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advanced sleep phase syndrome (ASPS)
circadian rhythm disorder in which the main sleep period is advanced in relation to the desired clocktime, that results in symptoms of compelling evening sleepiness, an early sleep onset, and an awakening that is earlier than desired. More here.
drug that binds to and causes a functional effect identical in quality and quantity to the endogenous ligand
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 hypobaropathy.
complete or partial loss of sensation, usually caused by artificially produced unconsciousness. More.
drug the binds but causes no functional effect; blocks the action of the natural ligand or agonist drug
drugs which combat the effects of histamine. (Histamine is a chemical released by certain cells of the body.) Used to reduce nausea and sickness. Drowinsess is a detrimental side-effect when used for these purposes, but this drowsiness is desired when the drugs are used to treat insomnia. Antihistimines are the active component in over-the-counter sleep aids.
awakening from sleep. Sleep scientists sometimes use this term to refer 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 confusional arousals.
arousal threshold
in scientific studies this is a parameter that measures how easily a sleeping person is awakened.
group of medicines called central nervous system (CNS) depressants. First developed in the 1950's, these drugs 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 tranquillisers".

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).

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. The biological clock of interest to sleep scientists is the circadian clock.
biphasic sleep
a pattern in which the main sleep period at night is broken into two rough halves, with a waking period of an hour of more in between. More.
brain waves
the brain's spontaneous electrical activity studied by electroencephalography (EEG).
teeth grinding or jaw clenching during sleep. The term clenching means you tightly clamp your top and bottom teeth together, especially the back teeth. More.
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’, cataplexy is not epileptic in nature. You are fully conscious, you just can't move.
light therapy. Use of bright light to affect a change in sleep patterns. Chronotherapy advocates claim it affects the same brain chemicals that antidepressant drugs do, with the advantages of being less expensive, working very soon, and having fewer side effects.
exhibiting a periodicity of 24 hours. Terms originated by Franz Halberg in the 1950s.
circadian disruption
also called circadian disturbance, a departure of the body's circadian cycle from the environment or habit. Results in a change in the time of day during which sleepiness and wakefullness happen. Often caused by jet lag (rapid travel across time zones), or transitions into and out of daylight savings time, or between workweeks and weekends.
circadian rhythm
relating to or exhibiting approximately 24-hour periodicity, especially related to fluctuation of behavioral and physiological functions, including sleep waking.
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). These are one of the major classifications of sleep disorders.
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 pressure. 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. More on CPAP machines.
delayed sleep phase syndrome
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 (under the old definition of stages. Under the new classification, just stage 3)
delayed sleep phase disorder
disorder in which the major sleep episode is delayed by two or more hours of the desired bedtime. This causes difficulty awakening at the desired time. More.
delta sleep
stage of sleep in which EEG delta waves are prevalent or predominant (sleep stages 3 and 4). 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.
delta waves
brain waves with a frequency of 1 to 3 hertz that emanate from the forward portion of the brain during deep sleep. Normal amplitude is 75 microvolts
in the context of sleep studies and disorder diagnosis, refers to lack of alignment between external signals and the biological clock. The cause of many circadian disorders.
difficulties initiating and maintaining sleep (DIS/DMS)
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.
active or occurring during the daytime; repeating once each day.
inhibitory neurotransmitter involved in voluntary movement and motivation.
When the body is in a state of desynchronization. In sleep medicine, this refers most often to people with circadian disorders.
electroencephalogram (EEG)
a measurement of the electrical activity associated with brain activity. First developed in the 1920s by Hans Berger.
electromyogram (EMG)
a measurement of the electrical activity associated with muscle movements. Part of the polysomnogram.
electrooculogram (EOG)
a measurement of the electrical activity associated with eye movements. Part of the polysomnogram.
endocrine system
the ductless glands in the body that secrete hormones.
to reset or align with the biological clock.
bed-wetting, also called nocturnal enuresis or nocturnal micturition or nocturnia
in polysomnography, an epoch is a
epworth sleepiness scale
a scale indicating propensity to sleep during the day as perceived by patients. From the subjective answers to eight questions. A sample is at
The scale was developed by researchers in Australia and is widely used by sleep professionals around the world to measure sleep deprivation. It has the benefits of being fast and simple.
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 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.
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 or propensity 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.
ford insomnia response to stress test (FIRST)
Quick test that is used to identify people at risk for insomnia. Developed at the Henry Ford Hospital. Available on-line at
free-running disorder (FRD)
a circadian disorder where the sleep cycle becomes disattached from the normal patterns observed by most of society, such as the rising and setting of the sun. Often afflicts blind people. More.
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.
a false and distorted perception of objects or events.
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 neurobiological signals mediating the pressure or urge to sleep.
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. Page on hypersomnia.
also "hypnogogia". Sleep paralysis during transition between waking and sleep. A parasomnia.
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. Everyone experiences them. Also called a sleep start or a hypnagogic jerk.
a graphical summary of the electrical activities occurring during a night's sleep. More.
medications that cause sleep or partial loss of consciousness. They include benzodiazepines and imidazopyridines.
a class of peptide hormones that function as neurotrasmitters. Important in the sleep-wake cycle. Also called orexin.
a general term for the fear of sleep. It isn’t classified as a sleep disorder ;it’s more properly thought of as an anxiety disorder. There are many phobias.  Treatment is the same as for other anxiety disorders.  Cognitive behavioral therapy is considered the best long-term solution, although it is expensive. Doctors also prescribe benzodiazepines for anxiety disorders. Hypnophobia may also refer to a fear of being hypnotized.
class of drugs prescribed for insomnia. Often called nonbenzodiazepines, although nonbenzodiazepines refers to a number of different classes of drugs. Imidazopyridines include zolpidem (Ambien) and alpidem.
infradian rhythm
biolpgical rhythm with a period longer than a day. Often seen in menstruation, breeding, or tidal rhythms
initial insomnia
sleep-onset 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
  • unrefreshing sleep.

Insomnia can cause problems during the day, such as sleepiness, fatigue, difficulty concentrating, and irritability. Page on insomnia.

person with insomnia, especially chronic insomnia. The word was coined in 1908.
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.
typical voltage patterns seen on EEG in light sleep. Happen every couple of minutues during Stage 2 sleep.
lark (morning lark)
person who naturally wakes up early and goes to bed early. See page on chronotypes.
sudden awakening in which the person feels as if he or she is suffocating and has a wheezing.
light sleep
term used in clincial 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 rather than narrative dreams. 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.
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.
partial awakening from sleep. An episode where a sleeper partially awakes, but is not aware of it
period lasting up to a few seconds during which people appear to be asleep in otherwise waking periods. Cause for concern for people in critical jobs like truck drivers or pilots.
middle insomnia
waking up during the night and not being able to fall back asleep quickly. Another name for sleep maintenance insomnia
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".
stage 1 of sleep (NREM Stage 1)
stage 2 of sleep (NREM Stage 2)
stage 3 of sleep (NREM Stage 3)
short period of sleep at a time separate from the major sleep period, especially during the day
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. See page on narcolepsy.
natural short sleeper
Person who normally sleeps less than five hours a night with no adverse effects. See page on short sleepers.
compounds with classical “endocrine” functions also affect the brain in a fashion that is not dependent upon the presence of the appropriate endocrine or physiological target organ
a chemical produced by neurons that carries messages to other neurons.
unpleasant and/or frightening dream. Unlike night terrors, nightmares occur during REM sleep.
night terrors
also known as sleep terrors, or pavor nocturnus. Incomplete arousal from slow wave sleep accompanied by a state of intense fear and agitation. 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.
urination at night especially when excessive
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
Bedwetting. Urinating during sleep
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. By larger, we mean greater amplitude in the EEG output.
one full cycle of night and day; often simply called a day or a 24-hour day.
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. See page on apnea.
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 substantial scientific research at this time. Deficiency is associated with narcolepsy.
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
night owl
person who naturally sleeps late and goes to bed late
paradoxical sleep
REM Sleep
the light/dark or day/night cycle.
the brainstem region critical for initiating REM sleep.
disorders that intrude into the sleep process and create disruptive sleep-related events. These behaviors and experiences occur usually during sleep, and are most often infrequent and mild. They may happen often enough or become so bothersome that medical attention is required. See page of parasomnias.
partial agonist
drug that causes functional effect that, at a maximum, is only a fraction of that of the endogenous ligand
periodic limb movement disorder
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)
continuous and simultaneous recording of physiological variables during sleep, i.e., EEG (brain waves) electromyography (major muscle activity), electrooculography (eye movement), EKG (heart activity), respiratory air flow, respiratory excursion, lower limb movement, and other electrophysiological variables. See polysomnograms.
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. Also called Pickwickian syndrome
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.
quiet sleep
Non-REM sleep
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. Patients often report an ongoing, hallucinatory REM dream episode. More on this.
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.
recurrent insomnia
chronic insomnia.
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 even when the legs are not moving. There is an almost irresistible urge to move the legs; the sensations are relieved by movement. See RLS page.
physiological tendency to remain constant
seasonal affective disorder (SAD)
a form of depression caused by inadequate bright light affecting the biological clock during the late autumn and winter. Treatment often involves the use of light therapy.
chemicals (sometimes medicines) tending to calm, and reduce nervousness or excitement and foster sleep. Many sleep medicines are sedatives. Sometimes sedation is an undesired side effect of drugs given for other purposes. Common sedatives include Antihistamines, Benzodiazepines, Z-drugs, and herbal sedatives.
neurotransmitter involved in several important body functions such as memory, emotions, moods, sleep and arousal.
shift work sleep disorder
disorder characterized by excessive sleepiness and caused by working hours that differ from the body's circadian cycle.
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 stages 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.
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 efficiency
percentage of time in bed spent sleeping
sleep fragmentation
sleep interruption due to frequent or sustained awakenings or early morning awakenings
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 hyperhidrosis or sleep hyperhydrosis
profuse sweating during 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. More on sleep inertia.
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 maintenance
the ability to remain asleep for a long period of time
sleep manners
See this page
sleep paralysis
temporary inability to talk or move when falling asleep or waking up.
sleep talking
utterence 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. See somniloquy page.
sleep spindle
event in the brain that produces a 12 to 14 Hz spike in voltage on an EEG. Characteristic of Stage 2 sleep.
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. Is sleepiness a state or a trait? That's a philosophical question.
somnambulism. 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).
slow wave sleep
stages 3 and 4 sleep. Called because the EEG readings show slow waves. Deep sleep.
talking while asleep
sleep disorder
fear of sleep, fear of falling asleep. An anxiety disorder.
general term for a sleep-promoting substance in the brain. Rarely used in scientidic literature as it is too non-specific.
drowsiness, especially when the person seems on the verge of falling asleep.
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.
stage 1 sleep
EEG readings show theta rhythms – (4-8 Hz). External observers see shallow eye movements
stage 2 sleep
EEG readings show frequencies in the (4-7 Hz range with slow waves (K complexes) and sleep spindles (rapid at 14-16 Hz)
stage 3 sleep
also called slow-wave sleep or slow-wave activity. EEG readings show delta waves (0.5-4 Hz) with high amplitude (>75 microvolt)
Stanford Sleepiness Scale
7-point rating scale consisting of seven numbered statements describing subjective levels of sleepiness/alertness. An example can be seen at
suprachiasmatic nucleus (SCN)
the part of the brain (in the hypothalamus) that contains the biological clock for the circadian cycle.
terminal insomnia
waking up too early
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.
the process of regulating body temperature.
thetha waves
EEG waves with frequency of 4–7 Hz.
ultradian rhythm
A biological perodicity of less than 24 hours. Compared to a circadian rhythm (about a day) and an infradian rhythm (longer than a day).
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.
wake state (stage 0)
characterized by fast frequency EEG beta  (>12 Hz) and alpha rhythm (8-12 Hz) if eyes are closed
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
worn out syndrome
a somewhat outdated term. Refers to the feeling of fatige resulting from too much sleep or more sleep than normal. Related to sleep inertia.
German for "time givers". External cues that affect the Circadian cycle. Examples include sunlight, familiar morning noises and sounds, and meals.

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