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Sleep Disorder Definitions and Terminology


Advanced Sleep Phase Syndrome (ASPS): Disorder in which the major sleep episode is advanced in relation to the desired clock time, that results in symptoms of compelling evening sleepiness, an early sleep onset, and an awakening that is earlier than desired. 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. The major complaint may concern either the inability to stay awake in the evening, or early morning awakening insomnia, or both.

Altitude Insomnia: Insomnia that occurs when people go to higher altitudes. Usually accompanied by headaches, loss of appetite, and fatigue. Twenty-five percent of individuals who go from sea level to 2,000 meters will have some symptoms. Also called Acute mountain sickness, Acosta`s disease, Alpine sickness, and hypobaropathie.

Anesthesia: Complete or partial loss of sensation, usually caused by artificially produced unconsciousness.

Antihistamines: Drugs which combat the effects of histamine. (Histamine is a chemical released by certain cells of the body.) Used to reduce nausea and sickness. Drowsiness is a detrimental side-effect when used for these purposes, but this drowsiness is desired when the drugs are used to treat insomnia.

Arousal: Awakening from sleep. Also sometimes refers to a change from a "deeper" stage of non-REM sleep to a "lighter" stage arousal disorder parasomnia disorder presumed to be due to an abnormal arousal function. Classical arousal disorders: sleepwalking, sleep terrors and confused arousal. arousal threshold in scientific studies this is a parameter that measures how easily a sleeping person is awakened.

Benzodiazepines: Group of medicines called central nervous system (CNS) depressants. First developed in the 1950's, these deugs tranquilize and sedate. They work by slowing down the activity of the central nervous system. They slow the messages going to and from the brain to the body, including physical, mental and emotional responses. Also referred to as 'minor tranquilizers'. Benzodiazepines include Diazepam (brand names Valium, Ducene) , Oxazepam (Alepam, Murelax, Serepax), Nitrazepam (Alodorm, Mogadon), Flunitrazepam (Rohypnol), and Temazepam (Normison, Euhypno). Benzodiazepines (ben-zoe-dye-AZ-e-peens) belong to the group of medicines called central nervous system (CNS) depressants (medicines that slow down the nervous system). Some benzodiazepines are used to treat insomnia.

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.

Brain Waves: The brain's spontaneous electrical activity studied by electroencephalography (EEG).

Bruxism: Teeth grinding or jaw clenching during sleep. The term "clenching" means you tightly clamp your top and bottom teeth together, especially the back teeth. The stressful force of clenching causes pressure on the muscles, tissues, and other structures around your jaw.

Cataplexy: Sudden muscle weakness associated with narcolepsy. It is often triggered by emotions such as anger, surprise, laughter, and exhilaration. No loss of consciousness is involved – i.e. it is not a black out or a faint, and, despite the phonetic similarity of ‘narcolepsy’ and ‘cataplexy’ with ‘epilepsy’, not epileptic in nature. You are fully conscious, you just can't move.

Cerebral Cortex: The brain’s outer layer of gray tissue that is responsible for higher nervous function.

Chronotherapy Light Therapy: Use of bright light to affect a change in sleep patterns. Scientists who work in the area feel chronotherapy targets the same brain chemicals that antidepressant drugs do, with the advantages of being less expensive, working very soon, and having fewer side effects.

Circadian Rhythm: Relating to or exhibiting approximately 24-hour periodicity, especially related to fluctuation of behavioral and physiological functions, including sleep waking. Sometimes to a different (e.g., 23 or 25 hour) periodicity when light/dark and other time cues are removed.

Circadian Rhythm Sleep Disorders: Disorders that are related to the timing of sleep within the 24-hour day. Some of these disorders are influenced by the timing of the sleep period that is under the individual's control (e.g., shift work or time zone change). Others in this group are disorders of neurological mechanisms (e.g., irregular sleep-wake pattern and advanced sleep phase syndrome).

CPAP - Continuous Positive Airway Pressure: A machine that helps a person who has apnea breathe more easily during sleep by sending blowing air at a constant, continuous pressure. During sleep, CPAP patients wear a face mask connected to a pump that forces air into the nasal passages at pressures high enough to overcome obstructions in the airway and stimulate normal breathing.

Delayed Sleep Phase Syndrome: A circadian-rhythm sleep disorder thought to result from the endogenous circadian pacemaker being “stuck” at a later-than-normal phase, relative to the desired sleep-wake schedule. The basic pathophysiology of DSPS remains poorly understood.

Deep Sleep: Refers to combined non-REM sleep stages 3 and 4, also known as delta sleep.

Delayed Sleep Phase: A disorder in which the major sleep episode is delayed by 2 or more hours of the desired bedtime. This causes difficulty awakening at the desired time. Symptoms include:

  • Complaint of insomnia or excessive sleepiness
  • Inability to fall asleep at the desired time
  • Iinability to wake up at the desired time
  • Depression (sometimes)
  • This sleep pattern has been present for 3 months

Delta Sleep: A stage of sleep in which EEG delta waves are prevalent or predominant (sleep stages 3 and 4, respectively). Called "slow wave" sleep because brain activity slows down dramatically from the "theta" rhythm of Stage 2 to a much slower rhythm of 1 to 2 cycles per second called "delta" and the height or amplitude of the waves increases dramatically. In most adults these two stages are completed within the first two 90 minute sleep cycles or within the first three hours of sleep. Contrary to popular belief, it is delta sleep that is the "deepest" stage of sleep and the most restorative.

Delta Waves: 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.

Desynchronization: In the context of sleep studies and disorder diagnosis, refers to lack of alignment between external signals and the biological clock.

Diagnostic Sleep Study: Monitoring of several physiological activities. 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 (home sleep testing).

Diurnal: Active or occurring during the daytime; repeating once each day.

Dopamine: Inhibitory neurotransmitter involved involuntary movement and motivation.

Electroencephalogram (EEG): A measurement of the electrical activity associated with brain activity.

Electromyogram (EMG): A measurement of the electrical activity associated with muscle movements.

Electrooculogram (EOG): A measurement of the electrical activity associated with eye movements.

Endocrine System: The ductless glands in the body that secrete hormones.

Entrain: To reset or align with the biological clock.

Enuresis: Bed-wetting.

Epworth Sleepiness Scale: A scale indicating propensity to sleep during the day as perceived by patients. From the subjective answers to 8 questions. The scale was developed by researchers in Australia and is widely used by sleep professionals around the world to measure sleep deprivation.

Excessive Daytime Sleepiness (EDS): (also "excessive daytime somnolence") - subjective report of difficulty in staying awake, accompanied by a ready entrance into sleep when the individual is sedentary EDS suggests the presence of a significant sleep disorder and is different from fatigue. Depression, anxiety, stress, and boredom are commonly thought to cause excessive sleepiness, but in fact these conditions cause fatigue and apathy.

Endogenous Rhythms: Rhythms driven by an internal, self-sustaining biological clock rather than by signals that are external to the organism (for example, light).

Exogenous Rhythms: Rhythms that are directly regulated by an external influence, such as an environmental cue. They are not generated internally by the organism itself.

Fatigue: Feeling of tiredness, weariness or lack or energy usually associated with lower performance (physical or mental). Fatigue is different from drowsiness. In general, drowsiness is feeling the need to sleep, while fatigue is a lack of energy and motivation. Drowsiness and apathy (a feeling of indifference or not caring about what happens) can be symptoms of fatigue. Fatigue often develops in response to physical exertion, emotional stress, boredom, or lack of sleep.

Free-Running Disorder (FRD): A circadian disorder where the sleep cycle becomes dis-attached from the normal patterns observed by most of society, such as the rising and setting of the sun. Often afflicts blind people.

GABA (Gamma-Aminobutyric Acid): An amino acid neurotransmitter (C4H9NO2) in the brain. Believed to be involved in muscle relaxation, sleep, diminished emotional reaction and sedation.

Hallucination: A false and distorted perception of objects or events.

Homeostasis: The ability or tendency of an organism or cell to maintain internal equilibrium by adjusting its internal processes. From the Greek "to remain the same".

Homeostatic Regulation of Sleep: Refers to the neurological signals mediating the pressure or urge to sleep.

Hypersomnia: Excessive sleep, characterized by recurrent episodes of unusual daytime sleepiness or prolonged nighttime sleep. Different from feeling tired due to lack of or interrupted sleep at night, persons with hypersomnia are compelled to nap repeatedly during the day, often at inappropriate times such as at work, during a meal, or in conversation. These daytime naps usually provide no relief from symptoms. The symptoms are typically treated, not the underlying problem. Stimulants, such as amphetamine, methylphenidate, and modafinil, may be prescribed. Other drugs that doctors sometimes use include clonidine, levodopa, bromocriptine, antidepressants, and monoamine oxidase inhibitors. Page on hypersomnia.

Hypnagogic Hallucination: A “greater-than-life-like” dream experience. Sometimes associated with narcolepsy.

Hypnic Jerk: A startle reaction as a person falls asleep; muscle jerks like an electric shock. Normal.

Hypnogram: A graphical summary of the electrical activities occurring during a night's sleep.

Hypothalamus: The part of the brain that lies below the thalamus and regulates body temperature and metabolic processes.

Hypnotics: Medications that cause sleep or partial loss of consciousness.

Hypocretin: Peptide involved in sleep cycle. Also called orexin.

Imidazopyridines: Relatively new class of drugs inducing sleepiness. related to benzodiazepines. They include zolpidem (Ambien) and alpidem

Insomnia : Complaint describing difficulty in sleeping. People with insomnia have one or more of the following:

  • Difficulty falling asleep
  • Waking up often during the night and having trouble going back to sleep
  • Waking up too early in the morning
  • Un-refreshing sleep.
  • Insomnia can cause problems during the day, such as sleepiness, fatigue, difficulty concentrating, and irritability.

Jet Lag: Describes a combination of symptoms induced by a major rapid shift in environmental time during travel to a new time zone. Called "jet" lag because of the often noticed after airplane flights. Fatigue, irritability, dehydration, and a broken sleep pattern are common symptoms of jet lag.

Light Sleep: Term used in clinical practice to describe non-REM stage 1, and sometimes, stage 2 sleep. People in light sleep drift in and out of sleep and can be awakened easily. Eyes move very slowly and muscle activity slows. People awakened from stage 1 sleep often remember fragmented visual images. Many also experience sudden muscle contractions called hypnic myoclonia, often preceded by a sensation of starting to fall.

Light Therapy: Form of therapy where the person is exposed to bright light at the appropriate time of day to effect the timing, duration and quality of sleep. Also used in the treatment of Seasonal Affective Disorder.

luteinizing hormone: A glycoprotein secreted by the pituitary gland. It stimulates the gonads to secrete sex steroids.

Melatonin: A hormone secreted by the pineal gland that is derived from the amino acid tryptophan, which helps synchronize biological clock neurons in the suprachiasmatic nucleus.

Micro-Arousal: Partial awakening from sleep. An episode where a sleeper partially awakes, but is not aware of it.

Micro-Sleep: Period lasting up to a few seconds during which people appear to be asleep in otherwise waking periods. Cause for concern for people in certain critical jobs.

Multiple Sleep Latency Test (MSLT): A common sleep test given at sleep labs in the diagnosis of sleep disorders. The multiple sleep latency test records brain waves (via EEG), heart rate (via EKG), muscle activity and eye movements. Often given as a series of "nap tests".

Nap: Short period of sleep at a time separate from the major sleep period, especially during the day.

Narcolepsy: Sleep disorder characterized by brief attacks of deep sleep, and with symptoms including excessive sleepiness, cataplexy, sleep paralysis, hypnogogic hallucinations, overwhelming daytime sleepiness (even after adequate nighttime sleep), and an abnormal tendency to pass directly from wakefulness into REM sleep. Not all narcoleptics have all of these symptoms.

Natural Short Sleeper: Person who habitually and spontaneously sleeps substantially less in a 24-hour period than is expected for a person in his or her age group, and does not have excessive sleepiness. Although there is a broad range of variation in the individual need for sleep, the typical adult requires an average of 7 to 10 hours each night. Short sleepers have a daily total sleep time of less than 75% of the age-related norm, and awaken spontaneously.

Neurotransmitter: A chemical produced by neurons that carries messages to other neurons.

Nightmare: Unpleasant and/or frightening dream that usually awakens the sleeper. Unlike night terrors, nightmares occur during REM sleep.

Night Terrors: Incomplete arousal from slow wave sleep accompanied by a state of intense fear and agitation sometimes experienced, especially by children, on awakening from a stage of sleep not associated with dreaming but characterized by extremely vivid hallucinations. The person awakens in terror with feelings of anxiety and fear but is unable to remember any incident that might have provoked those feelings. In contrast, people who wake up from nightmares often recall some of the dream.

Nocturia: Urination at night especially when excessive.

Nocturnal: Relating to or taking place at night.

Nocturnal Sleep-Related Eating Disorder (NS-RED): Eating while sleepwalking. Typically the person doesn't remember eating when he or she wakes up.

Nocturnal Enuresis: Bed wetting. Urinating during sleep.

NREM: Non-REM sleep - a normal part of sleep accounting for typically 75-80% of sleep time. Characterized by slower and larger brain waves than in REM.

Obstructive Sleep Apnea (OSA): A disorder in which breathing is frequently interrupted for brief intervals during sleep, resulting in intermittent decreases in blood oxygen levels and transient arousals from sleep, leading to poor sleep quality and excessive daytime sleepiness.

Orexin: Protein neurotransmitter or neuropeptide active in the sleep cycle and in appetite. Peptide family comprised of two peptides, orexin-A (hypocretin-1) and orexin-B (hypocretin-2). Subject of great research at this time. Deficiency is associated with narcolepsy.

OSLER: Oxford Sleep Resistance test. Designed as an objective vigilance test. Similar to the MSLT but without EEG monitoring in the sleep cycle and in appetite.

Photoperiod: The light/dark or day/night cycle.

Photoreceptor: A molecule or structure that can detect light.

Pons: The brain stem region critical for initiating REM sleep.

Parasomnias: Disorders that intrude into the sleep process and create disruptive sleep-related events. These behaviors and experiences occur usually while sleeping, and are most often infrequent and mild. They may happen often enough or become so bothersome that medical attention is required. The parasomnias are typically classified as: (1) arousal disorders (2) sleep-wake transition disorders (3) parasomnias usually associated with REM sleep and (4) other parasomnias.

Phase Advance: A shift earlier in time, for instance if someone starts going to bed earlier and waking up earlier.

Phase Delay: A shift later in time, for instance if one's sleep cycle moves ahead on the clock.

Polysomnogram (PSG): A continuous and simultaneous recording of physiological variables during sleep, i.e., EEG, EOG, EMG (the three basic stage scoring parameters), EKG, respiratory air flow, respiratory excursion, lower limb movement, and other electrophysiological variables. See polysomnograms.

Polysomnograph: A test of sleep cycles and stages through the use of continuous recordings of brain waves (EEG), electrical activity of muscles, eye movement (electrooculogram), breathing rate, blood pressure, blood oxygen saturation, and heart rhythm and direct observation of the person during sleep. See page on polysomnography.

Obesity-Hypoventilation syndrome: A condition related to obstructive sleep apnea in which a very obese person does not breathe sufficient air during sleep or while awake.

Obstructive Apnea: Apnea due to a mechanical obstruction, such as a very large uvula or tongue in the back of the mouth, or a problem with the trachea.

Rebound Insomnia: Sleep difficulties after discontinuing use of a hypnotic medication.

REM Latency: Period of time from sleep onset to the first appearance REM.

REM Motor Atonia: When the large skeletal muscles go limp during REM sleep.

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 Sleep Behavior Disorder (RBD): Very rare disorder in which sleeper acts out dreams, often violently, and has bodily movement. The body is usually paralyzed during REM sleep. People often report an ongoing, hallucinatory REM dream episode during their activity. May be a warning sign of Parkinson's disease. Often associated with neuropathologies including: vascular insult, tumors, and brain degenerative disorders.

Rapid Eye Movement (REM) Sleep: Deep sleep period with rapid eye movements. Normal part of sleep cycle. Recurs cyclically several times during a normal period of sleep. Characterized by increased neuronal activity of the forebrain and midbrain, by depressed muscle tone. Most dreaming occurs in this stage, which accounts for about 20% of sleep in adults.

REM Sleep Rebound: Increase in REM sleep following unnatural reduction. Extension of time in, and an increase in frequency and density of REM sleep episodes.

Restless Legs Syndrome (RLS): Sleep disorder characterized by a deep creeping, or crawling sensation in the legs that tends to occur when an individual is not moving. There is an almost irresistible urge to move the legs; the sensations are relieved by movement. The sensations have the following features: · Occur during periods of inactivity · Become more sensitive in the evening and at night · Are relieved by movement of the limb · Often cause difficulty staying or falling asleep, which leads to feelings of daytime tiredness or fatigue · May cause involuntary jerking of the limbs during sleep and sometimes during wakefulness Up to 8% of the adult population may have this condition.

Rheostasis: Physiological tendency to remain constant.

Seasonal Affective Disorder (SAD): A form of depression caused by inadequate bright light reaching the biological clock in the suprachiasmatic nucleus. Consequently, treatment often involves the use of light therapy.

Sedatives: Chemicals (sometimes medicines) tending to calm, and reduce nervousness or excitement and foster sleep. Many medicines are sedatives; sometimes they are administered for that purpose. Sometimes sedation is an undesired side effect. Common sedatives include Antihistamines, Benzodiazepines, Imidazopyridines, and herbal sedatives.

Serotonin: Neurotransmitter involved in several important body functions such as memory, emotions, moods, sleep and arousal.

Sleep: The natural periodic suspension of consciousness during which the powers of the body are restored, characterized by lessened consciousness and slowed-down metabolism.

Sleep Apnea: Condition where the sleeper repeatedly stops breathing for 10 or more seconds during sleep. The Greek word "apnea" literally means "without breath." There are three types of apnea: obstructive, central, and mixed; of the three, obstructive is the most common. Can be dangerous as people with sleep apnea sometimes stop breathing hundreds of times during the night and often for a minute or longer.

Sleep Architecture: NREM/REM stage and cycles of sleep and time spent in each stage. Also called sleep timing mechanism. One's sleep architecture changes with age. Middle-aged and elderly people tend to spend less time in deeper sleep than younger people. By age 60 or 70, many adults experience a decrease in the proportion of time spent in delta sleep. The percentage of REM sleep remains relatively stable.

Sleep Cycle: Term used by scientists and sleep researchers to describe the pattern of sleep stages, especially the NREM-REM cycle.

Sleep Debt: Physiological state that results from recurrent sleep deprivation occurs over time. When an individual does not experience sufficient restorative daily sleep required to maintain a sense of feeling rested and refreshed.

Sleep Deprivation: Acute or chronic lack of sufficient sleep.

Sleep Disorders: General term applied to a broad range of illnesses, 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: General description for a group of disorders that produce pauses in breath in the sleeper or that reduce the amount of air the person is getting. Apnea is a common type.

Sleep Fragmentation: Sleep interruption due to frequent or sustained awakenings or early morning awakenings.

Sleep Hyperhidrosis: Profuse sweating that occurs during sleep.

Sleep Hygiene: Conditions and practices that promote continuous and effective sleep. These include bedtime routines, regular bed and arise times. And regularly getting enough sleep to avoid sleepiness during the day. For some people, can also refer to limiting alcoholic and caffeinated beverages prior to bedtime and using exercise, nutrition, and environmental factors so that they enhance, not disturb, restful sleep.

Sleep Inertia: Feelings of grogginess and/or sleepiness that persist longer than 10 to 20 minutes after waking up. Symptoms include what goes under the scientific term is transitory "hypovigilance" or low vigilance, along with confusion, disorientation of behavior and impaired cognitive and sensory-motor performance. Happens often when a person is aroused from deep sleep in the first part of the night.

Sleep Hyperhydrosis: Profuse sweating during sleep.

Sleep Latency: The time between going to bed and sleep onset. Similarly, the term "REM sleep latency" refers to the time between sleep onset and the onset of the first episode of REM sleep. The term "sleep efficiency" refers to the proportion of time in bed that is spent sleeping. Also called "sleep onset latency".

Sleep Paralysis: Temporary inability to talk or move when falling asleep or waking up. It occurs normally during REM sleep.

Sleep Talking: Utterance of speech or sounds during sleep without awareness of the event. Takes place during stage REMS, representing a motor breakthrough of dream speech, or in the course of transitory arousals from NREMS and other stages. The person is not fully consciousness and retains no memory of the talking. Symptoms include:

  • speech or utterances during sleep
  • episodes are not associated with awareness of talking
  • polysomnography (sleep recording) shows episodes of sleep talking that can occur in any stage of sleep
  • possible association with psychiatric disorders such as anxiety disorders
  • possible association with medical disorders such as febrile illness (fever)
  • possible association with other sleep disorders such as sleepwalking, obstructive sleep apnea syndrome, or REM sleep behavior disorder

Sleepiness: Somnolence, drowsiness - state where the subject finds it difficult to maintain the wakeful state and falls asleep if not actively kept aroused. Differs from simply a feeling of physical tiredness or listlessness.

Sleepwalking : A sleep disorder where the person gets out of bed and walks around during sleep. Typically occurs in the first third of the night during deep NREM sleep (stages 3 and 4). This sleep disorder affects an estimated 10 percent of all humans at least once in their lives. Most common among children from the ages of 4 to 12. Boys sleepwalk more frequently than girls and that it is between the ages of 11 and 12 that the most cases of sleepwalking are reported.

Somniloquy: Talking while asleep.

Somniphobia: Fear of sleep, fear of falling asleep. Possibly related to anxiety disorder.

Snoring: Noise produced with inspiratory respiration during sleep owing to vibration of the soft palate and the pillars of the oropharyngeal inlet. Forty-five percent of normal adults snore at least occasionally, and 25 percent are habitual snorers. Problem snoring is more frequent in males and overweight persons, and it usually grows worse with age. Problem snorers may develop obstructive sleep apnea.

Stanford Sleepiness Scale: 7-point rating scale consisting of seven numbered statements describing subjective levels of sleepiness/alertness.

Suprachiasmatic Nucleus (SCN): The part of the brain (in the hypothalamus) that contains the biological clock.

Thalamus: A part of the brain consisting of two large ovoid structures at the base of the cerebrum. It acts as a vital relay station between the sensory nerves and the cerebral cortex.

Thermoregulation: The process of regulating body temperature.

Transient Insomnia: Non-chronic insomnia. Insomnia lasting less than three weeks.

Ultradian Rhythm: A periodicity of less than 24 hours.

Unihemispheric Sleep: 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).

Uvulopalatopharyngoplasty (UPPP): An operation on the throat to treat severe snoring and sleep apnea. Soft tissue on the back of the throat and soft palate (the uvula) is removed. The tonsils and possibly other excess tissue may also be removed, if present.

White Noise: Heterogeneous mixture of sound waves extending over a wide frequency range that may be used to mask unwanted noise that may interfere with sleep

Zeitgebers: German for "time givers". External cues that affect the Circadian cycle. Examples include sunlight, familiar morning noises and sounds, and meals.