Understanding Your AHI Score: A Complete Guide

What Is AHI?

AHI stands for Apnea-Hypopnea Index. It's the single most important number in your CPAP therapy. It measures how many breathing disruptions you have per hour of sleep.

Your CPAP machine calculates AHI automatically every night by counting two types of events:

• Apneas: Complete pauses in breathing lasting at least 10 seconds
• Hypopneas: Partial reductions in airflow (at least 30% reduction) lasting at least 10 seconds

The formula is simple:

AHI = (Total Apneas + Total Hypopneas) ÷ Hours of Sleep

For example, if you had 12 events over 6 hours of therapy, your AHI would be 2.0.

How AHI Is Calculated

Your CPAP machine uses its internal sensors to detect two distinct types of breathing disruptions, then combines them into a single number.

Apneas are complete cessations of airflow. When the machine detects that airflow has dropped to less than 10% of your normal breathing level for at least 10 seconds, it records an apnea event. Think of it as your airway closing almost entirely.

Hypopneas are partial reductions. The machine detects that airflow has dropped by at least 30% from baseline for at least 10 seconds, accompanied by a reduction in blood oxygen saturation (in clinical settings) or an arousal. On a CPAP machine, hypopneas are typically scored based on airflow reduction alone, since the machine does not measure oxygen levels.

The machine then divides the total number of these events by the number of hours of recorded therapy time. This is an important distinction: the denominator is hours of actual CPAP use, not total hours in bed. If you used your machine for 5 hours and had 10 events, your AHI is 2.0, even if you were in bed for 8 hours.

It is worth noting that your CPAP machine's AHI is a machine-scored estimate. It may differ slightly from AHI calculated during a clinical sleep study (polysomnography), where technicians use additional data including brain wave activity, oxygen levels, and chest movement. Research published in the Journal of Clinical Sleep Medicine suggests that CPAP-reported AHI generally correlates well with clinical measurements, but small differences are expected.

What's a Good AHI Score?

AHI Range	Classification	What It Means
Under 5	Normal	Your therapy is working well
5 – 15	Mild	Room for improvement. Check mask fit and settings
15 – 30	Moderate	Therapy needs adjustment. Talk to your provider
Over 30	Severe	Contact your sleep physician promptly

Most sleep physicians consider under 5 the target for effective CPAP therapy. Many well-treated patients achieve AHI under 2.

AHI Severity Classifications Explained

AHI Severity Classification

< 5

Normal

Normal~80%

5 - 14

Mild

Mild~13%

15 - 29

Moderate

Moderate~5%

≥ 30

Severe

Severe~2%

AASM classification. Prevalence from Peppard et al. 2013 (Wisconsin Sleep Cohort).

The severity ranges above come from the American Academy of Sleep Medicine (AASM) and are used by sleep physicians worldwide to guide treatment decisions.

Normal (AHI under 5). This is the therapeutic target for CPAP users. An AHI consistently below 5 generally indicates that your therapy is effectively controlling your obstructive sleep apnea. Many optimized patients regularly achieve AHI readings between 0.5 and 2.0. At this level, research suggests your risk of apnea-related health complications is significantly reduced.

Mild (AHI 5 to 14). At this level, you are experiencing some residual breathing disruptions despite therapy. Common causes include mask leak, suboptimal pressure settings, or positional factors. If your treated AHI consistently falls in this range, discuss it with your sleep physician. Adjustments to your mask, pressure, or sleep habits may help bring it into the normal range. An overnight pulse oximeter like the Wellue O2 Ring (opens in new tab) can catch oxygen desaturations your CPAP misses, which is useful evidence to bring to that conversation.

Moderate (AHI 15 to 29). Consistent readings in this range suggest that therapy is not adequately controlling your sleep apnea. This warrants a conversation with your sleep provider about potential causes and solutions. Research indicates that untreated moderate sleep apnea is associated with increased daytime sleepiness, impaired cognitive function, and elevated cardiovascular risk.

Severe (AHI 30 or higher). If your treated AHI is in this range, contact your sleep physician promptly. This level of residual apnea may indicate a significant issue with your equipment, settings, or an underlying condition that requires further evaluation. Untreated severe sleep apnea is associated with serious health consequences, including hypertension and increased cardiovascular risk, according to observational studies.

Types of Events in Your AHI

Not all respiratory events are the same. Understanding the breakdown helps you and your doctor fine-tune treatment.

Obstructive Apneas (OA)

Your airway physically collapses, blocking airflow. This is what CPAP primarily treats. Positive air pressure acts as a pneumatic splint to keep your airway open. If you're having many obstructive apneas, discuss pressure adjustment with your provider.

Central Apneas (CA)

Your brain temporarily stops sending the signal to breathe. These are not caused by airway obstruction, so increasing CPAP pressure won't help (and may worsen them). Central apneas can be related to sleep stage, medications, altitude, or sleep position. For a deeper look at how these two event types differ and what each means for treatment, see our guide to central vs obstructive apnea.

Hypopneas (H)

Partial airflow reduction. Your breathing becomes shallow but doesn't stop completely. These are the most common events for most CPAP users.

RERAs (Respiratory Effort-Related Arousals)

Very subtle breathing disruptions that cause a brief awakening. RERAs are not included in your AHI (they're counted in a separate metric called RDI), but they can still fragment your sleep.

Why Your AHI Changes Night to Night

It's completely normal for AHI to vary. Common factors include:

• Sleep position: Back sleeping typically produces more events
• Alcohol: Even moderate drinking relaxes throat muscles and increases events
• Allergies and congestion: Nasal congestion can increase mouth breathing and leaks
• Mask fit: A leaking mask means the machine can't maintain proper pressure
• Weight changes: Even small weight changes can affect airway anatomy
• Sleep stage: REM sleep naturally produces more airway relaxation

What Affects AHI Accuracy

Your CPAP machine's AHI is a useful approximation, but several factors can make it less accurate on any given night.

Mask leak. When air escapes around the mask seal, the machine's flow sensors lose accuracy. Large leaks can cause the machine to either miss events (because airflow data is noisy) or miscount events (because the leak itself creates airflow patterns that look like breathing disruptions). If your leak rate is consistently elevated, your AHI reading may not reflect your true respiratory status. See our leak fix guide for troubleshooting steps. If you suspect the mask category itself is wrong for your face or sleep position (a common cause of persistent leak), the Mask Finder Quiz takes 30 seconds to recommend a different style.

Sleep position. Sleeping on your back (supine position) typically produces more obstructive events than side sleeping. Research published in the journal Sleep indicates that positional obstructive sleep apnea (where AHI is at least twice as high when supine) affects an estimated 50% or more of OSA patients. A single night on your back might produce an AHI of 8, while a night on your side could be 2.

Alcohol consumption. Alcohol relaxes the muscles of the upper airway, increasing the likelihood and duration of obstructive events. Studies suggest that alcohol consumed within a few hours of bedtime can significantly increase AHI. If you notice AHI spikes after evenings with alcohol, this is likely the cause.

Medication effects. Certain medications, including sedatives, muscle relaxants, and some pain medications, can affect airway muscle tone and breathing patterns. Opioid medications in particular are associated with central apnea events. If you notice AHI changes after starting a new medication, discuss this with your prescribing physician.

Night-to-night variability. Even with identical conditions, AHI varies from night to night. A single-night AHI of 6 does not necessarily mean your therapy has failed. Look at trends over a week or more rather than focusing on individual nights. CPAP Clarity's trend charts help you see this bigger picture.

AHI vs RDI: What's the Difference?

You may encounter another metric called RDI (Respiratory Disturbance Index) on sleep study reports or in discussions with your sleep physician. RDI and AHI measure overlapping but different things.

AHI counts apneas and hypopneas only. This is what your CPAP machine reports.

RDI counts apneas, hypopneas, and RERAs (Respiratory Effort-Related Arousals). RERAs are subtle breathing disruptions that don't meet the full criteria for apnea or hypopnea but still cause brief arousals from sleep. Because RDI includes these additional events, it is always equal to or higher than AHI.

Some sleep labs report RDI instead of (or in addition to) AHI. If your sleep study report shows an RDI, don't compare it directly to your CPAP machine's AHI. They are measuring different things. Your CPAP machine does not detect RERAs, so its AHI will always be lower than a clinically measured RDI.

If your AHI looks good but you still feel excessively tired, RERAs could be a factor. Discuss this possibility with your sleep physician, who can evaluate whether a more detailed sleep study might be helpful.

When to Discuss AHI Changes With Your Doctor

Your AHI data is a tool for informed conversations with your sleep provider, not a self-diagnosis system. Here are specific situations where you should bring your data to your next appointment (or schedule one sooner).

Sudden AHI increase. If your AHI jumps from a typical range of 1 to 2 up to 8 to 10 or higher over several nights, and you have ruled out obvious causes (mask leak, alcohol, illness), contact your sleep provider. A sudden change may indicate equipment issues, weight changes, or other factors that warrant clinical evaluation.

Gradual upward trend. If your AHI has been slowly climbing over weeks or months (for example, from an average of 1.5 to an average of 4.5), this pattern is worth discussing even though individual readings may still be under 5. Gradual changes can reflect weight gain, aging-related airway changes, or equipment wear.

Treatment-related changes. If you recently changed masks, adjusted humidifier settings, or your provider changed your pressure prescription, monitor your AHI closely for the first week or two. Share the before-and-after data with your provider to confirm the changes are working as intended.

AHI consistently above 5. If your treated AHI rarely drops below 5 despite consistent mask use and good seal, your current therapy settings may need optimization. Bring your data to your sleep provider for review.

New central apneas. If your event breakdown shows a new or increasing pattern of central apneas (as opposed to obstructive events), report this to your provider. Central apneas have different causes and may require a different treatment approach. Learn more in our central vs obstructive apnea guide.

How to Improve Your AHI

1. Ensure a good mask seal. This is the #1 factor. If your mask leaks, your therapy is compromised. Replace cushions regularly (opens in new tab) (every 1-3 months).

2. Use your CPAP all night. Therapy is most effective with 7+ hours of use. Taking the mask off at 3am means you're unprotected during late-night REM sleep.

3. Check your pressure settings. If your AHI is consistently above 5, discuss pressure adjustments with your provider. Auto-adjusting (APAP) machines adapt in real-time.

4. Manage nasal congestion. Use your CPAP humidifier, consider saline rinse before bed, and treat allergies.

5. Sleep position. Side sleeping reduces obstructive events for most people.

How to Read Your AHI Data

Your ResMed machine stores detailed AHI data on its SD card. While the machine's screen shows a basic summary, tools like CPAP Clarity let you see the complete picture, including event types, timing, and trends over time.

Analyze your CPAP data for free →

Understanding your AHI is the first step to better sleep therapy. Track it consistently, look for patterns, and work with your provider to optimize your treatment. myAir users can learn more in Beyond myAir. For help interpreting the rest of your session data alongside AHI, see our guide on how to read CPAP data.

Some links in this article are Amazon Associates affiliate links. As an Amazon Associate I earn from qualifying purchases at no extra cost to you. See our full disclaimer.