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Advanced sleep phase syndrome (ASPS)
Circadian rhythm disorder is when the main sleep period is changed/advanced in relation to the desired clock-time, resulting in symptoms of compelling evening sleepiness, an early sleep onset, or an awakening that is earlier than desired.

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

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.

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.

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.

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.

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.

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.


Epworth sleepiness scale
A scale indicating propensity to sleep during the day as perceived by patients. From the subjective answers to eight questions. 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.

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.

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 associated with blindness.

A false and distorted perception of objects or events.

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.

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.

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.

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 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.

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.

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".

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.

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 diverse ranges of low to high voltage mixed frequency brain waves.

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 arousal from sleep, leading to poor sleep quality and excessive daytime sleepiness.

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.

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.

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.

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.

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

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.

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 even when the legs are not moving. There is an almost irresistible urge to move the legs; the sensations are relieved by movement.

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.

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 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 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 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.

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).

Talking while asleep

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.

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

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

Sleep Disorders

Breathing Disorders

Sleep breathing disorders include some of the most frightening and life-threatening conditions you or a loved one can experience. Snoring is an extremely common and treatable breathing disorder.

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Excessive Sleepiness

Narcolepsy is defined as constant sleepiness and the tendency to sleep at inappropriate times. Its impact on a person's life can be significant and even disabling.

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Restless Leg Syndrome

People experience restless legs in many ways, but all describe the feeling as an unpleasant "creepy, crawly" sensation when they are sitting or laying still, especially at bedtime.

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Nighttime Sleep Disorders

Almost 40% of all children will sleepwalk at some time, with peak occurrence between 3 and 7 years. Sleep walking often runs in families and most children outgrow it by adolescence. Sleepwalking can also be associated with night terrors.

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Insomnia is the inability to fall asleep or remain asleep. Trouble falling asleep or staying asleep plagues many adults. Insomnia will affect your waking and sleeping hours.

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Circadian Rhythm

Delayed sleep phase syndrome (DSPS) is a disorder in which the person's sleep-wake cycle (internal circadian rhythm clock) is delayed by two or more hours.

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