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Rapid Eye Movement (REM) Sleep

One of the two basic states of sleep. REM sleep, also known as dream sleep, is characterized by rapid eye movements, and more irregular breathing and heart rate compared to NREM 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 Density

A function that expresses the frequency of eye movements per unit of time during REM sleep.

REM Motor Atonia

The active suppression of activity in the antigravity and voluntary muscles during REM sleep. The muscles are completely flaccid and limp.

REM Onset

Designation for commencement of a REM period; used also as a shorthand term for a sleep-onset REM period.

REM Period

REM portion of a NREM-REM cycle; early in the night it may be as short as a half-minute, whereas in later cycles longer than an hour.

REM Rebound

Increased frequency, duration and percentage of dream sleep commonly seen after REM sleep deprivation and during CPAP titration study.

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 Behavior Disorder (RBD)

1) 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.

2) Disorder in which REM motor atonia is partially or completely absent and the individual acts out the ongoing dream. The behavior in REM behavior disorder is often correlates with the ongoing, hallucinatory REM dream episode.

REM Sleep Episode

REM sleep portion of a NREM-REM sleep cycle. Early in the first sleep period, episodes may be only several minutes in duration. Later REM episodes almost are always longer, 20 to 30 minutes up to an hour.

REM Sleep Intrusion

Brief interval of REM sleep appearing out of its usual positioning in the NREM-REM sleep cycle.

REM Sleep Latency

Designation for the first epoch of a REM sleep episode.

REM Sleep Onset

Designation for the first epoch of a REM sleep episode.

REM-Associated Disorders

Sleep disturbances that occur in REM sleep. REM sleep Onset – Beginning of REM sleep.

Restless Legs Syndrome (RLS)

1) 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.

2) A neurologic movement disorder that is often associated with a sleep complaint.

Restlessness (Referring to Quality of Sleep)

Persistent or recurrent body movements, arousals, and/or brief awakenings in the course of 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.


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).


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.


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.


1) Another name for sleep-sex.

2) Parasomnia of a sexual, sometimes, violent nature towards the bed partner.

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).


Another name for a nap.

Sigma Wave

Another name for a sleep spindle.


1) 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.

2) A state marked by lessened consciousness, lessened movement of the skeletal muscles, and slowed-down metabolism.

Sleep Apnea

1) 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).

2) Sleep disorder that occurs when a person’s breathing is interrupted during sleep.

Sleep Architecture

1) 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.

2) Sleep stages composition: NREM-REM cycles. Usually given as percentage per age.

Sleep Cycle

1) 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.

2) Synonymous with NREM-REM cycle.

Sleep Debt

1) 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).

2) Result of recurrent sleep deprivation which occurs over time when an individual does not experience a sufficient amount of the restorative daily sleep that is required to maintain a sense of feeling rested and refreshed.

Sleep Deprivation

1) The condition of not having (or not being allowed to have) sufficient sleep, which leads to sleep debt (also called sleep loss).

2) Acute or chronic lack of sufficient sleep.

Sleep Disorder(s)

1) 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).

2) Broad range of illnesses arising from many causes, including, dysfunctional sleep mechanisms, abnormalities in physiological functions during sleep, abnormalities of the biological clock, and sleep disturbances that are induced by factors extrinsic to the sleep process.

Sleep-Disordered Breathing

Any medical condition characterized by repeated episodes of not breathing (sleep apnea) or under-breathing (sleep hypopnea) during sleep.

Sleep Episode

Interval of sleep that may be voluntary or involuntary.

Sleep Extension

Extending sleep time by increasing the time in bed.

Sleep Fragmentation

Brief arousals occurring throughout the night, reducing the total amount of time spent in the deeper levels of 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

1) The practice of consciously following daily guidelines and adopting particular habits and routines in order to ensure a more restful and effective sleep.

2) Practices, habits, and environmental factors that are important for getting sound sleep.

Sleep Hyperhydrosis

Excessive sweating during 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

1) The feeling of grogginess, impaired alertness and decreased motor dexterity immediately following awakening.

2) Feelings of grogginess and/or sleepiness that persist longer than 10 to 20 minutes after waking up.

Sleepiness (Somnolence, Drowsiness)

Difficulty in maintaining the wakeful state so that the individual falls asleep if not actively kept aroused; not simply a feeling of physical tiredness or listlessness.

Sleeping Pills

Compounds that have a sedative effect, used to produce sleepiness.

Sleeping Sickness

An old name for narcolepsy.

Sleep Interruption

Breaks in the sleep architecture resulting in arousal and wakefulness.

Sleep Latency

1) 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).

2) Time from lights off to sleep onset.

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 (DIMS)

1) Difficulty staying asleep or going back to sleep once woken (also known as middle insomnia).

2) Disturbance in maintaining sleep once achieved; persistently interrupted sleep without difficulty falling asleep.

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 Mentation

Thoughts, feelings, images, perceptions, hallucinations, and active dreams taking place during sleep.

Sleep Onset

1) The transition from wakefulness into sleep, commonly referred to as “falling asleep.”

2) Conversion from wakefulness to sleep period after lights off.

Sleep Onset Imagery

Images and experiences during the moments following the transition from wake to sleep.

Sleep-Onset Insomnia

Difficulty getting to sleep in the first place (also known as initial insomnia).

Sleep-Onset REM Period

Atypical beginning of sleep by entrance directly into stage REM.

Sleep Paralysis

1) 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).

2) Symptom of narcolepsy; involves the temporary inability to move or speak while falling asleep or waking up.

Sleep Pattern

1) An individual’s overall schedule of sleep and waking times, including naps, sleep interruptions, etc.

2) Individual’s clock hour schedule of bedtimes and rise times as well as nap behavior: may also include time and duration of sleep interruptions.

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 Accidents

Accidents caused by individuals who were sleep deprived and who, as a result, had impaired judgment.

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

Limitation of the number of hours in bed.

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.


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

1) 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).

2) Spindle shaped EEG bursts. They are one of the signs for sleep initiation.

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 Structure

Similar to sleep architecture. Sleep structure, in addition to encompassing sleep stage and cycle relationships, assesses the within-stage qualities of the EEG and other physiological attributes.


1) 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).

2) Talking in sleep takes place during stage REMS, representing a motor breakthrough of dream speech, or in the course of transitory arousals from NREMS and other stages. Full consciousness is not achieved and no memory of the event remains.

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-Wake Shift (-Change, -Reversal)

Sleep wholly or partially moved to a time of customary waking activity, and the latter is moved to the habitual sleep period; common in jet lag and shift work.

Sleep-Wake Transition Disorder

Disorder occurring during the transition from wakefulness to sleep or from one sleep stage to another; a form of parasomnia.

Sleep-wake, 24 Hour Cycle

The clock hour relationships of the major sleep and wake phases in the 24 hour cycle: similar to sleep pattern.


1) 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).

2) Individual subject to somnambulism (one who walks while sleeping). Sleepwalking typically occurs in the first third of the night during deep NREM sleep (stages 3 and 4).

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).


1) 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.

2) Noise produced primarily with inspiratory respiration during sleep owing to vibration of the soft palate and the pillars of the oropharyngeal inlet. Many snorers have incomplete obstruction of the upper airway, and may develop obstructive sleep apnea.


Another name for sleep-walking.


Another name for sleep-talking.


Another name for sleep disorder.


Another name for hypnophobia.


1) Another name for excessive daytime sleepiness or stage 1 non-REM sleep.

2) Prolonged drowsiness or sleepiness.

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.

Spindle REMS

Condition in which sleep spindles persist atypically in REMS; seen in chronic DIMS conditions.


Causing or tending to cause sleep.

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).


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 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).

Subjective Sleepiness

Feelings of sleepiness.

Subwakefulness Syndrome

Syndrome defined as a defect in the CNS support system for waking. The few individuals reported with subwakefulness syndrome have daytime drowsiness and daytime sleep episodes that are always composed of NREMS stages 1 or 2. The naps occur repetitively.

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.


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).


Scheduling sleep to synchronize with the biological clock.


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).


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).


A hormone, actively suppressed during sleep, that stimulates the thyroid gland to produce other hormones used in metabolism (also called thyroid-stimulating hormone).


A feeling of a lessened capacity for work and reduced efficiency of accomplishment, usually accompanied by a sense of weariness and fatigue.

Total Sleep Period

Period of time measured from sleep onset to final awakening. In addition to total sleep time, it is comprised of the time taken up by arousals and movement time until wake-up.

Total Sleep Time

Amount of NREM and REM sleep from lights off to lights on.

Transient Arousals

Brief awakenings from sleep.

Transient Insomnia

1) Insomnia that persists for just a few days, usually following a stressful event or excessive use of caffeine or nicotine.

2) Difficulty sleeping for only a few nights.


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).


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.


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

1) 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).

2) A type of sleep in which one side of the brain is asleep while the other is awake. This phenomenon is observed most notably in birds (like those that make long, transoceanic flights) and aquatic mammals (like dolphins and porpoises).

Unintended Sleep Episode

Sleep episode that is not planned and may happen during an activity in which such an episode is hazardous, such as when driving a car or working with machinery.

Upper Airway Resistance Syndrome

A sleep disorder similar to, but milder than, sleep apnea.


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.


Wake Time

Total time that is scored awake in a polysomnogram occurring between sleep onset and final wake-up.


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.




An involuntary, deep, open-mouthed inhalation of air, usually during times of drowsiness, fatigue or boredom.



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”).

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