This glossary explains the meaning of the most common terms that may be used in connection with insomnia and sleep. If something crucial is missing, please help us to perfect this page and send us feedback.


Acceptance and commitment therapy (ACT): uses acceptance and mindfulness strategies mixed in different ways with commitment and behavior-change strategies, to increase psychological flexibility. The overall goal is to learn to live a rich and meaningful life accepting the pain that inevitably goes with it.

Actigraph: electronic device used to measure brain activity, sleep and wakefulness.

Active sleep: equivalent in infants to REM sleep.

Actometry (actigraphy): a wristwatch-like device measures body movements to help distinguish between sleep and wakefulness periods.

Acute insomnia: an  inability to consistently sleep well for a period of less than a month. Sleep problems occur despite adequate opportunity and circumstances for sleep and they must result in problems with daytime function.Acute insomnia is also known as short term insomnia or stress related insomnia.

Advanced Sleep Phase Syndrome (ASPS): circadian rhythm sleep disorder where the sufferer has problems staying awake in the evening and sleeping beyond the early hours.

Altitude Insomnia: a kind of insomnia a person suffers from when he or she goes to higher altitudes.

Antidepressants: Some of the older antidepressants like amitriptyline and dosulepin are themselves a sedative. As a result, they have been used a great deal to help depressed patients with sleep problems. Over the last two decades, doctors treating sleep disorders have prescribed more antidepressants than insomnia drugs, according to several published analyses.

Antihistamines: medicines usually used to treat allergies. As a side effect many antihistamines cause drowsiness and some people find these drugs helpful in treating insomnia. Most OTC (over-the-counter) sleeping pills contain antihistamine.

Arousal: an awakening from sleep. However, it may also refer to a change from a so-called “deeper” stage of non-REM (NREM) sleep to somewhat “lighter” stage.

Arousal disorders: sleep disorders, such as confusional arousals, sleepwalking, sleep terror, that emerge abruptly from deep NREM sleep.


Benzodiazepines: a group of sedative medicines that are used to treat anxiety and sleep problems. Some of these drugs are applied for the insomnia treatment. Diazepam, Flunitrazepam, Nitrazepam, Oxazepam, and Tempazepam are examples of drugs belonging to benzodiazepines family.

Bedwetting: Most children wet the bed occasionally or even nightly during the potty-training years. Controlling bladder function during sleep is usually the last stage of potty-training. Around 30% of 4-year-olds still wet their beds and 10% of 6-year-olds have occasional bedwetting.

Biological Clock: regulates our biological rhythm, like the sleep / awake cycle and other aspects of biological timing.

Brain Waves: show the spontaneous electrical activity of the brain. This is investigated by electroencephalography (EEG).

Bruxism: a person who grinds their teeth or clenches their jaw while asleep.


Cataplexy: sudden muscle weakness provoked by strong emotions such as laughter, surprise, fear or anger. It can cause a collapse or localized weakness lasting from a few second to minutes without loss of consciousness and with prompt recovery.

Cerebral Cortex: the outer layer of the grey tissue of the brain, which is responsible for higher nervous functioning.

Chronic fatigue syndrome (CFS): persistent or episodic fatigue and muscle pain of uncertain cause that does not lessen with sleep or rest.

Chronic Insomnia: the most severe and long-term type of insomnia, which occurs at least three days every week and lasts more than 1-3 months (according to most experts). It can be the result of another disorder, or it can be a primary disorder.

Chronotherapy: refers to the use of circadian or other rhythmic cycles in therapy. In sleep therapy, this means an attempt to move bed time and rising time later and later each day, around the clock, until the person is sleeping on a normal schedule. This could be combined with complementary light therapy.

Circadian Rhythm: internal body clock that controls the sleep/wake cycle. This is the daily rhythmic activity cycle, which is based on 24 – hour intervals.

Circadian Rhythm Sleep Disorders: are disorders of the body clock that affect an individual’s ability to sleep at usual times. Some of such disorders may be influenced by the timing of the sleep period, which is not under the control of a person, for example shift work, time zone change, etc. Other disorders are of the neurological character: irregular sleep – wake pattern, advanced sleep phase syndrome, etc.

Conditional insomnia: difficulty sleeping, the original cause of which no longer operates but which has led to habitual insomnia.

Cognitive Behavioral Therapy (CBT): treatment that targets behaviour and though processes.

Comfort object: an object such as a favourite toy or a piece of blanket that helps a child to feel secure and accept separation from parents.

Continuous Positive Airway Pressure (CPAP): a special machine which helps people suffering from sleep apnoea to breathe more easily during their sleep.


Delayed Sleep Phase Syndrome (DSPS):  a circadian rhythm disorder where the body clock is running late. The sufferer finds it difficult to fall asleep until very late and has trouble getting up.

Diuretic: a substance that promotes urination.

Deep Sleep: non-REM (NREM) sleep stages 3 and 4.

Delta Sleep=Slow-wave sleep (SWS): often referred to as deep sleep. Delta refers to the stage of sleep, when EEG delta waves are either prevalent or predominant. Delta sleep is considered to be the “deepest” stage of sleep, and the most restorative stage of sleep.

Delta Waves: called brain waves, which occur with a frequency of 1 up to 4 hertz. Delta waves, like other brain waves, are recorded with an electroencephalogram (EEG). Delta waves are usually associated with the deepest stages of sleep (3 NREM), also known as slow-wave sleep (SWS), and aid in identifying the depth of sleep.

Desynchronization: lack of alignments between external signals and the biological clock of a person.

Diagnostic Sleep Study is the procedure, which involves the monitoring of several physiological activities of a person. As a rule it is conducted with the aim of determining the absence or presence of any kind of sleep disorder.

Difficulty in breathing: Chronic obstructive pulmonary disease (COPD) is a term used for lung disorders such as emphysema, chronic bronchitis, and in some cases chronic asthma. People with COPD may have difficulty breathing, A chronic cough, fatigue, and chest tightening. COPD can also result in reduced blood oxygen levels, causing fatigue and leading to adverse health conditions.

Drowsiness: the first stage of sleep when your brainwaves begin to slow down.


Electroencephalogram (EEG): a procedure, during which the electrical activity associated with the brain activity, is measured.

Electromyogram (EMG): a procedure, during which the electrical activity associated with the muscle movements, is measured.

Electrooculogram (EOG): a procedure, during which the electrical activity associated with the eye movements, is measured.

Entrain: resetting or aligning someone’s biological clock.

Epworth Sleepiness Scale (ESS): a scale intended to measure daytime sleepiness during daytime activities. ESS is measured using a very short questionnaire. This can be helpful in diagnosing sleep disorders.

Excessive Daytime Sleepiness (EDS): is characterized by persistent sleepiness, and often a general lack of energy, even after apparently adequate or even prolonged night time sleep. EDS is a symptom of many sleep disorders.

Endogenous Rhythms: are any rhythms, which are given by an internal, self – sustaining biological clock of every single individual, rather than by any signals, such as light, noise, etc., which are external to the human organism.

Exogenous Rhythms: are any rhythms, which are regulated by some external influence, such as an environmental cue. These rhythms are not generated in an internal way by the organism.


Fatigue: is a term used to describe any feeling like a lack of energy, tiredness, weariness, and it is usually associated with lower performance (physical and/or mental). This condition can develop in response to boredom, emotional stress, lack of sleep, physical exertion, etc.


GABA (Gamma – Aminobutyric Acid): is the name of an amino acid, that acts as a chemical messenger in the brain, spinal cord, heart, lungs and kidneys and ‘tells’ the body to slow down.

GI (Glycaemic index): a ranking of foods according to the effect they have on blood sugars.


Hallucination: is any false and distorted perception of events and/or objects. It may possibly be the result of tiredness, caused by insomnia.

Homeostasis: is the tendency or the ability of a cell or an organism to keep internal equilibrium, having adjusted its internal processes.

Hyperarousal: stress response often called ‘fight of flight’. This happens when our survival instinct alerts us of an imminent danger and tells how to deal with it. When we are tired-but-wired we are in the state of hyperarousal non-stop and can’t sleep even when we are exhausted.

Hypersomnia: describes excessive sleep. This phenomenon may be characterized by recurrent episodes of some unusual daytime sleepiness and/or prolonged night-time sleep.

Hypnagogic Hallucination: is called a “greater – than – life – like” dream experience. It may possibly be associated with narcolepsy.

Hypnopompic hallucinations: unusual perceptual experiences when waking up.

Hypopnea syndrome: abnormally shallow breathing or slow respiratory rate while sleeping.

Hypnic Jerk: describes any startled reaction after a person has fallen asleep. The muscles can be jerking like under the electric shock influence. This phenomenon is considered to be quite usual and normal.

Hypnogram: diagram of the structure of overnight sleep showing the sequence of sleep stages and periods of waking.

Hypothalamus: the name of the part of the brain situated below the thalamus. The function of the hypothalamus is to regulate body temperature as well as the metabolic processes of every organism.

Hypnosis: a trance state characterized by extreme suggestibility, relaxation and heightened imagination. It’s not really like sleep, because the subject is alert the whole time. It is most often compared to daydreaming, or the feeling of “losing yourself”. Under hypnosis we are fully conscious, but we tune out most of the stimuli around us.

Hypnotics: medications which cause sleep or partial loss of consciousness.

Hypocretin (orexin): neurotransmitter in the brain involved in the control of sleep. Low levels are found in people with narcolepsy/cataplexy.


Imidazopyridines: is the name of the class of drugs for inducing sleepiness, which is quite a modern one. These drugs relate to benzodiazepines.

Insomnia: difficulty in falling or staying asleep.

Irregular sleep–wake rhythm: is characterized by numerous naps throughout the 24-hour period, no main nighttime sleep episode and irregularity from day to day. Sufferers have no pattern of when they are awake or asleep, may have poor quality sleep, and often may be very sleepy while they are awake.


Jet Lag: a major rapid shift in environmental time during a person’s travel to a new time zone. The word “jet” is used because this phenomenon is mostly noticed after flights by an airplane. Jet Lag may cause broken sleep pattern, dehydration, fatigue, irritability, etc.


Light Sleep: is the second stage of sleep (non – REM stage). People, who are in light sleep, keep drifting in and out of their sleep, and they can be awakened without any problem.

Light Therapy: the person is being exposed to bright light at the appropriate time of the day, in order to affect his or her timing, quality, as well as duration, of sleep.


Melatonin: a hormone produced in the pineal gland that promotes sleep.

Micro – arousal: describes partial awakening from sleep, when a person is partially awake, although he or she is not aware of it yet.

Micro – sleep: lasts up to several seconds. During these seconds people appear to be asleep in otherwise waking periods. This phenomenon is considered to be the reason of great concern for people on certain critical jobs.

Mindfulness meditation: is one style of meditation that has attracted attention in Western cultures as a possible approach to healing. Today it’s also been used for anxious and racing minds as well as for insomnia.

Multiple Sleep Latency Test (MSLT): an objective test of sleepiness in which the opportunity to fall asleep is given four or five times during the day. This test can help with the diagnosis of narcolepsy.


Nap: a short period of sleep taken during the day. Common recommendation is 5-45 minutes before 4pm.

Narcolepsy: a sleep disorder linked to excessive daytime sleepiness and insomnia. This condition also includes tendency to pass right from awake into REM sleep, cataplexy, excessive sleepiness, hypnogogic hallucination, overwhelming daytime sleepiness (which may take place even if a person had an adequate night-time sleep), and even sleep paralysis.

Narcotic: the name of any addictive drug, which acts by dulling all the senses and inducing sleep.

Natural Short Sleeper: a person, who can sleep much less in a 24 – hour period, in comparison to other people of his or her age group. This person would not experience any excessive sleepiness. Short sleepers have a daily total sleep time of less than 75% of their age – related norm. For middle aged people this can mean just 3 hours per night.

Nightmare: means any unpleasant and/or frightening dream, which usually makes the sleeper awaken. Unlike night terrors, nightmares take place during REM sleep and usually happen in the early hours of the morning.

Night Terror: incomplete arousal from slow wave deep sleep.  In contrast to nightmares, when a person can recall at least part of his or her dream, if someone has night terror, he or she usually awakens in terror with feelings of anxiety and intense fear, without any recall of the incident.

Nocturia: frequent urination at night. It differs from enuresis, or bed-wetting, in which the person does not arouse from sleep, but the bladder empties anyway. Nocturia is a common cause of sleep loss, especially among older adults. Nocturia could result simply from too much liquid intake before going to bed (usually the case in the young), or it could be a symptom of a larger problem (such as sleep apnea, bladder infection, diabetes). Nocturia can also be a sign of stress, too little rest daytime or an allergy.

Nocturnal: anything which relates to or takes place at night time.

Nocturnal Sleep – Related Eating Disorder (NS – RED): eating something while sleepwalking. As a rule the
person is not able to remember the process of eating when he or she wakes up.

Non–REM Sleep (NREM): is a normal part of every healthy person’s sleep (sleep stages 1-4). This sleep period accounts typically 75 – 80% of the entire sleep time. The NREM-sleep is characterized by slower and larger brain waves than in REM sleep. Also, NREM-sleep has no darting eye movements.


Obesity–Hypoventilation Syndrome: is directly related to Obstructive Sleep Apnea, in which people, who suffer from extreme obesity, are not able to breathe sufficient air during their sleep, as well as when they are awake.

Obstructive Apnea: occurs because of a mechanical obstruction, for example because of an excessively large uvula or tongue in the back of the mouth, and/or because of some problems with the trachea.

Obstructive Sleep Apnea: a sleep disorder, where the process of breathing gets interrupted for short intervals during sleep. This condition usually results in intermittent decreases in blood oxygen levels and transient arousals from sleep, which leads to poor quality of sleep and to an excessive daytime sleepiness.

OCT (Over-The-Counter medicine): The main ingredient in OTC sleeping pills is an antihistamine. Common OTC sleep medications include: Diphenhydramine, Doxylamine and Acetaminophen Others, such as NyQuil, combine antihistamines with alcohol.

Orexin: is applied to a protein neurotransmitter or neuropeptide that works during the sleep cycle and in appetite. In cases where a person suffers from the orexin deficiency, it is usually associated with narcolepsy.

OSLER (Oxford Sleep Resistance Test): a test designed as an objective vigilance test, and it is similar to MSLT (above). OSLER does not include EEG monitoring in the sleep cycle and in appetite.


Parasomnias: sleep disorders involving episodes of unusual behaviour and/or strange experience involving movement, or seeing, hearing or feeling things that don’t exist.

Periodic limb movements in sleep (PLMS): sleep disorder where sufferer moves limbs (repetitive, brief, rapid) involuntarily while asleep (mainly in legs).

Phase Advance: in a case where a person begins to go to bed earlier as well as wake up earlier, phase advance is considered to take place.

Phase Delay: in a case where a person’s sleep cycle moves ahead on the clock.

Placebo: an inactive substance (‘empty pill’) given to a patient to compare its effects with those of a treatment/substance, or so that participants can benefit from believing they have received a treatment and will therefore feel better.

Polysomnography (PSG): continuous recording of sleep by means of an EEG, EMG and EOG, with the possible addition of breathing or other physiological measures depending on the nature of the sleep problem.

Polysomnograph: a test of sleep cycles and stages, which is based on the application of continuous recordings of brain waves (EEG), blood pressure, blood oxygen saturation, breathing rate, electrical activity of muscles, eye movement, heart rhythm, as well as on the direct observation of the sleeping person.

Photoperiod: a name of the light / dark or day / night cycle.

Photoreceptor: a molecule and / or a structure, which can detect light.

Progressive relaxation: is a technique for learning to control the state of tension in one’s muscles.


Quiet sleep: equivalent in infants of NREM sleep.


Rapid Eye Movement (REM) Sleep: type of sleep that occurs during sleep stage 5 when the brain is most active and involves the eyes moving quickly whilst the eyelids are closed.  It reoccurs in a cyclical manner and can repeat up to several times during a normal period of sleep. The majority of dreaming occurs during this very stage of sleep, which accounts for about 20% of sleep in adult people. We have more REM sleep during the early hours of night.

Rebound Insomnia: repeating sleep difficulties.

REM Sleep Behavior Disorder (RBD): People, suffering from this disorder, act out their dreams, often in some violent way, and have bodily movements (in general very rare condition). REM Sleep Behavior Disorder may be considered as a warning sign of Parkinson’s disease.

Restless Legs Syndrome (RLS): a periodic limb movement disorder where the sufferer repeatedly moves the legs during the sleep.  It’s often characterized by a creeping sensation in the legs, which occurs while a person is in a still position, lying in his or her bed. This kind of a sensation results in an irresistible necessity to move and / or to jerk the leg.


Seasonal Affective Disorder (SAD): daytime fatigue and sleepiness, poor concentration and increased appetite and weight gain, mainly in the winter months. It is usually caused by an inadequate bright light reaching the biological clock. Light therapy can be an effective treatment.

Sleep apnoea: a condition where sufferer momentarily stops breathing during sleep.

Sleep bruxism: teeth grinding at night.

Sleep diary: daily written record of a person’s sleep-wake pattern and related events.

Sleep disorder: underlying condition causing a sleep problem.

Sedatives: the name of chemicals and medicines which have a tendency to calm and reduce the state of nervousness and / or excitement; in addition, they work by fostering sleep.

Serotonin: a neurotransmitter, which is involved in several important functions of the body. To these functions belong arousal, emotions, memory, moods and sleep.

Short-term insomnia: can last from a few days to a few weeks. Typical causes are stress, poor sleep habits, jet lag, etc.

Situational circadian rhythm sleep disorders: shift work sleep disorder (SWSD) and jet lag. Shift and irregular work have increased in modern society but human’s ability to adapt hasn’t changed. Human systems comply with a regular circadian rhythm where the main assumption is that we work daytime and sleep nightime.

Sleep: a natural period suspension of consciousness, during which all powers of the body get restored. Sleep is also characterized by lessened consciousness and slowed – down metabolic processes.

Sleep anxiety: excessive worrying about not being able to sleep. Worries related to sleep itself represent a biggest reason (along with stress) to insomnia.

Sleep architecture: general name of NREM and REM stages and cycles of sleep, as well as time of sleep, spent in every stage. As a rule everyone’s sleep architecture gets changed with the age. Thus, by age 60 – 70 most adults tend to experience some decrease in proportion of time, spent in deep delta sleep, though the percentage of REM sleep does not get changed significantly.

Sleep Cycle: is applied to describe the pattern of stages of sleep, especially the NREM – REM cycle.

Sleep Debt: a psychological condition, which can result from recurrent sleep deprivation, when a person does not receive sufficient restorative daily sleep that is considered to be necessary for the sense of feeling rested and refreshed.

Sleep Deprivation: a name for a process, in which the requirement for sleep is not met.

Sleep Disorder: underlying condition causing a sleep problem.

Sleep Fragmentation: sleep interruption caused by frequent sustained awakenings or early morning awakenings.

Sleep Homeostat: a mechanism controlled by brain chemicals such as melatonin, that ensures us to get enough sleep.

Sleep Hygiene: way of improving sleep or preventing poor sleep. Sleep hygiene guidance usually includes practices related to bedtime routines, regular bed and arise times, limiting beverages, containing alcohol and caffeine, before bedtime and using exercise, environmental factors and nutrition in such a way, that they enhance, and not disturb their restful sleep.

Sleep Hyperhidosis: extreme sweating during sleep.

Sleeplessness: difficulty getting to sleep or staying asleep. Equivalent to ‘insomnia’.

Sleep Inertia: combination of feelings of grogginess and / or sleepiness, which lasts longer than 10 to 20 minutes after a person has waked up.

Sleepiness: a condition during which the subject finds it difficult to keep the wakeful condition and fall asleep, in case if he or she is not kept aroused in some active way.

Sleep Latency: time interval between going to bed and the onset of sleep.

Sleep Onset Insomnia: difficulty in falling asleep.

Sleep problem: complaint of disturbed or absnormal sleep.

Sleep start: sudden brief movement or sensory experience when going off to sleep.

Sleep Maintenance Insomnia: difficulty in staying asleep.

Sleep Paralysis: brief inability to move or speak when falling asleep or waking up. It may be accompanied by a feeling of an inability to breathe and by hypnagogic hallucinations. It can be very frightening.

Sleep school: Sleep school / clinic is a reception center which studies sleep disturbances such as chronic insomnia and related diseases. A sleep clinic aims to find out causes for insomnia and to identify appropriate treatment(s) for a patient

Sleep Talking: uttering of sounds during sleep (normally REM) without being aware of this.

Sleepwalking: also called somnambulism. It belongs to sleep disorders, and occurs when some person gets out of his or her bed and walks around during sleep. In according to statistics, about 10 % of all people on earth suffer from sleepwalking. This phenomenon is considered to be most common among children (especially boys) of the age from 4 to 12.

Slow-wave sleep (SWS): The combination of stages 3 and 4 of NREM sleep, the deeper parts of sleep. Also called ‘delta sleep’ referring to the large, slow delta waves that predominate in SWS.

Sleep/wake cycle: pattern of sleep and wakefulness that is determined by the circadian rhythm and sleep homeostat.

Snoring: noise (can be very loud) that is produced by a large intake of breath at the end of an apnoea or partial apnoea. In accordance with the statistics, about 45% of adult people snore from time to time and more than 25% are considered to be habitual snores. In addition, males and overweight people are reported to suffer from snoring more.

Stimulus control therapy: a treatment that aims to help insomnia by encouraging the brain to link certain cues, e.g. the bedroom with sleep.

Somniloquy: talking whilst asleep.

Somnipathy: a sleep disorder.

Somniphobia: describes fear of sleep, and / fear of falling asleep. This is related to anxiety disorders.

Soothing: bringing comfort, calmness, peacefulness, quiety.

Stanford Sleepiness Scale: a rating scale, which consists of seven numbered statements; each of them describes subjective levels of sleepiness / alertness.

Suprachiasmatic Nucleon: the part of the brain, which is situated in the hypothalamus, where the biological clock exists.


Tolerance: describes how certain medicine becomes no longer effective, and the more time it is used, the less effective it becomes.

Transient Insomnia: insomnia that lasts for just a few nights and less than a week. It is usually caused by some outside influence, ie sleeping in a strange bed (hotel), worrying about the next day, high-peak stress at work, etc.

Tryptohpan: an amino acid used to make serotonin, which is turned into the ‘sleep hormone’ melatonin.


Ultradian Rhythm: periodicity of less than 24 hours.

Uvulopalatopharyngoplasty (UPPP): the name of the surgical intrusion, which is conducted on the throat with the aim of treating snoring and sleep apnea. During this surgery soft tissue of the back of the throat, as well as the soft palate is removed. In case if there is any excess tissue in the tonsils, it may also be removed.


White Noise: heterogeneous mixture of sound waves. It may be applied with the aim of masking some unwanted noise.


Z-drugs: are a group of non-benzodiazepine drugs with effects similar to benzodiazepines. They are used in the treatment of insomnia, and names usually start with the letter ‘Z’.

Zeitgeber: literally means “time giver” from German. It informs the brain whether it is day or night and therefore indicates whether it is time to be awake or asleep.

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