Glossary of Terms
Acute Insomnia - The inability to sleep well for a period of three weeks to 6
months. After this time the person stops experiencing insomnia and therefore has not progressed into the more severe form of chronic
Actigraph - An actigraph is an instrument that is used to measure body movement.
Advanced Sleep Phase Syndrome - The individuals sleeping and waking cycle is advanced well ahead of a conventional bedtime.
Consequently they want to fall asleep or wake up too early. It is a circadian rhythm disorder (see definition below).
Antidepressant - Antidepressants are a catagory of prescribed medications that are commonly used for the treatment of
depression. They are sometimes used as a treatment option for certain sleep disorders such as cataplexy and sleep paralysis. There is some
evidence that they may reduce REM sleep in some individuals.
Apnea - The cessation of airflow at the mouth and nostrils for a minimum of 10 seconds.
Apnea Index - This measures how severe the sleep apnea is and is a measure of the number of apnea events per hour.
Arousal - Arousal is the change from being asleep to waking up. The term can also be used when an individual moves from stage
3 or 4 sleep (deep sleep) to a lighter sleep stage (stage 1, 2 or REM).
Arousal Disorder - Arousal disorder is a parasomnia disorder (see definition below) that is related to certain unusual or
abnormal arousal activities. Somnumbalism ( sleepwalking ) and night terrors are some examples.
Arousal Threshold - The arousal threshold is defined as how easy it is to wake a sleeping individual at a particular
Bruxism - The grinding of ones teeth during sleep.
Cataplexy - This is a medical condition that most often affects people who have narcolepsy. Cataplexy is a muscular weakness
that can be something as minor as some slackening of the facial muscles to dropping of the jaw to in severe instances, total collapse.
Central Sleep Apnea - Sleep apnea caused by the brains inability to control the individuals breathing.
Chronic Insomnia - Insomnia is considered to be chronic insomnia if it occurs nightly for a minimum of one month.
Chronotherapy - Chronotherapy is used for the treatment of circadian rhythm sleep disorder (see definition). The
patient’s sleep and wake cycle are systematically changed so that there biological clock can be reset to more standard sleep times.
Circadian Rhythm - This is roughly a 24 hour cycle in the physiological
processes of an individual. It depends partially on external factors such as heat and sunlight.
Continuous Positive Airway Pressure - CPAP - This device is used by people with sleep apnea and it provides a constant
positive airway pressure while they are asleep to keep their airway open.
CPAP Pressure - This is the amount of pressure needed to keep the airway open in a sleep apnea patient who is using a CPAP
sleep machine. It is expressed in centimeters of water or cmH20. Depending on the patient this can range from 5 to 20 cmH20.
Deep Sleep - This is sleep stage 3 and 4 of non-REM sleep.
Delayed Sleep Phase Syndrome - This is a circadian rhythm disorder (see definition). The term is used when an individual’s
sleep wake cycle is delayed compared to what is considered a normal sleep cycle. Generally the individual goes to bed very late and has
difficulty getting up in the morning.
Dyssomnias - Sleep disorder that has the individual experiencing excessive sleepiness or insomnia.
EPAP - See Expiratory Positive Airway Pressure.
Epoch - During a sleep test this is a measurement of the duration of the sleep recording. It generally is 20 to 30 seconds
which is approximately one page of the polysomnogram.
Epworth Sleepiness Scale - Based on the patients perceptions the Epworth scale is an index of sleep propensity during the
day. There are 8 questions that are completed.
Esophageal Scale - This measurement determines respiratory effort and airway resistance. It is used in certain cases by sleep
disorders centers when they are doing polysomnographic testing.
Expiratory Positive Airway Pressure - EPAP - The prescribed pressure for the breathe out phase (expiratory) of an individual
on Bi-level CPAP therapy for obstructive sleep apnea.
Five Stages of Sleep - Stage 1 is the first sequence called drowsiness, stage 2 is a period of light sleep, stages 3 and 4
are the deep sleep stages with stage 4 the deeper sleep of the two, stage 5 sleep is REM ( Rapid Eye Movement ) sleep.
Gastroesophageal Reflux Disease - This disease causes the individual to awaken multiple times per night due to stomach acid
flowing upward into the persons throat while they are asleep.
Histogram - A histogram is a graph that shows frequency distributions and is used by sleep technicians as a measure and
evaluate a sleep study.
Hyperirritability - Individuals who are sleep deprived may experience extreme irritability.
Hypersomnia - The individual sleeps for extremely long periods of time that
are way above the sleep periods of a average individual.
Hypotonia - Temporary paralysis that is usually present during REM sleep. This prevents the individual from thrashing about
or acting on their dreams.
Hypoxia - An individual has an insufficient amount of oxygen reaching the tissues in their body.
Insomnia - Defined as difficulty sleeping. Generally, there are 3 types of insomnia, transient, acute and chronic.
Jet Lag - Jet lag is a physiological condition causes by an interruption of the individuals circadian rhythm (see
definition). The term is generally used when individuals travel by air across different time zones and their bodies are unable to immediately
adjust to the new daylight and darkness hours. Symptoms include loss of appetite, headache, fatigue and grogginess among others.
Leg Movement - During a sleep study an individuals leg movements are recorded to test for restless legs syndrome.
Maintenance of Wakefulness Test - This test is used to assess the individual’s ability to remain awake and generally involves
being placed in a darkened room in a semi-reclined position.
Melatonin - This is a hormone secreted by the pineal gland.
Narcolepsy - A neurological condition that includes excessive daytime sleepinness. An individual may fall asleep at
inopportune times and places. Their nightime sleep may be fragmented.
Night Terror - This is an extreme emotional episode in which the individual wakes up with fear and anxiety. Generally this
occurs in children and they are often unable to remember the details of the incident that provoked the fears.
Obstructive Sleep Apnea - The cessation of breathing while asleep of up to 10 seconds. Caused by obstruction in the upper
throat or airway.
Parasomnia - This term includes sleep arousal disorders such as sleepwalking, aggressive behavior while sleeping, and
nightmares. Generally, treatable by medical intervention.
REM Sleep - Rapid Eye Movement during sleep. This is sleep that is characterized by active brain waves and movement of the
eyes. Dreaming occurs during this sleep stage.
Restless Legs Syndrome - Also called RLS and is a sleep disorder caused by
itchiness or a crawling sensation in the legs. This most likely occurs while the individual is not active. The symptoms are alleviated by
Sleep Debt - Occurs when the individual over time does not get a sufficient amount of restorative sleep.
Sleep Deprivation - A chronic lack of sufficient restorative sleep.
Sleep Disorder - This includes a wide range of sleep illnesses that have multiple causes and include physiological and
psychological causes as well as a multitude of other sleep disturbances.
Sleep Hygiene - Activities related to getting a good night's sleep. Some positive sleep hygiene habits include going to
bed at the same time each night, reduced alcohol consumption, no large meals before bed and no caffeine 4 hours before going to sleep.
Sleep Latency Test - A sleep latency test measures how easily a person is able to fall asleep.
Sleep Paralysis - An individual awakens and is unable to move for a short period of time. They were likely awakened during
the REM sleep stage where sleep paralysis is naturally induced by the body.
Sleepwalking - Walking while asleep generally occurs during stages 3 and 4 sleep which is the deep sleep stage. Also called
Somnambulism - Walking while asleep.
Transient Arousal - Briefly awakening from sleep.
Transient Insomnia - This is the mildest and probably the most common type of insomnia. It lasts from one night to several
weeks and can be caused by jet lag or anxiety.
Unintended Sleep Episode - An unplanned sleep episode that occurs such as falling asleep while riding a bus or subway or
perhaps during a more dangerous activity such as driving.
Upper Throat or Upper Airway - Part of the respiratory system that includes the palate, tongue, sinus passages, and nose.
Uvula> - The small soft tissue that hangs down at the back of the tongue before your throat. You can see your uvula
by opening your mouth wide and looking close up in the mirror.
White Noise Machines - In relation to sleep white noise machines produce a random sound that soothes the individuals and can
mask other sounds such as conversation or other noises. The sound is generally like the sound of air escaping a balloon. Other sounds such as
rain or ocean waves are also used to soothe the user.
See Also: Sleep With Your Eyes Open