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A-Z Sleep Definitions


A wrist watch device to assess movement and sleep/wake cycles.

AHI (Apnoea/Hypopnoea Index)

The number of Apnoea/Hypopnoea Events per hour.

AI (Apnoea Index) 

The number of Apnoea events per hour.


Automatic Positive Airway Pressure Machine (Automatic CPAP), which automatically adjusts the level of pressure needed on a breath-by-breath basis.   (Useful for people on high pressures or who find it difficult to tolerate the pressure needed and used for identifying correct pressures).


Without Breath (also spelt Apnea in some countries). Therefore, Sleep Apnoea means pauses in breathing whilst sleeping.


Going from sleep to wakefulness, or from a deeper to lighter stage of sleep.

ASV (Adaptive Servo Ventilation)

A non-invasive ventilator machine, that is used to treating central sleep apnoea, mixed sleep apnoea and other conditions.  These machines support regular breathing with a back-up rate to ensure people continue breathing during central events.


Bi-level Pressure Machine with 2 pressures – a lower pressure for exhalation and a higher pressure for inhalation.  (Useful for people who find it difficult to exhale against the pressure). 


Teeth grinding during sleep.


A combination of Central and Obstructive Sleep Apnoea. (Also known as Mixed Sleep Apnoea).


Continuous Positive Airway Pressure Machine with one fixed pressure giving constant same-pressure therapy to keep the airways open.

CSA (Central Sleep Apnoea)   

Absence of breath caused by irregularity in the brain’s control of breathing.


Expiratory Positive Airway Pressure prescribed for the expiratory (breathing out) phase of a person on NIV therapy, such as BiPAP/ASV.

FL (Flow Limitation)

Partial closure of the upper airway impeding the flow of air into the lungs.

GERD (Gastroesphageal Reflux Disease)

Flow of stomach acid upwards into the oesophagus that can cause arousals and disrupt sleep.

HST (Home Sleep Study)

A sleep study that can be taken in a person’s own home.


Excessive daytime sleepiness


Sleep-inducing medication.


Shallow breathing, often accompanied by oxygen desaturation.


Decreased rate of breathing.


Difficulty in falling and/or staying asleep.

IPAP (Inspiratory Positive Airway Pressure)

Pressure prescribed for the inspiratory phase of a person on Bi-level CPAP therapy

LAUP (Laser Assisted Uvulopalatoplasty)

Surgery using a laser to make some small cuts in your soft palate to make it firmer and hopefully reduce snoring. 

MAD, MRD or MAS These are all mouth/oral appliances which should be made by a dentist trained in sleep and can be useful for mild sleep apnoea to create more space at the airways.


Sleep episode lasting from a fraction of a second up to 30 seconds – normally as a result of sleep deprivation and sadly responsible for many traffic accidents!


A combination of Central and Obstructive Sleep Apnoea. (Also known as Complex Sleep Apnoea).

MMA (Maxillomandibular Advancement) Surgery to advance the upper and lower jaws forward to create more space to the airways.  A major surgery, so often considered when other surgery or CPAP has failed.

MSLT (Multi Sleep Latency Testing)

Hospital tests via recurrent naps to check for sleep onset and REM, which is useful for identifying narcolepsy and other sleep issues.


A sleep disorder that causes excessive sleepiness and frequent daytime sleep attacks. 

NIV (Non-Invasive Ventilator)

A term used for machines such as BiPAP, VPAP and ASV, when people need more ventilation support for breathing.


Excessive, sometimes frequent, urination during the night.

OSA (Obstructive Sleep Apnoea)

Sleep disorder where the sufferer stops breathing for at least 10 seconds, caused by an obstruction, such as the uvula or tongue blocking the windpipe, soft palate dropping etc.

OSAS (Obstruction Sleep Apnoea Syndrome)

As for OSA (above) although some people refer to OSAS meaning suffering with daytime symptoms.  However, it is rarer to have untreated OSA without daytime symptoms!


An episode of overly shallow breathing or an abnormally low respiratory rate, which is different from an apnoea as there does remain some flow of air.   It is also caused by the same obstructions as OSA.

ODI (Oxygen Desaturation Index)

The number of times per hour oxygen levels dip (desaturate) below the baseline by 3% – often coinciding with apnoeas/hypopnoeas.

PAP (Positive Air Pressure)

A slang term for CPAP, APAP and BiPAP machines.

PMLD (Periodic Limb Movement Disorder) A sleep disorder where the sufferer involuntarily moves their limbs during sleep.

PSG (Polysomnography)

Sleep Study Testing for sleep apnoea and other disorders.

PULSE OXIMETER Medical device which monitors the oxygen saturation and heart rate of the wearer by attaching a probe to the finger or earlobe.
RERA (Respiratory Effort-Related Arousals) Reduction in airflow events which disturb sleep, but do not fulfill  the criteria to be classed as a hypopnea or apnea.
RAINOUT Term used to describe a build-up of condensation/water in the CPAP or NIV hose tube from the humidification.  (A hose cover or heated hose normally fixes this).
RDI (Respiratory Disturbance Index)  

Includes all respiratory events per hour (apnoeas, hypopnoeas and RERA’s).

REM SLEEP (Rapid Eye Movement Sleep)

The deep sleep stage where there is rapid eye movement (hence its name) and where there is loss of body movement, more brain activity and dreaming occurs.  Most apnoeas happen during REM sleep.

RLS (Restless Legs Syndrome)

A tingling, aching, crawling and itching of the limbs which can last for hours when ‘at rest’ and usually gets worse at night, preventing the sufferer from getting the sleep they need. There is an almost irresistible urge to move the legs to ease the discomfort.  During sleep this is called PMLD (as above).


Registered Polysomnography Technologist or ‘Sleep Tech’ for short.

SEPTOPLASTY Surgery to straighten the nasal septum (dividing the nasal passage).  A deviated septum can cause nasal blockage and sinus problems.  Whilst it does not often cure sleep apnoea, it helps to breathe easier in the masks.
SLEEP APNOEA A common sleep disorder caused by the cessation of breathing for 10 or more seconds during sleep, and takes the form of Obstructive Sleep Apnoea, Central Sleep Apnoea or Mixed/Complex Sleep Apnoea.

A build-up (often accumulated over many years) of sleep deprivation, which the body requires to be paid back in order for healing and restoration to take place.  This debt does not usually start to get paid back until the sufferer is on successful therapy.

SLEEP PARALYSIS An experience of waking and not being able to move at all for (usually) a short period of time.  Whilst this is not harmful, it can be very distressing.  It can sometimes be a symptom of sleep apnoea.

Titration carried out during sleep with a CPAP or NIV – adjusting the pressures as needed to eliminate events, and to ensure the patient is on the correct pressures.

TONSILLECTOMY Surgical removal of the tonsils which can alleviate one of the causes of obstructive sleep apnoea.  In children there is a good success rate of a cure from OSA.
TRACHEOTOMY Surgical procedure to create an opening in the trachea (windpipe) to enable breathing (by-passing the upper airways). At present this is the only known complete cure for obstructive sleep apnoea.
UARS  (Upper Airways Resistance Syndrome)  A sleep disorder with and without snoring, causing airflow resistance and RERA’s.  Symptoms are the same as for sleep apnoea, but the AHI index does not fulfil the criteria for OSA.  Narrow airways is common with UARS and sufferers are often of average or slim build.
UPPP (Uvulopalatopharyngoplasty) Surgical procedure to remove the uvula to help improve the cause of OSA.