Sleep-related breathing disorders are conditions in which airflow, breathing effort, ventilation, or blood oxygen becomes abnormal during sleep. Sleep apnea is only part of this category. The International Classification of Sleep Disorders, Third Edition, Text Revision, or ICSD-3-TR, groups the disorders into obstructive sleep apnea, central sleep apnea, sleep-related hypoventilation, and sleep-related hypoxemia 1.
These are not interchangeable labels. A person can snore without apnea, have apnea without obvious snoring, retain carbon dioxide despite a modest apnea count, or have low oxygen for a reason other than apnea. Some people meet criteria for more than one family, and the pattern can change with sleep stage, body position, altitude, medication exposure, illness, or treatment 1.
This page maps the category and the diagnostic logic. For broader apnea basics, see our sleep apnea guide. The focused guides cover obstructive sleep apnea, central sleep apnea, hypoventilation, and obesity hypoventilation syndrome in more detail.
The four major clinical families
A sleep study does more than count pauses. It asks what changed during an event, whether breathing effort continued, how oxygen and carbon dioxide responded, and whether sleep was disrupted.
| Clinical family | What is happening | Measurements that help distinguish it |
|---|---|---|
| Obstructive sleep apnea (OSA) | The upper airway narrows or closes while the person continues trying to breathe. | Airflow falls while chest and abdominal effort continues; the study also records oxygen changes and sleep disruption. |
| Central sleep apnea (CSA) | Airflow falls because breathing effort is reduced or absent, rather than because the upper airway alone is blocked. | Airflow and respiratory-effort signals fall together; the event pattern and medical context help identify the cause. |
| Sleep-related hypoventilation | Ventilation is inadequate for a sustained period, causing carbon dioxide to rise during sleep. | Transcutaneous or end-tidal carbon dioxide during sleep, sometimes paired with an awake blood gas; oxygen may also fall. |
| Sleep-related hypoxemia | Oxygen remains abnormally low during sleep without enough carbon dioxide elevation to classify the problem as hypoventilation. | Sustained oxygen saturation plus carbon dioxide assessment and evaluation for lung, heart, vascular, altitude, or other causes. |
The obstructive and central distinction rests heavily on respiratory effort. The hypoventilation and hypoxemia distinction rests on carbon dioxide. Oximetry alone cannot make either distinction 12.
Obstructive sleep apnea
In OSA, airflow is reduced or stops while the chest and abdomen show continued breathing effort. A study can score obstructive apneas, hypopneas, and related arousals, then place them in the context of actual sleep time, oxygen change, sleep stage, and body position.
OSA can occur in adults and children, but the presentation and scoring criteria differ. Adult thresholds should not be applied to a young child, and a child's snoring should not be classified with an adult consumer sleep test.
Central sleep apnea
In CSA, reduced airflow occurs with reduced or absent respiratory effort. Brief central events can occur during the transition into sleep, after ascent to altitude, or when ventilation changes during positive airway pressure treatment. A central event on a report is therefore not automatically a central sleep apnea disorder 1.
Clinicians look at the central event burden, symptoms, breathing pattern, and cause. Relevant settings include heart failure with Cheyne-Stokes breathing, recent altitude exposure, opioids or other respiratory depressants, neurological disease, treatment-emergent central apnea, and less common primary forms.
Sleep-related hypoventilation
Hypoventilation is inadequate effective ventilation. Carbon dioxide rises because the lungs are not clearing it sufficiently. This is a sustained gas-exchange problem, not simply a high apnea-hypopnea index.
Possible settings include obesity hypoventilation syndrome, neuromuscular weakness, chest-wall restriction, lung disease, impaired central breathing control, and medication or substance effects. Awake carbon dioxide can be normal in some sleep-related hypoventilation disorders. Obesity hypoventilation syndrome is different because its definition requires awake daytime hypercapnia after other causes are considered 13.
Sleep-related hypoxemia
Sleep-related hypoxemia means sustained low oxyhemoglobin saturation during sleep without hypercapnia that explains the pattern. Causes can include ventilation-perfusion mismatch in lung disease, shunting, pulmonary vascular or cardiac disease, low inspired oxygen at altitude, or a combination of mechanisms 1.
A person can have both hypoventilation and hypoxemia. The distinction matters because supplemental oxygen may improve saturation without correcting airway obstruction or inadequate ventilation.
What a diagnostic study measures
The AASM scoring manual supplies standardized rules for sleep stages, arousals, airflow events, respiratory effort, oxygen changes, carbon dioxide patterns, and adult versus pediatric scoring. It also distinguishes an apnea-hypopnea index, or AHI, calculated per hour of measured sleep from the respiratory event index, or REI, commonly reported by home testing when sleep itself may not be measured in the same way 4.
An attended polysomnogram can include:
- brain, eye, and chin-muscle signals to identify sleep and arousals;
- nasal and oral airflow;
- chest and abdominal effort belts;
- pulse oximetry;
- heart rhythm and body position;
- leg-muscle signals and video; and
- transcutaneous or end-tidal carbon dioxide when hypoventilation is a concern.
Not every study includes every channel. The test needs to match the clinical question. The National Heart, Lung, and Blood Institute notes that sleep studies can determine which type of sleep apnea is present and how severe it is, while other testing may be needed to identify the medical cause 5.
AHI, oxygen, and carbon dioxide answer different questions
The AHI counts scored apneas and hypopneas per hour of sleep. A central apnea index isolates scored central apneas. Oxygen metrics describe desaturation depth, duration, or frequency. Carbon dioxide measurements assess ventilation.
None of those numbers replaces the others. Two people with the same AHI may have different event types, oxygen exposure, sleep disruption, symptoms, or underlying disease. A low event count also does not rule out sustained hypoventilation or hypoxemia.
Why symptoms and home devices cannot classify the disorder
Loud snoring, witnessed pauses, gasping, morning headache, unrefreshing sleep, insomnia symptoms, and daytime sleepiness can justify evaluation. They cannot show whether a recorded event is obstructive or central, whether carbon dioxide rises, or why oxygen is low.
Snoring indicates vibration in the upper airway. It does not measure airflow cessation, effort, sleep state, or gas exchange. A bed partner may also miss quiet events. In children, habitual snoring plus other concerning features warrants pediatric evaluation, but objective testing is needed to distinguish primary snoring from OSA 6.
Overnight oximetry
Pulse oximetry estimates oxygen saturation and pulse rate. It does not measure airflow, respiratory effort, sleep stages, arousals, or carbon dioxide. Repetitive dips may raise suspicion for apnea, while sustained low values may suggest lung disease, altitude effects, hypoventilation, or hypoxemia. A normal-looking tracing can miss events that cause arousal without a large oxygen drop.
Readings also have technical limits. The U.S. Food and Drug Administration lists poor circulation, skin pigmentation, skin temperature, tobacco use, nail polish, and other factors that can affect accuracy. Many general-wellness oximeters have not been evaluated for clinical decision-making 7.
Consumer wearables
A watch, ring, phone, or mattress sensor may estimate oxygen, pulse, movement, breathing rate, sound, or sleep. Even a device that issues a sleep-apnea notification is a screening tool for a defined use, not a full classification of OSA, CSA, hypoventilation, or hypoxemia 1.
Bring a repeated alert or concerning trend to a clinician, along with the device name and report. Do not treat the absence of an alert as proof that breathing is normal, and do not use a wearable result to select oxygen or a PAP mode.
How medical context changes the test
The AASM allows either polysomnography or a technically adequate home sleep apnea test for certain uncomplicated adults who have signs suggesting moderate to severe OSA. If a home test is negative, inconclusive, or technically inadequate while concern remains, polysomnography is recommended 8.
Polysomnography is preferred over routine home testing when the differential is broader. The AASM specifically identifies significant cardiorespiratory disease, possible respiratory muscle weakness from a neuromuscular condition, awake hypoventilation or suspected sleep-related hypoventilation, chronic opioid use, previous stroke, and severe insomnia 8.
Heart and lung disease
Heart failure can be associated with obstructive apnea, central apnea with Cheyne-Stokes breathing, or both. Chronic obstructive pulmonary disease, interstitial lung disease, pulmonary vascular disease, and other cardiopulmonary conditions can produce sustained hypoxemia, hypoventilation, or events that resemble OSA on limited channels.
Testing may therefore combine polysomnography with carbon dioxide monitoring, blood gas assessment, lung-function testing, cardiac evaluation, or other cause-specific studies. A home oximeter cannot decide which of these is needed.
Neuromuscular or chest-wall weakness
Respiratory muscle weakness can first become more apparent during sleep, particularly when breathing depends more heavily on the diaphragm. The diagnostic question is not only whether apneas occur, but whether ventilation falls and carbon dioxide rises. A laboratory study with carbon dioxide monitoring and separate respiratory assessment is often more informative than a routine OSA home test 82.
Opioids and other respiratory depressants
Chronic opioid use raises concern for central apnea and sleep-related hypoventilation as well as OSA. Sedatives, alcohol, and other substances can further alter breathing and arousal. Tell the clinician exactly what is taken, the dose, and the timing. Do not stop a prescribed opioid, benzodiazepine, or other dependence-forming medicine abruptly without medical guidance.
If a person using opioids cannot be awakened, has slow or shallow breathing, makes choking or gurgling sounds, or develops discolored lips or nails, treat the situation as a possible overdose. Call local emergency services, give naloxone if available, try to keep the person breathing, place them on their side if appropriate, and stay until help arrives 9.
Obesity
Obesity increases OSA risk, but obesity plus sleepiness or low overnight oxygen does not establish obesity hypoventilation syndrome. OHS requires obesity, sleep-disordered breathing, and awake hypercapnia after excluding other causes.
The American Thoracic Society advises direct arterial blood gas measurement when clinical suspicion is high. In lower-probability situations, serum bicarbonate can help decide who needs an arterial measurement, but awake oxygen saturation alone is not an established screening substitute 3.
Altitude
Recent ascent can lower inspired oxygen and provoke periodic breathing or central apneas. The sleeping altitude, usual residence, timing of ascent, symptoms, and whether the pattern improves with acclimatization or descent all affect interpretation. A sea-level OSA result should not automatically be used to explain a new high-altitude pattern 15.
Children and infants
Children require pediatric history, sensors, scoring rules, and interpretation. Habitual snoring, labored breathing, pauses, restless sleep, bedwetting, morning headaches, or attention and behavior changes can prompt evaluation. The AAP recommends polysomnography or specialist referral when a child regularly snores and has other OSA features 6.
Infants have additional developmental, airway, neurological, and feeding considerations. Use the focused guides on children and sleep apnea and sleep apnea in infants and newborns rather than adult thresholds or a consumer monitor.
Treatment follows the mechanism
The category name does not identify one treatment. Detailed treatment decisions belong in the disorder-specific guides and with the treating sleep, pulmonary, cardiac, neurological, pediatric, or ear, nose, and throat team.
- OSA: CPAP or APAP, an appropriately fitted oral appliance, surgery, positional therapy, and weight-directed care can each have a role in selected people. See sleep apnea treatment for the treatment map.
- CSA: The underlying cause and central-apnea subtype drive treatment. The 2025 AASM guideline makes conditional, cause-specific recommendations involving CPAP, bilevel positive airway pressure with a backup rate, adaptive servo-ventilation, low-flow oxygen, acetazolamide, or transvenous phrenic nerve stimulation. It advises against bilevel therapy without a backup rate for several CSA causes because it can worsen central breathing instability 10.
- Hypoventilation: Care may require treatment of the medical or medication cause and ventilatory support that assists breathing and controls carbon dioxide. In stable ambulatory OHS with severe coexisting OSA, CPAP may be first-line; other OHS and hypoventilation presentations may need noninvasive ventilation 3.
- Hypoxemia: Treatment targets the cause. Prescribed oxygen may be appropriate in some settings, but it does not splint an obstructed airway, provide respiratory effort, or remove carbon dioxide.
CPAP, bilevel PAP with or without a backup rate, adaptive servo-ventilation, volume-assured modes, supplemental oxygen, and invasive ventilation perform different jobs. Their indications, contraindications, settings, and monitoring differ. Do not borrow equipment, add oxygen to PAP, change ventilator modes, or select settings without the prescribing clinical team.
When to seek care
Arrange a medical evaluation for repeated snoring with pauses or gasping, witnessed absent breathing effort, persistent low overnight oxygen readings, morning headaches with breathing symptoms, unexplained excessive sleepiness, or symptoms arising after a medication change or altitude exposure.
Do not drive or operate dangerous equipment when struggling to stay awake. Stop in a safe place and arrange another way home if sleepiness develops while driving.
Call emergency services for severe or rapidly worsening breathing difficulty while awake, blue or gray lips or face, new confusion or unresponsiveness, chest pain with breathing distress, or a suspected opioid or sedative overdose. A sleep-clinic appointment is not the right route for an acute breathing emergency.
The bottom line
Sleep-related breathing disorders are separated by mechanism, not by one symptom or one oxygen number. Airflow and effort distinguish obstructive from central events. Carbon dioxide distinguishes hypoventilation from isolated hypoxemia. Sleep signals show whether events occur during sleep and whether they cause arousal.
The safest next step is a test chosen for the full clinical context. That is especially important with heart or lung disease, neuromuscular weakness, obesity with possible awake hypercapnia, opioid exposure, altitude, or childhood symptoms. Once the mechanism is clear, treatment can be matched to the disorder instead of guessed from snoring, a wearable alert, or an oximeter trace.
