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Glossary of Terms Used In Sleep Disorders

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Abbreviations

A

Alpha activity

The presence of alpha waves or alpha rhythm (i.e., electrical oscillations in the alpha frequency) in the EEG of humans. See Alpha rhythm.

Alpha intrusion (-infiltration, -insertion, -interruptions "riddled with" alpha)

A brief interposition of alpha activity during a stage of sleep.

Alpha rhythm (-activity, -frequency)

EEG oscillations with a frequency of 8-13 Hz in adults, prominent over the occipital cortex: indicative of the awake state in humans; present in most, but not all, normal individuals most consistent and predominant during relaxed wakefulness, particularly with reduction of visual input. The alpha frequency has a range in each individual: the low end is exhibited in drowsiness or sleep and the upper end with alertness. The frequency range also varies with age; it is slower in children and older age groups relative to young and middle-aged adults.

Arousal

An abrupt change from a "deep" stage of NREM sleep to a "lighter" stage, or from REMS to awake, with the possibility of awakening as the final outcome. Arousal may be accompanied by increased tonic EMG activity and heart rate, as well as body movements.

Awakening (full arousal)

The return to the polysomnographically defined awake state from any of the NREM stages or REMS: characterized by alpha and beta waves, rise in tonic EMG, voluntary REMS, and eye blinks. This definition of awakening is valid only insofar as the polysomnogram is matched by a resumption of a reasonably alert state of consciousness.

B

Base line

The normative (i.e., typical) state of an individual or of an investigative parameter prior to an experimental manipulation.

Bedtime

Defined as the time when one attempts to fall asleep (as distinguished from the time when one gets into bed).

Beta rhythm (-waves, -activity)

EEG frequency in the range of 13-35 Hz; when the predominant frequency, beta rhythm is usually associated with alert wakefulness or vigilance and is accompanied by a high tonic EMG.

C

Cataplexy

A sudden, dramatic decrement in muscle tone and loss of deep reflexes leading to muscle weakness, paralysis, or postural collapse: usually precipitated by an outburst of emotional expression-notably laughter, startle, or sudden physical exercise; one of the tetrad of symptoms of narcolepsy. During cataplexy, respiration is not compromised.

Cheyne-Stokes respiration

A breathing pattern characterized by regular "crescendo-decrescendo" fluctuations in respiratory rate and tidal volume.

Circadian rhythm

An innate, daily, fluctuation of physiological and behavioral functions, including sleep waking, generally tied to the 24 hour day-night cycle but sometimes to a measurably different (e.g., 23 or 25 hour) periodicity when light/dark and other time cues are removed.

Conditioned insomnia

An easily overlooked form of chronic insomnia (sometimes a component of psychophysiological DIMS) caused by the development-during an earlier experience of sleeplessness -- of a negative association between characteristics of the customary sleep environment and sleeping.

D

"Deep" sleep stage

Common term for NREM stages 3 and 4 sleep. In some European sleep literature, "deep" sleep is applied to REM sleep because of its high awakening threshold. Set, "Intermediary" sleep stage: "Light" sleep stage.

Delayed sleep phase

A condition that occurs when the clock hour at which sleep normally occurs is moved back in time in a given, 24 hour sleep-wake cycle. This results in a temporarily displaced (delayed) occurrence of sleep within the 24 hour cycle. The same term denotes a chronic sleep schedule disturbance.

Delta sleep stage(s)

Indicative of the stage(s) of sleep in which EEG delta waves are prevalent or predominant (sleep stages 3 and 4, respectively). See Slow wave sleep.

Delta waves

EEG activity with a frequency of less than 4 Hz. In human sleep scoring, the minimum characteristics for scoring delta waves is conventionally 75 uV (peak-to-peak) amplitude, and 0.5 second duration (2 Hz).

Dyssomnia

Any disorder of sleep or wakefulness per se; not a parasomnia.

E

Early a.m. arousal

Synonymous with premature morning awakening.

Electroencephalogram (EEG)

A recording through the scalp of the electrical potentials from the brain and the moment-to-moment changes in these potentials. With the EMG and EOG, the EEG is one of the three basic variables used to score sleep stages and waking. Sleep recording in humans utilizes surface electrodes to record potential differences between brain regions and a neutral reference point, or simply between brain regions. Either the C3 or C4, central region, placement in the International 10-20 system is the standard electrode from which stage scoring is done.

Electromyogram (EMG)

A recording of electrical activity from the muscular system; in sleep recording, synonymous with resting muscle activity or potential. The chin/cheek EMG, along with EEG and EOG, is one of the three basic variables used to score sleep stages and waking. Sleep recording in humans utilizes surface electrodes to measure activity from the submental or masseter muscles. These reflect maximally the changes in resting muscle activity. The chin/cheek EMG is tonically inhibited during REM sleep.

Electro-oculogram (EOG)

A recording of voltage changes resulting from shifts in position of the eyeball-possible because each globe is a positive (anterior) and negative (posterior) dipole; along with the EEG and the EMG, one of the three basic variables used to score sleep stages and waking. Sleep recording in humans utilizes surface electrodes placed near the eyes to record the movement (incidence, direction, and velocity) of the eyeballs. Rapid eye movements in sleep indicate a certain stage of sleep (REM sleep).

Excessive daytime sleepiness or somnolence

A subjective report of difficulty in maintaining the awake state, accompanied by a ready entrance into sleep when the individual is sedentary; may be quantitatively measured by use of subjectively defined rating scales of sleepiness.

F

Fragmentation (pertaining to sleep architecture)

The interruption of any stage of sleep due to appearance of another stage or waking, leading to disrupted NREMS-REMS cycles; often used to refer to the interruption of REMS by movement arousals or stage 2 activity. Sleep fragmentation connotes repetitive interruptions of sleep by arousals and awakenings.

G

H

Hertz (Hz)

A unit of frequency; synonymous with cycles per second (cps).

Hypercapnia

Elevated carbon dioxide level in blood

Hypersomnia

Excessive or prolonged sleep. Sometimes associated with difficulty in awakening or sleep drunkenness.

Hypnagogic imagery (-hallucinations)

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

Hypnagogic startle

A "sleep start" or sudden body jerk, observed normally just at sleep onset and resulting in at least momentary awakening.

I

Insomnia

Difficulty in sleeping. A confusing term-though ubiquitously employed because it is used to indicate any and all gradations and types of sleep loss.

J

K

K complex

A sharp, negative, high-voltage EEG wave, which is followed by a slower, positive component. K complexes occur spontaneously during NREM sleep, beginning in (and defining) stage 2. They are thought to be CNS-evoked responses to internal stimuli. They can be elicited during sleep by external (particularly auditory) stimuli as well.

L

"Light" sleep stage

Common term for NREMS stage I (and sometimes stage 2). See "Deep" sleep stage.

M

Microsleep(s)

A period lasting up to a few seconds during which the polysomnogram suddenly shifts from waking characteristics to sleep and external stimuli are not perceived; associated with excessive daytime sleepiness and automatic behavior, which are symptoms of DOES.

Movement arousal

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

Movement time

The term used in sleep record scoring to denote when EEG and EOG tracings are obscured for more than 15 seconds because of movement. Usually combined with awake time.

Multiple sleep latency test (MSLT)

A series of measurements of the interval from "lights out" to sleep onset that is utilized in the assessment of excessive daytime sleepiness. Subjects are allowed a fixed number of opportunities to fall asleep during their customary awake period. Long latencies are helpful in distinguishing physical tiredness or fatigue from true sleepiness.

Muscle tone

A term sometimes used for resting muscle potential or resting muscle activity. See Electromyogram (EMG).

Myoclonus

Muscle contractions in the form of "jerks" or twitches. In sleep-related (nocturnal) myoclonus, the jerks are primarily of the flexor groups in the lower extremities and have a characteristic frequency of 20-40 seconds.

N

Nightmare

Used to denote a dream anxiety attack, not a sleep (night) terror. In the past-and still in the European sleep literature-nightmare is used to indicate both sleep terror and anxiety dream.

Nocturnal confusion

Episodes of delirium and disorientation close to or during nighttime sleep; often seen in the elderly and indicative of organic CNS deterioration.

Nocturnal dyspnea

Respiratory distress, minimal during the day but becoming quite disturbing in sleep.

Nocturnal sleep

Indicative of the typical "nighttime," or major, sleep period dictated by one's circadian rhythm of sleep and wakefulness; the conventional time for sleeping.

Non-rapid eye movement sleep

(NREMS, also written as non-REMS) See Sleep stages.

NREMS intrusion

An interposition of NREM sleep, or a component of NREMS physiology (e.g., elevated EMG, K-complex, sleep spindle, delta waves), in REMS; a portion of NREMS not appearing in its usual sleep cycle position.

NREMS period

The NREMS portion of NREMS - REMS cycle; such a period consists primarily of sleep stages 3/4 early in the night and of sleep stage 2 later. See Sleep cycle; Sleep stages.

NREMS-REMS cycle

(synonymous with sleep cycle) A period during sleep composed of a NREMS period and the subsequent REMS period; each NREMS-REMS couplet is equal to one cycle. Any NREM sleep stage suffices as the NREMS portion of a cycle. A sleep period of 6.5-8.5 hr generally consists of four to six cycles.

O

P

Parasomnia

Not a disorder of sleep or wakefulness per se; rather, an event happening during sleep, or induced or exacerbated by sleep, such as sleepwalking or asthma; not a dyssomnia.

Paroxysmal nocturnal dyspnea (PND)

Respiratory distress and shortness of breath due to pulmonary edema, which appear suddenly and often awaken the sleeping individual.

Period length

The duration of time encompassed by an individual's full, daily sleep-wake cycle; conventionally, but not always, 24 hours. See Circadian rhythm.

Phase advance

The movement to a position earlier in the 24 hour sleep - wake cycle of a period of sleep or wake; for example, a shift of the sleep phase from 11 p.m. - 7 a.m. to 8 p.m. - 4 a.m. See Phase delay.

Phase delay

Phase delay is exactly the opposite of phase advance, i.e., a shift later in time. (Confusion is sometimes introduced into these concepts because clock language is reversed; to effect a phase delay, the clock is moved ahead or advanced. Contrariwise, as in the example in phase advance, the change from the 11 p.m. - 7 a.m. to the 8 p.m. - 4 a.m. position requires a movement of the clock backward.) See Phase advance.

Phase transition

One of the two junctures of the major sleep and wake phases in the 24 hour sleep-wake cycle.

Phasic event (-activity)

Brain, muscle, or autonomic events of an episodic or fluctuating nature occurring in sleep; characteristic of REMS (e.g., eye movements, muscle twitches); usually enduring for milliseconds to 1 -2 seconds.

Polysomnogram

The continuous and simultaneous recording of physiological variables during sleep, i.e., EEG, EOG, EMG (these are the three basic stage scoring parameters), EKG, respiratory air flow, respiratory excursions, lower limb movement, and other electrophysiological variables.

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, typifies the failure to accomplish a normal length of nocturnal sleep because of interference at the end rather than at the commencement of sleep: the DIMS characteristic of depressed individuals.

Q

R

Rapid eye movement sleep (REMS)

See, Sleep stages.

REM density (-intensity)

A function that expresses the frequency of eve movements during sleep stage REM.

REMS intrusion

A brief interval of REMS appearing out of its usual position in the NREMS-REMS cycle, an interposition of REMS in NREMS; sometimes appearance of a single, dissociated component of REMS (e.g., eye movements, or "drop out" of muscle tone) rather than all REMS parameters.

REMS latency

The period of time in the sleep period from sleep onset to the first appearance of stage REMS.

REMS onset

The designation for commencement of a REMS period. Sometimes also used as a shorthand term for a sleep-onset REMS period. See Sleep onset: Sleep-onset REMS period.

REMS percent

The proportion of total sleep time constituted by the REM stage of sleep.

REMS period

The REMS portion of a NREMS-REMS cycle; early in the night it may be as short as a half-minute, whereas in later cycles longer than an hour. See Sleep stage REM.

REMS rebound or recovery

Lengthening and increase in frequency and density of REMS periods, which results in an increase in REMS percent above base line. REMS rebound follows REMS deprivation once the inhibitory influence is removed.

Restlessness (referring to a quality of sleep)

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

S

Sleep architecture

The NREMS/REMS stage and cycle infrastructure of sleep understood from the vantage point of the quantitative relationship of these components to each other. See Sleep structure.

Sleep cycle

Synonymous with NREMS-REMS cycle.

Sleep efficiency (or sleep efficiency index)

The proportion of sleep in the period potentially filled by sleep-, that is, the ratio of total sleep time to time in bed.

Sleep hygiene

The conditions and practices that promote continuous and effective sleep. These include regularity of bedtime and arise time: conformity of time spent in bed to the time necessary for sustained and individually adequate sleep (i.e., the total sleep time sufficient to avoid sleepiness when awake); restriction of alcohol and caffeine beverages in the period prior to bedtime; employment of exercise, nutrition, and environmental factors so that they enhance, not disturb, restful sleep.

Sleep interruption

Breaks in the sleep architecture resulting in arousal and wakefulness. See Fragmentation; Restlessness.

Sleep latency

The period of time measured from "lights out," or bedtime, to the commencement of sleep.

Sleep log (-diary)

A daily, written record of an individual's sleep-wake pattern containing such information as time of retiring and arising, time in bed, estimated total sleep period, number and duration of sleep interruptions, quality of sleep, daytime naps, use of medications or caffeine beverages, nature of waking activities, and other data.

Sleep-maintenance DIMS or insomnia

A disturbance in maintaining sleep once achieved; persistently interrupted sleep without difficulty falling asleep. Synonymous with sleep continuity disturbance.

Sleep mentation

The imagery and thinking (and emotion) experienced during sleep. Sleep mentation consists of individual representations-but usually combinations of-images and thoughts. Imagery is vividly expressed in dreams during REMS in all the senses in approximate proportion to their waking representations. Mentation is experienced generally less distinctly in NREM sleep, but it may be quite vivid in stage 2 sleep, especially toward the end of the sleep period.

Sleep onset

The transition from the awake to the sleep state, normally into NREM stage I (but in certain conditions, such as infancy and narcolepsy, into stage REMS). Most polysomnographers accept EEG slowing, reduction, and eventual disappearance of alpha activity, presence of EEG vertex spikes, and slow rolling eye movements (the components of NREM stage 1) as sufficient for sleep onset. others require appearance of stage 2 wave forms. Consciousness has been shown to be lost as alpha activity fragments. See Sleep latency, Sleep stages.

Sleep-onset REMS period

The atypical beginning of sleep by entrance directly into stage REMS.

Sleep pattern (24 hour sleep-wake pattern)

An individual's clock hour schedule of bedtimes and rise times as well as nap behavior: may also include time and duration of sleep interruptions. See Sleep-wake, 24-hour cycle; Circadian rhythm; Sleep log.

Sleep spindle

An episodically appearing, spindle-shaped aggregate of 12-14 Hz waves with a duration of 0.5-1.5 seconds, one of the identifying EEG phenomena of NREM stage 2 sleep; may persist into NREM stages ¾: not seen in REMS.

Sleep stage demarcation

The significant polysomnographic characteristics that distinguish the boundaries of the sleep stages. In certain conditions and with drugs, sleep stage demarcations may be blurred or lost, making it difficult to identify certain stages with certainty or to distinguish the temporal limits of sleep stage lengths.

Sleep stage period

A sleep stage interval that represents the stage in a NREMS - REMS cycle; easiest to comprehend in relation to REMS, which is a homogeneous stage, i.e., the fourth REMS period is in the fourth sleep cycle. If one interval of REMS separated from another by more than 20 minutes, they constitute separate REMS periods (and are in separate sleep cycles); a sleep stage period may be any duration.

Sleep stages:

Sleep stage NREM (NREMS)

The other major sleep state apart from REMS; comprises sleep stages 1-4, which constitute areas in the spectrum of NREMS "depth" or physiological intensity.

Sleep stage I (NREMS stage 1)

A stage of NREM sleep that ensues directly from the awake state. Its criteria consist of a low-voltage EEG with slowing to theta frequencies, alpha activity less than 50%, EEG vertex spikes, and slow rolling eye movements; no sleep spindles, K complexes, or REMS. Stage I normally assumes 4-5% of total sleep.

Sleep stage 2 (NREMS stage 2)

A stage of NREM sleep characterized by the advent of sleep spindles and K complexes against a relatively low-voltage, mixed-frequency EEG background; high-voltage delta waves may comprise up to 20% of stage 2 epochs; usually accounts for 45-55% of total sleep time.

Sleep stage 3 (NREMS stage 3)

A stage of NREM sleep defined by at least 20 and not more than 50% of the period consisting of EEG waves less than 2 Hz and more than 75 uV (high-amplitude delta waves); a "delta" sleep stage; with stage 4, it constitutes "deep" NREM sleep; often combined with stage 4 into NREMS stage 3./4 because of the lack of documented physiological differences between the two; appears usually only in the first third of the sleep period; usually comprises 4-6% of total sleep time.

Sleep stage 4 (NREMS stage 4)

All statements concerning NREMS stage 3 apply to stage 4 except that high-voltage, slow EEG waves, cover 50% or more of the record: NREMS stage 4 usually takes up 12-15% of total sleep time. Somnambulism, sleep terror, and sleep-related enuresis episodes generally start in stage 4 or during arousals from this stage. See Sleep stage 3.

Sleep stage REM (REMS)

The stage of sleep (i.e., state of the CNS) found in all mammals studied, including man, in which brain activity is extensive, brain metabolism is increased, and vivid hallucinatory imagery, or dreaming occurs (in humans). It is also called "paradoxical sleep" because, in the face of this intense excitation of the CNS and presence of spontaneous rapid eye movements, resting muscle activity is suppressed. The EEG is a low-voltage, fast-frequency, nonalpha record. Stage REMS is usually 20-25% of total sleep time.

Sleep structure

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

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. See Excessive daytime Sleepiness.

Sleep talking

Talking in sleep takes place during stage REMS, at which time it represents 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-wake, 24 hour cycle

Basically, the clock hour relationships of the major sleep and wake phases in the 24 hour cycle: similar to sleep pattern. See Phase transition; Circadian rhythm.

Sleep-wake shift (-change, -reversal)

When sleep-wholly or partially-is 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.

Slow wave sleep (SWS)

Synonymous with sleep stages 3 and 4. See Delta sleep stage(s).

Snoring

A 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 in time develop frank obstructive sleep apnea.

Spindle REMS

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

Subwakefulness syndrome

A syndrome postulated 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 I or 2. The naps occur repetitively.

T

Theta waves

EEG activity with a frequency of 4-8 Hz, maximal over temporal cortex.

Total sleep period

The 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. See Sleep efficiency.

Total sleep time

The amount of actual sleep time in a sleep period; equal to total sleep period less movement and awake time. Total sleep time is the total of all REMS and NREMS in a sleep period.

Tumescence (penile)

Hardening and expansion of the penis: penile erection. Commonly referred to as nocturnal penile tumescence (NPT) in sleep recordings.

Twitch (body twitch)

A very small body movement such as a facial grimace or finger jerk: not usually associated with arousal.

U

V

W

Wake time

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

X

Y

Z

List of Abbreviations