Definition of Terms
The following definitions are excerpted from ICSD-23 and are reprinted here with the permission of the American Academy of Sleep Medicine.
Actigraph: A biomedical instrument used to measure body movement.
Apnea: Cessation of airflow at the nostrils and mouth lasting at least 10 seconds. The three types of apnea are obstructive, central, and mixed. Obstructive apnea is secondary to upper-airway obstruction; central apnea is associated with a cessation of all respiratory movements; mixed apnea has both central and obstructive components.
Apnea-Hypopnea Index: The number of apneic episodes (obstructive, central, and mixed) plus hypopneas per hour of sleep, as determined by all-night polysomnography.
Apnea Index: The number of apneic episodes (obstructive, central, and mixed) per hour of sleep, as determined by all-night polysomnography. A separate obstructive apnea index or central apnea index sometimes is stated.
Arousal: An abrupt change from a “deeper” stage of non-REM (NREM) sleep to a “lighter” stage, or from REM sleep toward wakefulness, with the possibility of awakening as the final outcome. Arousal may be accompanied by increased tonic electromyographic activity and heart rate, as well as by an increased number of body movements.
Arousal Disorder: A parasomnia disorder presumed to be due to an abnormal arousal mechanism. Forced arousal from sleep can induce episodes. The “classic” arousal disorders are sleepwalking, sleep terrors and confusional arousals.
Cataplexy: A sudden decrement in muscle tone and loss of deep tendon reflexes, leading to muscle weakness, paralysis, or postural collapse. Cataplexy usually is precipitated by an outburst of emotional expression – notably laughter, anger, or startle; one of the symptoms of narcolepsy. During cataplexy, respiration and voluntary eye movements are not compromised.
Chronobiology: The science relating to temporal, primarily rhythmic, processes in biology.
Circadian Rhythm: An innate daily fluctuation of physiologic or behavior functions, including sleep-wake states, generally tied to the 24-hour daily dark-light cycle. This rhythm sometimes occurs at a measurably different periodicity (e.g., 23 or 25 hours) when light-dark and other time cues are removed.
Cycle: A characteristic of an event that exhibits rhythmic fluctuations. One cycle is defined as the activity from one maximum or minimum to the next.
Diagnostic Criteria: Specific criteria established in the International Classification of Sleep Disorders to aid in determining the unequivocal presence of a particular sleep disorder.
Diurnal: Pertaining to the daytime.
Drowsiness: A state of quiet wakefulness that typically occurs before sleep onset. If the eyes are closed, diffuse and slowed alpha activity usually is present, which then gives way to early features of stage 1 sleep.
Entrainment: Synchronization of a biologic rhythm by a forcing stimulus such as an environmental time cue (zeitgeber). During entrainment, the frequencies of the two cycles are the same or are integral multiples of each other.
Excessive Sleepiness (Somnolence, Hypersomnia, Excessive Daytime Sleepiness): A subjective report of difficulty in maintaining the alert awake state, usually accompanied by a rapid entrance into sleep when the person is sedentary. Excessive sleepiness may be due to an excessively deep or prolonged major sleep episode. It can be quantitatively measured by use of subjectively defined rating scales of sleepiness or physiologically measured by electrophysiologic tests such as the multiple sleep latency test (see MSLT). Excessive sleepiness most commonly occurs during the daytime, but it may be present at night in a person, such as a shift worker, who has the major sleep episode during the daytime.
Extrinsic Sleep Disorders: Disorders that either originate, develop, or arise from causes outside of the body. The extrinsic sleep disorders are a subgroup of the dyssomnias.
Fragmentation (Pertaining to Sleep Architecture): The interruption of any stage of sleep due to the appearance of another stage or to wakefulness, leading to disrupted NREM-REM sleep cycles; this term is often used to refer to the interruption of REM sleep by movement arousals or stage 2 activity. Sleep fragmentation connotes repetitive interruptions of sleep by arousals and awakenings.
Free Running: A chronobiologic term that refers to the natural endogenous period of a rhythm when zeitgebers are removed. In humans, it most commonly is seen in the tendency to delay some circadian rhythms, such as the sleep-wake cycle, by approximately 1 hour every day; this delay occurs when a person has an impaired ability to entrain or is without time cues.
Hypersomnia (Excessive Sleepiness): Excessively deep or prolonged major sleep period, which may be associated with difficulty in awakening. The term is primarily used as a diagnostic term (e.g., idiopathic hypersomnia) and the term excessive sleepiness is preferred to describe the symptom.
Hypnagogic: Occurrence of an event during the transition from wakefulness to sleep.
Hypnagogic Imagery (Hallucinations): Vivid sensory images that occur at sleep onset but are particularly vivid with sleep-onset REM periods. Hypnagogic imagery is a feature of narcolepsy, in which REM periods occur at sleep onset.
Hypnagogic Startle: A “sleep start” or sudden body jerk (hypnic jerk), observed normally just at sleep onset and usually resulting, at least momentarily, in an awakening.
Hypnopompic (Hypnopomic): Occurrence of an event during the transition from sleep to wakefulness at the termination of a sleep episode.
Hypopnea: An episode of shallow breathing (airflow reduced by at least 50%) during sleep, lasting 10 seconds or longer, usually associated with a fall in blood oxygen saturation.
ICSD Sleep Code: A code number of the International Classification of the Sleep Disorders that refers to modifying information of a diagnosis, such as associated symptom, severity, and duration of a sleep disorder.
Insomnia: Difficulty in initiating or maintaining sleep. This term is employed ubiquitously to indicate any and all gradations and types of sleep loss.
Intrinsic Sleep Disorders: Disorders that either originate or develop from within the body or that arise from causes within the body. The intrinsic sleep disorders are a subgroup of the dyssomnias.
Maintenance of Wakefulness Test (MWT): A series of measurements of the interval from “lights out” to sleep onset that are used in the assessment of an individual’s ability to remain awake. Subjects are instructed to try to remain awake in a darkened room while in a semi-reclined position. Long latencies to sleep are indicative of the ability to remain awake. This test is most useful for assessing the effects of sleep disorders or of medication upon the ability to remain awake.
Major Sleep Episode: The longest sleep episode that occurs on a daily basis. This sleep episode typically is dictated by the circadian rhythm of sleep and wakefulness; also known as the conventional or habitual time for sleeping.
Microsleep: An episode lasting up to 30 seconds during which external stimuli are not perceived. The polysomnogram suddenly shifts from waking characteristics to sleep. Microsleeps are associated with excessive sleepiness and automatic behavior.
Multiple Sleep Latency Test (MSLT): A series of measurements of the interval from “lights out” to sleep onset that is used in the assessment of excessive sleepiness. Subjects are allowed a fixed number of opportunities (typically 4 or 5) to fall asleep during their customary awake period. Excessive sleepiness is characterized by short latencies. Long latencies are helpful in distinguishing physical tiredness or fatigue from true sleepiness.
Myoclonus: Muscle contractions in the form of abrupt “jerks” or twitches that generally last less than 100 milliseconds. The term should not be applied to the periodic leg movements of sleep that characteristically have a duration of 0.5 to 5 seconds.
Nap: A short sleep episode that may be intentionally or unintentionally taken during the major episode of habitual wakefulness.
Nightmare: This term is used to denote an unpleasant and frightening dream that usually occurs in REM sleep. Nightmares are occasionally called dream anxiety attacks and are distinguished from sleep (night) terrors. In the past, the term nightmare has been used to indicate both sleep terrors and dream anxiety attacks.
Nocturnal Dyspnea: Respiratory distress that may be minimal during the day but becomes quite pronounced during sleep.
Obesity-Hypoventilation Syndrome: A term applied to obese individuals who hypoventilate during wakefulness. Because the term can apply to several different disorders, its use is discouraged.
Parasomnia: A disorder of arousal, partial arousal, or sleep-stage transition. It represents an episodic disorder in sleep (such as sleepwalking) rather than a disorder in the quantity or timing of sleep or wakefulness per se. A parasomnia may be induced or exacerbated by sleep; a parasomnia is not a dyssomnia.
Paroxysm: Phenomenon of abrupt onset that rapidly attains a maximum level and terminates suddenly; paroxysm is distinguished from background activity. This term commonly refers to an epileptiform discharge on the electroencephalogram.
Paroxysmal Nocturnal Dyspnea (PND): Respiratory distress and shortness of breath that are due to pulmonary edema; the dyspnea appears suddenly and often awakens the sleeping individual.
Period: The interval in time between the recurrence of a defined phase or moment of a rhythmic or periodic event; the time that occurs between one peak or trough and the next.
Periodic Leg Movement (PLM): A rapid partial flexion of the foot at the ankle, extension of the big toe, and partial flexion of the knee and hip that occurs during sleep. The movements occur with a periodicity of 20 to 60 seconds in a stereotyped pattern, lasting 0.5 to 5.0 seconds. PLMs are a characteristic feature of the periodic limb movement disorder.
Periodic Movements of Sleep (PMS): See Periodic Leg Movement.
Phase Advance: The shift of an episode of sleep or wake to an earlier position in the 24-hour sleep-wake cycle, a shift of sleep from 11 pm–7 am to 8 pm–4 am represents a three-hour phase advance. See Phase Delay.
Phase Delay: A shift of an episode of sleep or wake to a later position of the 24-hour sleep-wake cycle. A shift of sleep from 11 pm–7 am to 2 am–10am represents a three-hour phase delay. See Phase Advance.
Photoperiod: The duration of light in a light-dark cycle.
Pickwickian: A term applied to an individual who snores, is obese and sleepy, and has alveolar hypoventilation. The term has been applied to many different disorders and, therefore, its use is discouraged.
PLM-Arousal Index: The number of sleep-related periodic leg movements per hour of sleep that are associated with an electroencephalographic arousal. See Periodic Leg Movement.
PLM Index: The number of periodic leg movements per hour of total sleep time as determined by all-night polysomnography; the index is sometimes expressed as the number of movements per hour of NREM sleep because the movements are usually inhibited during REM sleep. See Periodic Leg Movement.
PLM Percentage: The percentage of total sleep time occupied with recurrent episodes of periodic leg movements.
Polysomnogram: The continuous and simultaneous recording of multiple physiologic variables during sleep, i.e., electroencephalogram, electrooculogram, electromyogram (these are the three basic stage-scoring parameters), electrocardiogram, respiratory air flow, respiratory movements, leg movements, and other electrophysiologic variables.
Polysomnograph: A biomedical instrument used to measure physiologic variables of sleep.
Premature Morning Awakening (Early Morning Awakening): Early termination of the sleep episode, accompanied by an inability to return to sleep, sometimes after the last of several awakenings. It reflects interference at the end rather than at the commencement of the sleep episode. This awakening is a characteristic sleep disturbance of some people with depression.
Respiratory-Disturbance Index (RDI) (Apnea-Hypopnea Index): The number of apneas (obstructive, central, or mixed) plus hypopneas per hour of total sleep time, as determined by all-night polysomnography.
Restlessness (Referring to a Quality of Sleep): Persistent or recurrent body movements, arousals, and brief awakenings that occur in the course of sleep.
Rhythm: An event that occurs at an approximately constant period length.
Sleep Architecture: The NREM-REM sleep-stage and cycle infrastructure of sleep understood from the vantage point of the quantitative relationship of these components to each other. Often plotted in the form of a histogram.
Sleep Efficiency (or Sleep-Efficiency Index): The proportion of sleep in the episode potentially filled by sleep (i.e., the ratio of total sleep time to time in bed).
Sleep Episode: An interval of sleep that may be voluntary or involuntary. In the sleep laboratory, the sleep episode occurs from the time of “lights out” to the time of “lights on.” The major sleep episode is usually the longest daily sleep episode.
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 before bedtime; and employment of exercise, nutrition, and environment factors so that they enhance, not disturb, restful sleep.
Sleepiness (Somnolence, Drowsiness): Difficulty in maintaining alert wakefulness so that the person falls asleep if not actively kept aroused. The sleepiness is not simply a feeling of physical tiredness or listlessness. When sleepiness occurs in inappropriate circumstances, it is considered to be excessive sleepiness.
Sleep Latency: The duration of time from “lights out,” or bedtime, to the onset of sleep.
Sleep Log (Diary): A daily, written record of a person’s sleep-wake pattern that contains such information as time of retiring and arising, time in bed, estimated total sleep time, number and duration of sleep interruptions, quality of sleep, daytime naps, use of medications or caffeine-containing beverages, and the nature of waking activities.
Sleep Maintenance DIMS (Insomnia): A disturbance in maintaining sleep after sleep onset is achieved; persistently interrupted sleep without difficulty falling asleep; a disorder characterized by sleep-continuity disturbance.
Sleep Paralysis: Immobility of the body that occurs in the transition from sleep to wakefulness (i.e., atonia); this is a partial manifestation of REM sleep.
Sleep Talking: Talking in sleep that usually occurs in the course of transitory arousals from NREM sleep. The talking can occur during stage REM sleep, at which time it represents a motor breakthrough of dream speech. Full consciousness is not achieved, and no memory of the event remains.
Snoring: A noise produced primarily with inspiratory respiration during sleep that is due to vibration of the soft palate and the pillars of the oropharyngeal inlet. All snorers have incomplete obstruction of the upper airway, and many habitual snorers have complete episodes of upper-airway obstruction.
Twitch (Body Twitch): A very small body movement such as a local foot or finger jerk; this movement usually is not associated with arousal.
Zeitgeber: An environmental time cue, such as sunlight, noise, social interaction, alarm clocks, that usually helps an individual entrain to the 24-hour day.