Sleep Tests: Home Sleep Apnea Testing, Polysomnography & Understanding Your Results

Written and medically reviewed by Kwaku Osafo-Mensah, MD
Pulmonary Medicine | Sleep Medicine | Diplomate, ABSM
Medically reviewed: August 2026

Sleep testing helps determine whether breathing, sleep quality, movements, or other physiologic abnormalities are disrupting sleep.

For suspected obstructive sleep apnea, testing may be performed at home or in an accredited sleep laboratory. These tests are not interchangeable in every situation, and understanding what each test measures can help you make better sense of your results.

This guide explains the major types of sleep studies, common measurements such as AHI and RDI, oxygen levels, sleep stages, and important questions to ask after receiving a sleep-study report.

Why Is a Sleep Study Performed?

A sleep study may be recommended when symptoms or medical history suggest a sleep disorder.

Common reasons include:

Loud habitual snoring
Witnessed pauses in breathing
Gasping or choking during sleep
Excessive daytime sleepiness
Unexplained fatigue
Morning headaches
Resistant or difficult-to-control hypertension
Suspected obstructive or central sleep apnea
Unusual movements or behaviors during sleep
Possible periodic limb movements
Certain forms of insomnia
Evaluation of PAP therapy or other sleep-disorder treatments

The appropriate test depends on the suspected disorder and the individual’s medical circumstances.

The Two Main Tests for Sleep Apnea

Home Sleep Apnea Testing (HSAT)

A Home Sleep Apnea Test, or HSAT, allows selected patients to undergo testing in their own home.

Depending on the device, an HSAT may record signals such as

Airflow
Respiratory effort
Blood oxygen saturation
Pulse or heart rate
Body position
Snoring

Some newer devices measure additional physiologic signals.

HSAT can be convenient and less expensive than laboratory polysomnography, but it has important limitations.

In-Laboratory Polysomnography

Polysomnography (PSG) is a comprehensive overnight sleep study performed in a sleep laboratory.

A typical PSG may record the following:

Brain-wave activity (EEG)
Eye movements
Chin muscle activity
Airflow
Chest and abdominal respiratory effort
Blood oxygen saturation
Heart rhythm
Leg movements
Snoring
Body position
Video and audio when appropriate

Because brain activity is recorded, polysomnography can determine when you are actually asleep and identify different stages of sleep.

HSAT vs. Polysomnography: What’s the Difference?

One of the most important differences is how sleep time is determined.

Most conventional home sleep apnea tests do not measure the same complete set of sleep-stage signals used during laboratory polysomnography.

As a result, some home studies calculate respiratory events using recording or monitoring time rather than precisely measured total sleep time.

If someone spends a substantial portion of the recording awake, the calculated respiratory-event frequency may underestimate the severity of sleep-disordered breathing.

An HSAT can therefore be very useful in appropriately selected patients, but a negative or inconclusive home study does not always exclude clinically important sleep apnea.

When Might an In-Lab Sleep Study Be Preferred?

Laboratory polysomnography may be particularly useful when:

A home study is negative or inconclusive despite significant clinical suspicion
Central sleep apnea is suspected
Significant cardiopulmonary disease complicates interpretation
Hypoventilation or sleep-related hypoxemia is suspected
Neuromuscular disease affects breathing
Other sleep disorders are suspected
Unusual movements or behaviors occur during sleep
Detailed sleep staging is important
PAP titration or more complex treatment assessment is required

The appropriate study should be selected based on the clinical situation rather than convenience alone.

For a detailed comparison of the two testing approaches, see Home Sleep Apnea Test vs. Lab Sleep Study: Which One Do You Need?

Understanding Your Sleep Study

Sleep reports contain many abbreviations and numbers. Several are especially important.

What Is Apnea?

An apnea is a substantial or complete reduction in airflow that meets defined duration and scoring criteria.

Apneas may be classified as the following:

Obstructive Apnea

Airflow stops or markedly decreases while respiratory effort continues.

Central Apnea

Both airflow and respiratory effort are absent or markedly reduced during the event.

Mixed Apnea

An event contains features of both central and obstructive apnea.

What Is a Hypopnea?

A hypopnea is a partial reduction in breathing that meets specific scoring criteria and is associated with an oxygen desaturation and sleep arousal depending on the scoring rule being used.

Differences in scoring criteria are one reason numbers from different sleep studies may not always be directly comparable.

What Is AHI?

AHI stands for Apnea-Hypopnea Index.

It represents the average number of apneas and hypopneas occurring per hour of sleep during polysomnography.

In adults, AHI is commonly categorized approximately as follows:

Less than 5 events/hour: generally within the normal range
5–14.9: mild sleep apnea
15–29.9: moderate sleep apnea
30 or greater: severe sleep apnea

These categories are useful, but AHI should not be interpreted in isolation.

For a more detailed explanation of AHI severity ranges, REM and positional AHI, oxygen levels, RDI, and the limitations of AHI, see AHI Explained: What Your Apnea-Hypopnea Index Means.

What Is REI?

Home sleep apnea testing frequently reports a Respiratory Event Index (REI) rather than a true polysomnographic AHI.

Because many HSAT devices use monitoring or recording time rather than EEG-confirmed sleep time, REI can sometimes be lower than the AHI that might have been measured during polysomnography.

What Is RDI?

RDI stands for Respiratory Disturbance Index.

Its precise definition can vary depending on the testing system and reporting convention.

In some settings, RDI includes apneas, hypopneas, and respiratory effort-related arousals (RERAs).

For this reason, AHI and RDI should not automatically be assumed to represent exactly the same measurement.

For a detailed comparison of these respiratory indices, see AHI vs. RDI vs. REI: What’s the Difference on a Sleep Study?

What Is a RERA?

A Respiratory Effort-Related Arousal is a sequence of breaths characterized by increasing respiratory effort or airflow limitation that leads to an arousal from sleep but does not meet criteria for an apnea or hypopnea.

RERAs can contribute to sleep fragmentation even when the AHI is relatively low.

Understanding Oxygen Levels

Oxygen Saturation

Sleep studies commonly monitor peripheral oxygen saturation, often displayed as SpO₂.

Important measurements may include:

Baseline oxygen saturation
Average oxygen saturation
Lowest recorded saturation
Frequency and depth of desaturations
Amount of time spent below specified oxygen thresholds

A single lowest oxygen value should be interpreted cautiously because artifacts can occasionally produce misleading readings.

Oxygen Desaturation Index

Some reports include an Oxygen Desaturation Index (ODI).

ODI represents the number of oxygen-desaturation episodes per hour using a specified threshold.

ODI and AHI are related but are not interchangeable measurements.

Learn how to interpret oxygen nadir, ODI, T90, and sustained versus intermittent desaturation in Sleep Apnea and Oxygen Levels: How Low Is Too Low?

Understanding Sleep Stages

Laboratory polysomnography divides sleep into several stages.

N1 Sleep

N1 is the lightest stage of non-REM sleep and commonly occurs during the transition from wakefulness to sleep.

N2 Sleep

N2 usually accounts for a substantial portion of adult sleep.

N3 Sleep

N3 is often called slow-wave or deep sleep and is associated with important restorative physiological processes.

REM Sleep

Rapid Eye Movement (REM) sleep is associated with vivid dreaming, characteristic brain activity, and substantial reduction in skeletal-muscle tone.

Obstructive sleep apnea can become more severe during REM sleep in some individuals.

Why Body Position Matters

Some people have substantially more obstructive events while sleeping on their back.

A sleep report may, therefore, provide separate values for the following:

Supine AHI — events while sleeping on the back.

Non-supine AHI—events while sleeping in other positions.

This information can help determine whether sleep apnea has an important positional component.

Why REM AHI Matters

A report may also calculate an AHI specifically during REM sleep.

Someone can have a relatively modest overall AHI but much more significant obstruction during REM sleep.

This phenomenon is another reason the overall AHI does not tell the entire story.

What Is Sleep Efficiency?

Sleep efficiency is the percentage of time in bed or in the recording period that is actually spent asleep, depending on how it is calculated.

Low sleep efficiency can occur with insomnia, discomfort in the laboratory, frequent awakenings, medical conditions, medications, or other factors.

What Does Sleep Latency Mean?

Sleep latency describes how long it takes to fall asleep after the study begins.

Very short or prolonged sleep latency can sometimes provide useful information, although it must be interpreted within the overall clinical context.

What Is a PAP Titration Study?

A PAP titration study evaluates positive airway pressure during sleep.

Pressure is adjusted while respiratory events, oxygen levels, sleep stages, leaks, and other physiologic variables are monitored.

Depending on the clinical situation, titration may involve CPAP, bilevel PAP, or other forms of positive airway pressure.

What Is a Split-Night Study?

During a split-night polysomnogram, the first portion of the night is used to diagnose sleep apnea.

If sufficient criteria are met, the second portion of the same night is when PAP therapy is introduced.

This can sometimes allow diagnosis and treatment assessment during a single laboratory visit.

Questions to Ask About Your Sleep Study

When reviewing your results, useful questions may include:

What type of study did I have?
Was I actually asleep for most of the recording?
What was my AHI or REI?
Was an RDI reported?
Were the events primarily obstructive or central?
Was sleep apnea worse during REM sleep?
Was it worse while sleeping on my back?
How low did my oxygen saturation fall?
How much time did I spend at reduced oxygen levels?
Were there significant heart-rhythm abnormalities?
Were periodic limb movements identified?
Was the test technically adequate?
Could the study have underestimated my sleep apnea?
What treatment options are appropriate for me?
Is additional testing necessary?

A Sleep Study Is More Than One Number

It is tempting to reduce an entire sleep study to the AHI.

But meaningful interpretation may require consideration of the following:

Symptoms + AHI/REI + oxygenation + event type + sleep stage + body position + arousals + medical history + technical quality of the study.

Two people with identical AHIs can have entirely unique symptoms, oxygen patterns, medical risks, and treatment needs.

Where to Go Next

If your study demonstrates sleep apnea, continue with the following:

Sleep Apnea—Understand the condition, symptoms, health implications, and treatment choices.

CPAP & PAP Therapy — Understand CPAP, APAP, bilevel PAP, masks, leaks, humidification, and troubleshooting.

Reviews & Comparisons — practical evaluations of sleep-testing technology, PAP equipment, and related products.

References & Further Reading

Kapur VK, Auckley DH, Chowdhuri S, et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea. Journal of Clinical Sleep Medicine. 2017;13(3):479–504.
Berry RB, Quan SF, Abreu AR, et al. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology, and Technical Specifications. American Academy of Sleep Medicine.
American Academy of Sleep Medicine (AASM). International Classification of Sleep Disorders, Third Edition, Text Revision (ICSD-3-TR). American Academy of Sleep Medicine; 2023.
Gottlieb DJ, Punjabi NM. Diagnosis and management of obstructive sleep apnea: a review. JAMA. 2020;323(14):1389–1400.
https://doi.org/10.1001/jama.2020.3514American Academy of Sleep Medicine. Sleep Education — Sleep Studies. https://sleepeducation.org/patients/sleep-study/

Medical Author & Reviewer
Kwaku Osafo-Mensah, MD
Pulmonary Medicine | Sleep Medicine
Diplomate, American Board of Sleep Medicine (ABSM)
More than 20 years of experience in sleep medicine

Medically reviewed: August 2026
About the medical reviewer · Editorial Policy

Medical Disclaimer: This information is provided for general educational purposes and is not a substitute for individualized medical evaluation, diagnosis, sleep-study interpretation, or treatment. An appropriately qualified healthcare professional should interpret sleep-study results in conjunction with symptoms, medical history, physical findings, and other relevant clinical information.