement sleep, that part of the sleep cycle (subdivided into 3 or 4 sleep stages, ranging from light sleep to deep slow-wave sleep) in which little or no eye movement occurs, there is no muscle atonia, and dreams are possible but relatively rare
norepinephrine	a neurotransmitter in the autonomic and sympathetic nervous system affecting blood pressure, etc, and a stress hormone affecting parts of the brain that control attention (also called noradrenaline)
normal hypersomnia	a term sometimes applied to naturally long sleepers (not necessarily reflecting clinical hypersomnia)
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obstructive sleep apnea (OSA)	the most common form of sleep apnea, in which breathing during sleep is disrupted by a physical block to the airflow caused by a collapse of the soft tissues in the throat and respiratory tract, causing constant awakenings
oneiric Darwinism	the dream theory that new ideas may be created through the generation of random thought mutations during dreaming, most of which may be discarded as useless, but some of which may be retained as potentially valuable
orexin	a neurotransmitter produced in the hypothalamus that regulates arousal, wakefulness and appetite, and the lack of which causes narcolepsy (also called hypocretin)
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paradoxical insomnia	genuine complaints of little or no sleep that are not corroborated by objective evidence of sleep disturbances (a kind of sleep state misperception)
paradoxical sleep	another name for REM sleep
paradoxical therapy	a psychological approach to insomnia treatment, which asks the insomniac to do the exact opposite of trying to fall asleep, in an attempt to eliminate any subconscious resistance to treatment
parasomnia	a type of sleep disorder involving abnormal or unnatural movements, behaviours, emotions, perceptions or dreams during sleep, often involving partial awakenings during the transitions between sleep and full wakefulness
parietal lobe	the central part of the cerebral cortex of the brain, responsible for bringing together all the different senses of the brain, and damage to which can lead to a complete absence of dreams)
periodic limb movement disorder (PLMD)	a sleep disorder where a sleeper moves limbs involuntarily during non-REM sleep, often manifested as quite violent involuntary kicking during sleep, thus disrupting normal sleep patterns (also known as nocturnal myoclonus)
phase advance	a variation in an individual’s circadian rhythm such that sleep onset and awakening typically occur significantly earlier than average (e.g. in older people)
phase delay	a variation in an individual’s circadian rhythm such that sleep onset and awakening typically occur significantly later than average (e.g. in adolescents and teenagers)
pineal gland	a small pine cone-shaped gland near the centre of the brain that produces several important hormones including melatonin
polyphasic sleep	segmented and broken-up sleep patterns, common in many animals and still found in many undeveloped or nomadic human societies (also called polycyclic sleep, and sometimes referred to as extreme napping when implemented as a planned sleep pattern)
polysomnography (PSG)	continuous monitoring of multiple measures of physiological activity during sleep, including brain activity, eye movement, muscle activity, respiration and heart rhythm (a polysomnogram is the physical output of the process)
pons	a thick band of nerve fibres in the brainstem, linking the spinal cord with the forebrain and containing nuclei dealing with REM sleep, respiration, swallowing, sense perception and many other functions
post-lunch dip	a tendency to sleepiness around 2:00-3:00pm, as a result of a natural dip in circadian alerting system activity, and not due to the effects of eating lunch (also called the mid-afternoon slump)
prefrontal cortex	the front part of the brain, involved in working memory, attention, personality expression, decision-making, logical reasoning and self-control, notably inactive during REM sleep
primary hypersomnia	hypersomnia symptoms that continue unabated for months or even years
primary insomnia	insomnia not due to any specific physical or mental imbalance, and having no obvious other cause
prion	a tiny malformed protein capable of infecting organisms in much the same way as a virus, leading to diseases like BSE in cattle and fatal familial insomnia in humans
progressive dreams	related dreams that occur in a sequence over a several nights, with one night’s dream continuing where the previous night’s dream left off
protein	one of a large class of large and complex organic macromolecules (including enzymes, hormones, antibodies, hemoglobin, etc), composed of amino acids, which are essential for the proper functioning of an organism, including metabolism, the immune system, growth and tissue repair
psychophysiological insomnia	insomnia arising from to a variety of psychological and behavioural stressors and/or environmental and situational causes
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rebound sleep	lengthened and deepened recovery sleep following a period of sleep deprivation (this may apply to both REM sleep and slow-wave non-REM sleep)
recurrent hypersomnia	hypersomnia symptoms that recur several times during the year, in between periods of relatively normal sleep-wake cycles, sometimes accompanied by other disorders such as hypersexuality or compulsive eating
recurrent isolated sleep paralysis	another name for sleep paralysis
recurring dreams	dreams, often having a nightmare quality, that repeat themselves night after night with little variation, possibly due to some unresolved conflict depicted by the dream
REM sleep	rapid eye movement sleep, that part of the sleep cycle characterized by rapid and random eye movement, high frequency (theta, alpha and even beta) brain waves similar to those during wakefulness, muscle paralysis or atonia, and frequent and vivid dreams
REM sleep behaviour disorder (RBD)	a relatively rare parasomnia, in which the protective muscle atonia of REM sleep is absent, so that sufferers try to physically act out their dreams, often suffering (or causing) physical injuries in the process
restless legs syndrome (RLS)	a sleep disorder characterized by a constant urge to move the legs in order to stop uncomfortable or odd sensations (tingling, creeping, crawling or aching), mainly during the time before sleep onset, leading to disrupted sleep
reticular activating system	a network of neurons in the reticular formation of the brainstem, which is responsible for regulating alertness and arousal and mediating transitions between sleep and wakefulness (also called the ascending reticular activating system)
reverse learning	the theory of dreams, similar to active unlearning, that synapses activated during dreams actually become weakened rather than strengthened, so that the content of dreams is unlearned and eliminated from memory
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secondary insomnia	insomnia occurring as a side-effect of another disease or complaint (e.g. hyperthyroidism, congestive heart failure, chronic obstructive pulmonary disease, dementia, pain-related conditions, or other underlying sleep disorders)
sedative	a sleeping pill or medication that causes drowsiness, induces sleep onset, and/or maintains sleep (also called a hypnotic)
selective mood regulatory theory	a theory of dreams whereby dreams function to contain and absorb negative emotional surges, and to expunge them from waking memory, allowing the dreamer to remain soundly asleep despite the inner turmoil, while simultaneously working out emotional problems
serotonin	a neurotransmitter in the central nervous system, derived from tryptophan, involved in sleep, depression, memory and other neurological processes, as well as in feelings of well-being and happiness
sexsomnia	another name for sleep-sex
shift work	unusual day shifts, evening shifts or night shifts, in which the constantly changing sleep patterns can disrupt workers’ circadian clocks and seriously impact the quantity and quality of their sleep (a formal sleep disorder known as shift worker disorder is sometimes recognized)
siesta	another name for a nap
sigma wave	another name for a sleep spindle
sleep	a naturally-occurring, reversible, periodic and recurring state in which consciousness and muscular activity is temporarily suspended or diminished, and responsiveness to outside stimuli is reduced
sleep apnea	a potentially life-threatening sleep disorder characterized by abnormal pauses in breathing during sleep, the most common variant of which is obstructive sleep apnea (spelled apnoea in England)
sleep architecture	the structure and pattern of sleep, including overall sleep duration, sleep depth, sleep stages and phases, time spent in non-REM and REM sleep, the organization of sleep cycles, etc
sleep cycle	the recurring progression of sleep stages, through different levels of non-REM sleep to REM sleep and then back again, repeated four or five times a night
sleep debt	the cumulative effect of insufficient sleep, which leads to mental, emotional and physical fatigue, and problems in effectively performing day-to-day activities (also known sleep deficit)
sleep deprivation	the condition of not having (or not being allowed to have) sufficient sleep, which leads to sleep debt (also called sleep loss)
sleep disorder	any medical disorder which negatively affects a person’s healthy sleep patterns, to the extent that it interferes with the person’s normal physical, mental and emotional functioning (also known as somnipathy or dyssomnia)
sleep-disordered breathing	any medical condition characterized by repeated episodes of not breathing (sleep apnea) or under-breathing (sleep hypopnea) during sleep
sleep gate	the point encountered during the late evening when the circadian drive for arousal falls off, melatonin production increases, and the homeostatic sleep drive becomes dominant enough to allow sleep to occur (also known as the primary sleepiness zone or sleep onset zone)
sleep hygiene	the practice of consciously following daily guidelines and adopting particular habits and routines in order to ensure a more restful and effective sleep
sleep hypopnea	a sleep disorder similar to sleep apnea, although less common, in which excessively shallow breathing leads to constant awakenings, as well as loud heavy snoring and choking sounds or snorts
sleep inertia	the feeling of grogginess, impaired alertness and decreased motor dexterity immediately following awakening
sleeping sickness	an old name for narcolepsy
sleep latency	the length of time taken to transition from full wakefulness to sleep onset, i.e. from “lights out” until sleep (also called sleep onset latency)
sleep learning	the memorization of facts, transference of information or acquisition of new knowledge during sleep, such as by the playing of audio recordings, etc, now largely discredited as a technique for enhanced learning (also called hypnopedia)
sleep-maintenance insomnia	difficulty staying asleep or going back to sleep once woken (also known as middle insomnia)
sleep medication	pills or other medicinal treatments for transient insomnia and other sleep disorders, including benzodiazepines and similar agents, sedative antidepressants, antihistamines, etc
sleep medicine	the medical specialty devoted to the diagnosis and therapy of sleep disturbance and sleep disorders
sleep onset	the transition from wakefulness into sleep, commonly referred to as “falling asleep”
sleep-onset insomnia	difficulty getting to sleep in the first place (also known as initial insomnia)
sleep paralysis	a parasomnia manifesting as a complete inability to move or speak during the times of falling asleep or waking up, due to disrupted REM sleep and the muscle atonia that usually accompanies it “leaking” into wakefulness (also called recurrent isolated sleep paralysis)
sleep pattern	an individual’s overall schedule of sleep and waking times, including naps, sleep interruptions, etc
sleep period	the total time from sleep onset until awakening, including any short mid-sleep arousals, which may therefore be longer than the actual total time spent sleeping
sleep-related eating disorder (SRED)	a potentially dangerous parasomnia in which the sufferer sleep-walks to the kitchen and eats and drinks at random, often including unhealthy fatty foods, raw foods, strange combinations of foods, or even toxic substances, cleaning fluids, etc (also called sleep eating or night eating syndrome)
sleep restriction therapy	a treatment for insomnia based on very gradually increasing sleep time, over a period of weeks or months, from an initial achievable low level
sleep-sex	a parasomnia in which sexual acts (from masturbation and sexual vocalizations to full intercourse and actual sexual assault) are performed while still asleep and not remembered afterwards (also called sexsomnia)
sleep spindle	a short burst of increased brain activity, in the region of 12-14 Hz, lasting maybe half a second, typically experienced during stage 2 non-REM sleep (also known as a sigma wave)
sleep stages	the different levels and depths of non-REM (stage 1, stage 2 and stage 3) and REM sleep experienced within each sleep cycle, which can be measured by electroencephalography and represented graphically by a hypnogram
sleep start	another name for hypnic jerk
sleep state misperception (SSM)	the mistaken perception of sleep as wakefulness, leading to severe underestimates of total sleep time (also known as paradoxical insomnia, pseudo-insomnia or subjective insomnia)
sleep-talking	a common parasomnia due to transitory arousals from non-NREM sleep or “motor breakthroughs” during REM dreams, the talk varying from gibberish or indistinct mumblings to complex monologues and dialogues (also called somniloquy)
sleep terror	another name for night terror
sleep time	the total time spent actually sleeping during a sleep period i.e. excluding any short mid-sleep arousals
sleep-wake cycle	the daily biological pattern of alternating sleep and wakefulness (roughly 8 hour of nighttime sleep and 16 hours of daytime wakefulness in humans)
sleep-wake homeostasis	an internal biochemical system, mediated by sleep-regulating substances like adenosine, that generates a drive or pressure for sleep as a function of the amount of time elapsed since the last adequate sleep episode
sleep-walking	a common parasomnia in which sleepers rise from slow-wave sleep and perform activities usually performed during full consciousness, such as talking, walking, cleaning, cooking, even driving (also called somnambulism)
slow-wave sleep	deep non-REM sleep, from which it is difficult to awaken, characterized by slow delta wave brain activity (also known as deep sleep, delta sleep or stage 3 sleep)
snore	an involuntary hoarse snorting sound made during sleep, caused by vibration of the muscles and soft tissues in the back of the throat, soft palate and uvula due to obstructed air movement while breathing during sleep
somnambulism	another name for sleep-walking
somniloquy	another name for sleep-talking
somnipathy	another name for sleep disorder
somniphobia	another name for hypnophobia
somnolence	another name for excessive daytime sleepiness or stage 1 non-REM sleep
spinal cord	a thick cord of nerve tissues extending through the spinal column, and linking the brain to the nerves that branch off to other parts of the body
stage 1 sleep (NREM1)	the first short period of very light, easily disrupted non-REM sleep, between wakefulness and sleep proper, characterized by alpha waves and theta waves and often punctuated by sudden twitches or hypnic jerks (sometimes called somnolence or drowsy sleep)
stage 2 sleep (NREM2)	the second stage of non-REM sleep, and the stage in which most time is spent, characterized by theta waves punctuated by higher voltage sleep spindles and K-complexes
stage 3 sleep (NREM3)	the third and deepest stage of non-REM sleep, during which the sleeper is largely unaware of and unresponsive to the outside environment, characterized by delta waves (according to older definitions of sleep stages, where delta sleep accounts for less than 50% of the total) along with some sleep spindles (also called deep sleep or slow-wave sleep)
stage 4 sleep (NREM4)	according to older definitions of sleep stages, the deepest part of slow-wave non-REM sleep, in which delta waves exceed 50% of the total (also called deep sleep or slow-wave sleep)
stimulant	a substance, drug or medication that stimulates the central nervous system, often used to treat excessive daytime sleepiness
stimulus control therapy	a treatment for insomnia/span> based on the strict rule that an individual has just ten minutes to fall asleep and, if sleep is not achieved in that time, they must get up, go somewhere else to relax, and only return to bed again when they feel sleepy (also called the 10-minute rule)
suprachiasmatic nucleus (SCN)	a tiny region in the hypothalamus of the brain responsible for controlling circadian rhythms, sending out the alerting pulses of the circadian alerting system, and regulating melatonin production in response to the light-dark cycle
synapse	an electro-chemical mechanism that allows neurons to connect to each other by transmitting and receiving electrical or chemical signals across a small gap (synaptic gap or synaptic cleft) between neurons by means of neurotransmitters (also called synaptic connection)
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teeth grinding	a common parasomnia, often caused by stress or anxiety, in which teeth grinding during the light stages of non-REM sleep can cause sleep disruption as well as tooth damage and jaw pain (also called bruxism)
terminal insomnia	difficulty staying asleep until a reasonable time in the morning, or waking up very early in the morning and not being able to get back to sleep (also called late insomnia)
thalamus	a region at the base of the brain through which sensory and motor signals pass to reach the cerebral cortex
theta waves	brain waves with a frequency of 4-7 Hz (typical of light non-REM sleep)
thyrotropin	a hormone, actively suppressed during sleep, that stimulates the thyroid gland to produce other hormones used in metabolism (also called thyroid-stimulating hormone)
transient insomnia	insomnia that persists for just a few days, usually following a stressful event or excessive use of caffeine or nicotine
tryptophan	an essential amino acid (which must be obtained through diet, such as from chocolate, dairy products, bananas, nut butters, seeds, etc), which is used to produce serotonin and melatonin and may help cause drowsiness
two-process model of sleep-wake regulation	the widely-accepted theory, usually attributed to Alexander Borbély, that sleep is regulated by interactions between two distinct processes, the circadian rhythm (Process C) and sleep-wake homeostasis (Process S)
tyramine	a naturally occurring compound, derived from the amino acid tyrosine (and found in many processed meats, fermented foods such as cheese, tofu and soy sauce, as well as in eggplant, raspberries, avocado, nuts, etc), which may have the effect of inhibiting sleep
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ultradian rhythms	recurrent periods or cycles, repeated multiple times within a 24-hour period, such as the repeated sleep cycles within a full sleep period
unihemispheric sleep	the ability of some aquatic mammals and birds to sleep with just one half of their brain at a time, while the other half (complete with one open eye) remains alert for predators (sometimes called unihemispheric slow-wave sleep)
upper airway resistance syndrome	a sleep disorder similar to, but milder than, sleep apnea
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ventrolateral preoptic nucleus (VLPO or VLPN)	a small area in the hypothalamus, sometimes referred to as the sleep switch because neurons in this area help to promote sleep by inhibiting activity in areas of the brainstem that maintain wakefulness
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wakefulness	the absence of sleep (sometimes also, in general usage, alertness or the inability to sleep), in which muscle tone is high, body movement is voluntary and brain waves are in the beta and gamma range
wake maintenance zone	the period, about 2-3 hours before one’s habitual bedtime, during which the circadian alerting system‘s pulses emitting from the suprachiasmatic nucleus reach their peak, offsetting the accumulated homeostatic sleep drive and allowing for continued alertness late into the evening
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yawn	an involuntary, deep, open-mouthed inhalation of air, usually during times of drowsiness, fatigue or boredom
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Zeitgeber	an external environmental time cue, such as daylight, that synchronizes an organism’s internal circadian clock to the Earth’s 24-hour light-dark cycle (from the German word meaning “time-givers”)