View Letters R-Z



An important neurotransmitter in the peripheral and central nervous system, involved with the transmission of nerve impulses throughout the body.


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.


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.


Another name for epinephrine.

Advanced Sleep Phase Syndrome (ASPS)

1) 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.

2) Phases of the daily sleep/wake cycle are advanced with respect to clock time. This is classified as a circadian rhythm disorder. The sleep phase occurs well ahead of the conventional bedtime and the tendency is to wake up too early.


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 Intrusion

Brief occurrence of alpha activity during a stage of sleep. Alpha rhythm – EEG oscillations, prominent over the occipital cortex, with a frequency of 8-13 Hz in adults; indicative of the awake state; present in most, but not all, normal individuals; most consistent and predominant during relaxed wakefulness.

Alpha Waves

Brain waves with a frequency of 8-13 Hz (typical of resting wakefulness light non-REM sleep).


A structure in the limbic system of the brain, involved with memory and emotions (particularly unpleasant emotions like anger and fear).


A constructive metabolic process during which new molecules are constructed and built up, and more complex substances are synthesized from simpler ones, requiring energy.


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).


A substance or drug that counteracts, blocks or inhibits the effects of another substance or drug (the opposite of agonist).


1) a pause in breathing of at least 10 seconds during sleep, which occurs as a result of sleep apnea (spelled apnoea in England)

2) An apnea is a period of time during which breathing stops or is markedly reduced. There are two types of apneas, the more common obstructive sleep apnea and the less common central sleep apnea.

Apnea-Hypopnea Index

Number of pauses in breathing or insufficient breathing usually with decrease in oxygenation events per hour of sleep.


1) 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.

2) Swift change of EEG frequency for at least 3 seconds taking place in any sleep stage.

Arousal Disorder

Type of parasomnia involving atypical arousal from sleep.

Arousal Threshold

1) The relative ease or difficulty with which a sleeper can be awakened at a particular time (e.g. by a noise, by shaking, etc).

2) Ease with which a sleeping person is awakened.


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 Behavior

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.


1) The transition from sleep into wakefulness (also called arousal).

2) Ease with which a sleeping person is awakened.


Basic Sleep Cycle

Progression through orderly succession of sleep states and stages. For the healthy adult, the first cycle begins by going from wakefulness to non-REM sleep. The first REM period follows the first period of non-REM sleep, and the two sleep states continue to alternate throughout the night with an average period of about 90 minutes. A night of normal human sleep usually consists of 4-6 non-REM/REM sleep cycles.


Clock time when one attempts to fall asleep, as differentiated from clock time when one gets into bed.


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).

Biological Clock

A collection of cells that regulates an overt biological rhythm, such as the sleep/wake cycle, or some other aspect of biological timing, including reproductive cycles or hibernation.

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).

Body Position

Four positions are identified in which a patient may be sleeping; back, left side, right side or abdomen. The time spent sleeping in each position and the number of respiratory events in a particular position are tabulated.


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.


Teeth grinding during sleep.


Caretaker Insomnia

Sleep disruption experienced by parents of newborn babies, or adults who care for family members with medical disorders, etc.


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.


1) 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).

2) Symptom of narcolepsy; consists of a sudden loss of muscle tone that leads to feelings of weakness and a loss of voluntary muscle control.


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).


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

1) 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.

2) Sleep disorder in which the airway is not blocked but the brain fails to signal the muscles to breathe.

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).

Cheyne Stokes Respiration

Breathing pattern characterized by regular waxing and waning of respiratory rate and tidal volume.

Chronic Insomnia

Ongoing insomnia symptoms that recur at least two days a week for at least a month (also called persistent insomnia).


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.


1) 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

2) A behavioral technique in which bedtime is systematically adjusted; used in cases when the patient’s sleep-wake pattern is out-of-synch with the external environment.


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(s)

1) 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).

2) Biological rhythms that include the internal clock which influences when, how much, and how well people sleep.

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 Therapy

In some cases of insomnia, this therapy includes interventions that are meant to help people identify and correct inappropriate thoughts and beliefs that may contribute to insomnia.

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.


A state of prolonged unconsciousness and unresponsiveness to external stimuli, from which arousal is not possible.

Conditioned Insomnia

1) Insomnia perpetuated by stress and anxiety about difficulties sleeping.

2) Type of psychophysiological insomnia produced by the development, during an earlier experience of sleeplessness.

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.


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.


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.

CPAP (Continuous Positive Airway Pressure)

An effective treatment for sleep apnea patients; delivers air into airways through a specially designed nasal mask or pillows.


Animal behaviour characterized by activity mainly during the twilight hours of dawn and dusk.


Proteins (including interleukin and interferon) produced by the immune system while fighting an infection, which also act as powerful sleep-inducing chemicals.



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

A condition occurring when the clock hour at which sleep normally occurs is moved back in time in a given, 24 hour sleep-wake cycle. The result is a temporarily displaced (delayed) occurrence of sleep within the 24 hour cycle.

Delayed Sleep Phase Syndrome (DSPS)

1) 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.

2) A clock problem, when the sleep-wake cycle is delaying regular bedtime. A Circadian Rhythm Disorder.

Delta Sleep

Also called slow-wave sleep, this deep sleep occurs in stages 3 and 4 of NREM sleep.

Delta Waves

1) Brain waves with a frequency of 0.5-4 Hz (typical of deep slow-wave non-REM sleep).

2) Brain waves with a frequency of 1 to 3 hertz that emanate from the forward portion of the brain during deep sleep in normal adults.

Diagnostic Sleep Study

Monitoring of several physiological activities in a sleeping individual. Usually performed to determine the absence or presence of a specific sleep disorder. The sleep study can occur in a sleep disorders center or in a patient’s home with portable recording equipment.


Animal behaviour characterized by activity during the daytime and sleep at night.


A neurotransmitter in the central nervous system that helps regulate movement and emotion, and is also involved in dreams.


Images, ideas, emotions and sensations that occur involuntarily in the mind during sleep, mainly during periods of REM sleep.


1) Another name for sleep disorder.

2) A primary disturbance of sleep initiation, maintenance or excessive sleepiness.


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.


Naturally produced by the body itself, i.e. originating, growing or developing from within an organism, rather than externally.


To reset or align with the biological clock.




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).


A standard 30 second duration of the sleep recording that is assigned a sleep stage designation; for special purposes, occasionally longer or shorter epochs are scored.

Epworth Sleepiness Scale

A quick and cheap diagnostic tool for sleep disorders, consisting of just eight simple questions about daytime sleep propensity.


1) 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).

2) Summer hibernation when animals sleep to get away from heat, no water, or their food source disappears.

Excessive Daytime Sleepiness (EDS)

1) 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).

2) Subjective report of daytime over- propensity to fall asleep.


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).

Extrinsic Sleep Disorders

Sleep disorders that arise from an out of the body cause.


Fatal Familial Insomnia (FFI)

A very rare fatal sleep disorder in which malformed proteins called prions attack the sufferer’s thalamus.


Feeling of tiredness or weariness usually associated with performance decrements.

Final Awakening

Time of wakefulness that occurs after the final wake-up time until lights on.

First Night Effect

Altered sleep physiological variables caused by unfamiliar sleep environment and/or recording equipment.

Forbidden Zone

The period of strongest clock-dependent alerting, usually in the evening. Prevents falling asleep.


The main part of the brain (including the cerebral cortex, limbic system, thalamus and hypothalamus, and excluding the brainstem).

Fragmentation (Pertaining to Sleep Architecture)

Interruption of a sleep stage as a result of the appearance of a lighter stage, or to the occurrence of wakefulness, which leads to disrupted non-REM-REM sleep cycles.

Fragmented Sleep

Repeated arousals and/or awakenings causing interrupted and insufficient sleep. One of the most common findings in child and adolescent sleep disorders.

Free Running Rhythm

Irregular circadian rhythm with no 24 hrs wake-sleep cycle. Possible causes are genetic and mostly unknown origin.

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).


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.


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).


The most common excitatory neurotransmitter in the nervous system, with a key role in learning and memory among other things.


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.


Habitual Snorers

Those who snore nearly every night.


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.


1) The habit of some mammals, reptiles and amphibians of passing the winter in a dormant condition with a greatly reduced metabolism.

2) Winter hibernation where the animal’s temperature, heart rate, and body slow down so much that it might appear to be dead.


A neurotransmitter released by the body during allergic reactions and immune responses, as well as in sleep modulation.


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 Regulation of Sleep

Refers to the neurobiological signals mediating the pressure or urge to sleep.

Homeostatic Sleep Drive

The pressure to sleep, generated by the process of sleep-wake homeostasis (also called homeostatic sleep pressure).


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.


Typical behavior in a child with a sleep disorder which is causing lack of quality sleep.


1) 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).

2) Synonym to excessive sleepiness.


Relating to the drowsy state just before sleep onset.

Hypnagogic Hallucination

A “greater-than-life-like” dream experience that occurs during sleep. Hypnagogic hallucinations are sometimes associated with narcolepsy.

Hypnagogic Imagery

Vivid sensory images occurring at sleep onset but particularly vivid with sleep-onset REM periods; feature of narcoleptic REM naps.

Hypnagogic Startle

“Sleep start” or sudden body jerk, observed normally just at sleep onset, resulting in at least momentary awakening.

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).


Sleep inducing (e.g. a substance in the body, or a drug).


1) A simplified graph representing the stages of sleep (based on electroencephalography recordings over the period of a night) as a function of time.

2) A graphical summary of the electrical activities occurring during a night’s sleep.


Another name for sleep learning.


1) An anxiety disorder manifesting as an abnormal fear of sleeping or going to bed (also called somniphobia or negative sleep conditioning).

2) Morbid fear of falling asleep.


Relating to the drowsy semi-conscious state just before complete wakefulness.


Sleep-inducing drugs.

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.


Another name for orexin.


Referred to 50% reduction of oro-nasal airflow for 10 seconds or more during sleep, usually associated with an arousal and/or oxygen desaturation.


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.



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.

Inappropriate Sleep Episodes

Unplanned sleep periods often occurring in an unsafe situation (i.e., while driving). These episodes are always due to sleep deprivation.


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).


1) A general term for any difficulty in falling asleep or staying asleep, such that the sufferer is still tired, unrefreshed and unrested on waking.

2) Sleeplessness; chronic difficulty with sleep onset or maintenance of sleep, or a perception of nonrefreshing sleep.

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).


Jet Lag

1) 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).

2) A temporary disorder that causes fatigue, insomnia, and other symptoms as a result of rapid air travel across time zones.



1) 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.

2) An EEG wave. One of the markers for stage 1 sleep, light 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).


Leg Movement

Leg movements are recorded in both diagnostic sleep studies and titration studies.

Light-Dark Cycle

The natural daily cycle between sunlight and darkness resulting from the Earth’s turning on its axis.

Light Sleep

1) 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.

2) Usually refereed to stage-1 sleep.

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.

Limit-Setting Sleep Disorder

Disorder due to child’s difficulty in falling asleep by delaying and refusing to go to bed.

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.


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.


A light-sensitive pigment (most sensitive to short wavelength “blue light”) in the ganglion cells of the retina of the eye.


1) 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.

2) The hormone that is considered to regulate the sleep-wake cycle to the outside world’s time giver signs, mainly light.

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.


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.


Animal behaviour characterized by activity (and sleep) partly during the day and partly at night.


A sleep occurrence lasting up to 10 seconds during which no external stimulus is perceived.


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.


1) 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.

2) Partial awakening from sleep.

Mixed Sleep Apnea

1) A relatively rare combination of central sleep apnea and obstructive sleep apnea.

2) A combination of central sleep apnea and obstructive sleep apnea.


A single major sleep period and a single major wake period in a 24-hour day.

Monophasic Sleep

Sleep organized into just one long period each day (also called monocyclic sleep).

Motor Activity in Sleep

Any muscular movement during sleep.

Motor Atonia

Absence of muscle activity during sleep.

Movement Arousal

A body movement associated with arousal or awakening; a sleep scoring variable.

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).



1) A brief period of sleep taken outside of the main sleep period, i.e. usually during the daytime (also known as a siesta).

2) A short sleep episode during routine wakefulness.


1) A chronic sleep disorder characterized by uncontrollable excessive daytime sleepiness, sleep attacks at inappropriate times, hallucinations, automatic behavior, episodes of sleep paralysis and/or cataplexy, and an abnormal tendency to pass directly into REM sleep from wakefulness.

2) A neurological condition in which people experience excessive daytime sleepiness, cataplexy, sleep paralysis, hallucinations and intermittent, uncontrollable sleep attacks during the day.

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.


A neurotransmitter (such as dopamine, serotonin, histamine, acetylcholine, etc) which diffuses through large areas of the nervous system, thereby affecting many different neurons.


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.


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).


1) 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.

2) Unpleasant and/or frightening dream occurring in REM sleep (different from a night terror).

Night Terror

1) 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).

2) Also known as sleep terrors, or pavor nocturnus. Night terrors are characterized by an incomplete arousal from slow wave sleep. If, the individual is awakened during a night terror, he/she is usually confused and does not remember details of the event. Night terrors are different from nightmares; if an individual is awakened during a nightmare, he/she functions well and may have some recall of the nightmare.


1) An excessive need for nighttime urination, particularly common in pregnant women and the elderly (also called nycturia).

2) Excessive, often frequent, urination during the night.


1) Animal behaviour characterized by activity during the night and sleep during the daytime.

2) Relating to or taking place at night.

Nocturnal Confusion

Episodes of delirium and/or disorientation near or during nighttime sleep; often seen in victims of Alzheimer’s Disease and more common in the elderly.

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 Enuresis (Bedwetting)

Urinating while asleep.

Nocturnal Groaning

Another name for catathrenia.

Nocturnal Myoclonus

Another name for periodic limb movement disorder.

Nocturnal Sleep-Related Eating Disorder (NS-RED)

Getting up during the night and eating while sleepwalking. No recall in the morning.

Non-24-Hour Sleep-Wake Syndrome (Non-24)

1) 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).

2) A circadian rhythm disorder in which the sleep-wake pattern does not conform to the usual 24-hour cycle.

Non-REM Sleep (NREM)

1) 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.

2) One of the two basic states of sleep; consists of Stages 1, 2 (light sleep) and 3, 4 (deep sleep).

NREM Sleep Intrusion

Brief period of NREM sleep patterns appearing in REM sleep; a portion of NREM sleep not appearing in its usual sleep cycle position.


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).


Obstructive Hypopnea

Periodic and partial closure of the throat during sleep resulting in reduced air exchange at the level of the mouth and/or nostril.

Obstructive Sleep Apnea (OSA)

1) 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.

2) The most common kind of sleep apnea. It is caused by a blockage of the upper airway.

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.

Optimum Sleep

Average amount of sleep needed every night by an individual.


A neurotransmitter produced in the hypothalamus that regulates arousal, wakefulness and appetite, and the lack of which causes narcolepsy (also called hypocretin).


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.


1) A type of sleep disorder involving abnormal or unnatural movements, behaviors, emotions, perceptions or dreams during sleep, often involving partial awakenings during the transitions between sleep and full wakefulness.

2) Abnormal behaviors during sleep that interrupt sleep and can result in injury, insomnia, and/or excessive daytime sleepiness.

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).

Paroxysmal Nocturnal Dyspnea (PND)

Respiratory distress and shortness of breath due to pulmonary edema, appearing suddenly and often awakening the sleeping individual.

Pathological Sleep

Abnormal sleep patterns.

Perceptual Disengagement

Change in consciousness at the onset of sleep when environmental stimuli are no longer perceived, and there is no longer any conscious, meaningful interaction with the environment.

Periodic Limb Movement Disorder (PLMD)

1) 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).

2) A disorder in which rhythmic jerking of the legs interrupts sleep, causing insomnia and/or excessive daytime sleepiness.

Persistent Insomnia

Continuing insomnia responding poorly to treatment.

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).

Phasic (Event/Activity)

Brain, muscle, or autonomic related event of a brief and episodic nature occurring in sleep. Usually occur during REM sleep, such as eye movements and/or muscle twitches.

Pineal Gland

A small pine cone-shaped gland near the centre of the brain that produces several important hormones including melatonin.


Multiple sleep periods and wake periods in a 24-hour day.

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).


1) 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.

2) The brainstem region critical for initiating REM sleep.

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).

Post-Prandial Drowsiness

Sleepiness that occurs after a meal, usually lunch.

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.

Premature Morning Awakening

Early termination of the sleep period in a sleep maintenance DIMS due to inability to return to sleep after the last of several awakenings.

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.


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.


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 behavioral stressors and/or environmental and situational causes.


Quite Sleep

The term used to describe NREM sleep during infancy.

View Letters R-Z