Plain-language sleep reference
Sleep Apnea, Sleep Test and CPAP Glossary
Understand the terms used in sleep-health articles, home sleep-test reports, CPAP prescriptions and equipment discussions—then follow the links for the clinical context behind each definition.
Definitions are a starting point—not an interpretation
The same term can be calculated differently by a laboratory study, home test or CPAP device. A number also means something different depending on recording quality, symptoms, health history, treatment use and the question the test was intended to answer.
Use this glossary to understand the language, not to diagnose yourself or change treatment. Ask the professional responsible for the report or prescription which definition, denominator, threshold and device rules apply in your situation.
A
- Adherence
- How consistently a treatment is used, often described with days and hours of CPAP use. Adherence is important, but use time alone does not prove that therapy is effective or comfortable.
- AHI (apnea–hypopnea index)
- The average number of apneas and hypopneas per hour of sleep in a study that measures sleep time. Severity categories provide context but do not replace symptoms, oxygen information, health history or clinical interpretation. See Understanding AHI and Sleep Study Results.
- Apnea
- A temporary pause or near-complete reduction in airflow that meets defined duration and scoring criteria. An obstructive apnea occurs despite breathing effort; a central apnea occurs with reduced or absent effort.
- APAP (auto-adjusting positive airway pressure)
- A PAP mode that varies pressure within a prescribed range in response to detected breathing patterns. It is not automatically appropriate for every person, and the range is a prescription—not permission to self-adjust.
- Arousal
- A brief shift toward lighter sleep or wakefulness. Arousals may follow breathing events, movement, noise or other causes, and a person may not remember them.
B
- Bilevel PAP (BPAP or BiPAP)
- Positive airway pressure that provides a higher pressure during inhalation and a lower pressure during exhalation. “BiPAP” is also used as a brand term. Bilevel therapy is selected for defined clinical needs, not simply because it has more settings.
- Body-position data
- Information showing whether a person was on their back, side, stomach or upright during recorded periods. Position can help explain patterns, but a sensor estimate and one night may not represent usual sleep.
C
- Central sleep apnea (CSA)
- Repeated pauses in breathing associated with reduced or absent respiratory effort because the brain’s breathing-control signals temporarily decrease. Its causes, testing and treatment differ from obstructive sleep apnea.
- Circadian rhythm
- An approximately 24-hour biological timing system influenced strongly by light and darkness. It helps regulate sleep timing, alertness, hormones and other body functions. See Understanding Your Body Clock.
- cmH2O
- Centimetres of water pressure, the unit commonly used for PAP pressure. A CPAP setting such as 10 cmH2O describes delivered pressure—not sleep apnea severity.
- CPAP (continuous positive airway pressure)
- PAP therapy that delivers one prescribed pressure to help keep the upper airway open during sleep. See How CPAP Works.
- Compliance
- A payer, program or supplier term sometimes used for documented CPAP use. It may have a specific administrative threshold. Clinical benefit and a person’s treatment experience are broader than compliance rules.
E
- Excessive daytime sleepiness
- Difficulty maintaining expected wakefulness or an increased tendency to doze during the day. It is different from feeling low in energy and can have many causes. Sleepiness affecting driving or safety-sensitive work needs prompt attention.
- Expiratory pressure relief
- A comfort feature that reduces pressure during exhalation on some PAP devices. Names and behaviour vary by manufacturer; it should be used according to the prescribed setup and device-specific guidance.
H
- Home sleep apnea test (HSAT)
- A limited sleep-related breathing study completed outside a laboratory, usually at home, for appropriately selected adults with suspected obstructive sleep apnea. It does not measure every sleep disorder. See Home Sleep Test in Canada.
- Humidification
- Moisture added to PAP airflow by an integrated or compatible humidifier. It may improve some dryness or congestion, but leak, mouth breathing, room conditions and nasal symptoms can also contribute.
- Hypopnea
- A partial reduction in airflow associated with a qualifying oxygen change, arousal or other criterion under the scoring rules used. Definitions can vary, so ask which criteria apply to the report.
I
- Insomnia
- Persistent difficulty falling asleep, staying asleep or obtaining restorative sleep despite adequate opportunity, with meaningful daytime effects. Insomnia and sleep apnea can occur together.
- Interface
- The mask or other patient-contact component that delivers PAP airflow. Common CPAP interfaces include nasal pillows, nasal masks and full-face masks.
L
- Leak
- Air escaping intentionally through a mask vent or unintentionally around the cushion, through the mouth or from a connection. Device leak values may report total or unintentional leak differently. See CPAP Mask Leaks.
- Light sleep
- A plain-language description often used for non-REM stages N1 and N2. Consumer devices may estimate sleep stages, but those estimates are not equivalent to laboratory scoring.
O
- Obstructive sleep apnea (OSA)
- Repeated narrowing or closure of the upper airway during sleep despite continued breathing effort. OSA is the form of sleep apnea most commonly evaluated with a home sleep apnea test in suitable adults.
- Oxygen desaturation
- A decrease in blood oxygen saturation from a preceding level. The size, duration, frequency and clinical context matter; one lowest value should not be interpreted alone.
- Oxygen desaturation index (ODI)
- The average number of qualifying oxygen drops per hour, based on the definition and time denominator used by the report. ODI and AHI or REI describe related but different measurements.
P
- PAP (positive airway pressure)
- The treatment family that uses pressurized air to support breathing during sleep. CPAP, APAP and bilevel PAP are different PAP modes.
- Polysomnography (PSG)
- A laboratory sleep study that records sleep stages and multiple body signals, commonly including breathing, oxygen, heart rhythm and limb movement. It provides more information than a typical home sleep apnea test.
- Prescribed pressure or pressure range
- The CPAP pressure, APAP range or bilevel settings selected for an individual treatment plan. Pressure is measured in cmH2O and should not be changed independently.
- Pulse oximetry
- A non-invasive estimate of blood oxygen saturation and pulse. Movement, poor sensor contact, circulation, nail products and other factors can affect the signal.
R
- Ramp
- A PAP comfort feature that begins at a lower pressure or delays the full treatment pressure while a person settles. Device behaviour varies, and ramp is not the same as the prescribed therapy setting.
- Recording time
- The time a home-test device collects usable data. It may include time awake and can be longer than actual sleep time.
- REI (respiratory event index)
- The average number of scored respiratory events per hour of recording or monitoring time, commonly used in home testing when sleep time is not directly measured. It may underestimate event frequency if a person is awake for substantial recorded time.
- Residual AHI or residual events
- A PAP device estimate of breathing events that remain during use. Manufacturer algorithms differ, and this value is not identical to a diagnostic AHI measured in a sleep laboratory.
S
- Sleep architecture
- The pattern and sequence of non-REM and REM sleep stages across the night. Alcohol, medicines, sleep disorders, age and many other factors can influence it.
- Sleep efficiency
- The percentage of time in bed that is spent asleep. It is one part of sleep assessment and should not be treated as a universal performance score.
- Sleep hygiene
- Habits and environmental practices that support sleep, such as consistent timing, appropriate light exposure and a suitable bedroom. Sleep hygiene does not treat every sleep disorder. See Sleep Hygiene.
- Sleep inertia
- Temporary grogginess, slower thinking or reduced performance after waking, particularly from deeper sleep or at an unfavourable circadian time.
- Sleep latency
- The time from attempting to sleep to sleep onset. How it is measured differs between personal estimates, wearables and polysomnography.
- Sleep study
- A general term for testing that records sleep or sleep-related signals. A laboratory polysomnogram and a home sleep apnea test are not interchangeable.
- Snoring
- Sound created by vibration of tissues as air moves through a narrowed upper airway. Snoring can occur without sleep apnea, and sleep apnea can occur without prominent snoring.
T
- Titration
- A process used to determine or evaluate PAP settings that adequately support breathing while considering sleep, oxygen, events and comfort. It may occur in a laboratory or through another clinician-directed pathway.
- Treatment-emergent central sleep apnea
- Central breathing events that appear or persist when obstructive sleep apnea is treated with PAP. It requires professional assessment and should not be managed by self-adjusting pressure.
How to read an unfamiliar report term
- Write down the exact term, abbreviation, value and unit.
- Check whether it came from a laboratory study, home test, PAP device or consumer wearable.
- Identify the denominator: sleep time, recording time, usage time or another period.
- Review technical-quality notes and the definition used by that laboratory or manufacturer.
- Ask how the value fits your symptoms, health history and the reason for testing or treatment.
- Confirm what action, if any, the responsible professional recommends.
Continue learning
Put the definition back into context
Use the linked guide for the full explanation, then ask the responsible professional how the term applies to your report, prescription or equipment.
Medical disclaimer
This information is educational and does not replace assessment, diagnosis, prescribing, emergency care or individualized advice from a licensed healthcare professional.