How to Read a Sleep Study Report Without Getting Lost in the Numbers

First, identify the type of study

An overnight laboratory study, or polysomnography, commonly records brain waves, breathing, oxygen, heart activity, and movements. [1]

A home sleep apnea test usually collects fewer signals and focuses on breathing. Do not expect every report to include measured sleep stages or the same indices.

Also check whether the study was:

• Diagnostic: Looking for a disorder.
• A PAP titration: Assessing treatment pressures.
• Split-night: Combining an initial diagnostic period with treatment testing later in the night.

Numbers collected before treatment and during treatment answer different questions. Ask which part of the night each result describes.

Sleep time and continuity

These terms describe the recorded night. They are not independent diagnoses or targets you must perfect.

For example, six hours of sleep during an eight-hour time-in-bed period gives a sleep efficiency of 75%. That calculation describes continuity; it does not explain why wakefulness occurred.

Tell your clinician if the night was very different from usual. Stage percentages can vary, and one low value does not establish a particular disease.

AHI, REI, and RDI: related, but different

AHI: apnea–hypopnea index

AHI is the number of scored apneas and hypopneas per hour of sleep. Apneas involve a marked interruption of airflow; hypopneas involve reduced airflow meeting specified scoring criteria. [2]

REI: respiratory event index

Many home tests use monitoring time rather than directly measured sleep time. If monitoring includes substantial wakefulness, the resulting REI can underestimate event frequency during actual sleep. [3]

RDI: respiratory disturbance index

Under the AASM definition, RDI includes apneas, hypopneas, and respiratory effort-related arousals, or RERAs, per hour of sleep. A RERA is a breathing disturbance that leads to an arousal without meeting apnea or hypopnea criteria. [4]

Ask which definition your report uses. These indices should not be assumed to mean exactly the same thing.

Why might a report show two AHIs?

Reports may apply different hypopnea scoring rules. One incorporates an arousal or at least a 3% oxygen drop; another requires at least a 4% oxygen drop. The resulting totals can differ. [4]

Ask your clinician to explain both the clinical interpretation and any coverage requirements.

Common adult AHI severity ranges

These ranges help classify findings, but diagnosis and treatment also consider symptoms, event types, and other health information. They are not pediatric thresholds. [2]

Oxygen measurements: look beyond the lowest number

Oxygen saturation, often labeled SpO₂, estimates the percentage of hemoglobin carrying oxygen.

A report may include:

• Mean saturation: The average during the stated period.
• Nadir: The lowest recorded value.
• T90: Time or percentage below 90%; check the units.
• Oxygen desaturation index (ODI): The frequency of drops meeting the report's specified definition.

Ask whether low readings were brief or sustained, associated with breathing events, or affected by a signal problem. A lowest value without its context can be misleading.

Do not start oxygen or change prescribed treatment based on a number in isolation.

Other terms you may encounter

Obstructive, central, and mixed events

These describe different breathing-event patterns. Obstructive events involve airflow blockage despite breathing effort. Central events involve absent breathing effort during the event. Mixed events contain features of both.

A central apnea index is not, by itself, enough to diagnose a central sleep apnea syndrome.

REM-related or positional findings

Breathing-event rates may differ during REM sleep or while lying on your back, also called the supine position. Ask how much time was recorded in each condition.

A high rate from a very brief period needs context, and little time in a particular position can limit what the study reveals.

Arousal index

An arousal is a brief shift in brain activity toward wakefulness. You may not remember it. The arousal index counts these shifts per hour of sleep.

Ask whether arousals were linked to breathing, movements, or had no clear identified trigger. An arousal count is not automatically an explanation for every symptom.

Periodic limb movement index (PLMI)

This describes repeated limb movements per hour of sleep. A separate index may count movements associated with arousals. Movement findings alone do not establish restless legs syndrome or prove that treatment is needed.

Questions for your follow-up

Bring the report and a few examples of your daytime symptoms. Start with these questions:

• What is the main conclusion?
• Was the study technically adequate and representative?
• Which findings best explain my symptoms?
• Were breathing events worse in REM or on my back?
• How significant were the oxygen findings?
• Were movements or arousals clinically important?
• What treatment or follow-up do you recommend?
• What should happen if I remain unrefreshed?

“Could you explain which findings matter most for me, what the study does not answer, and how we will know whether treatment is helping?”

Common questions

Use the numbers to support a conversation

Start with the interpretation, clarify unfamiliar terms, and connect the findings with your symptoms. The most useful outcome is a clear plan for what happens next.

Related: When a Sleep Study Looks Normal but You Still Feel Unrefreshed