Understanding Sleep Apnea

If you've just been told you have sleep apnea — or you suspect it because you snore, wake up gasping, or drag through every afternoon — this guide is your starting point. Sleep apnea is one of the most common sleep disorders in adults, yet it's also one of the most misunderstood, partly because the single number most people fixate on (the AHI) tells only part of the story. Below, you'll find a plain-language map of the whole topic: what sleep apnea is, the different types, the metrics that score it, who's at risk, why untreated apnea matters for your heart, and how it's diagnosed. Each section links to a deeper guide so you can follow the threads that matter most to you.

What Sleep Apnea Actually Is

Sleep apnea is a condition in which your breathing repeatedly stops or becomes shallow while you sleep. Each pause — called an apnea (a full stop) or hypopnea (a partial reduction in airflow) — can last ten seconds or longer, often ending with a brief arousal you never remember. These micro-awakenings fragment your sleep and can starve your blood of oxygen, which is why people with untreated apnea so often feel exhausted despite "sleeping" eight hours.

The disorder isn't all-or-nothing. It ranges from mild to severe, and it comes in distinct forms depending on why the breathing stops. For a foundational walkthrough of the condition, the types, and how severity is graded, start with our overview: What Is Sleep Apnea? Types, AHI, and Severity Explained.

The Types of Sleep Apnea

There isn't one sleep apnea — there are several, and they behave differently both in your body and in your device data.

• Obstructive sleep apnea (OSA) is by far the most common. The airway physically collapses or is blocked, but your brain is still signaling you to breathe — you're trying to inhale against a closed throat.
• Central sleep apnea (CSA) is different: the airway is open, but the brain briefly stops sending the signal to breathe. There's no effort, no struggle — just an absent breath.
• Mixed (or complex) sleep apnea combines both patterns in a single event, often starting central and ending obstructive.

Understanding which type you have shapes everything about treatment. Our guide to obstructive vs central vs mixed sleep apnea breaks down the mechanics and — importantly — what your CPAP device data can and can't tell you about each.

CPAP therapy reliably treats obstructive events because pressurized air splints the collapsing airway open. It does not directly correct the unstable breathing drive behind central events. If "clear airway" (CA) events start showing up or rising on your reports, that's worth understanding rather than panicking over — our explainer on central vs obstructive apnea and what CA events mean walks through it. Sometimes central events appear after a person starts CPAP, a pattern called treatment-emergent central sleep apnea (TECSA), covered in depth in central apneas showing up on CPAP.

The Numbers: AHI, RDI, Hypopnea, and RERA

Sleep apnea is scored, and the scores can be confusing because several metrics measure overlapping things.

The AHI is the number you'll see everywhere. Severity is generally graded as:

• Normal: fewer than 5 events per hour
• Mild: 5 to under 15
• Moderate: 15 to under 30
• Severe: 30 or more

But AHI has blind spots. The RDI adds RERAs to the count, which is why a person can have a "normal" AHI and still have an elevated RDI — and still feel awful. Our guide on AHI vs RDI explains why two numbers exist and when the gap between them matters.

Then there are the events that don't even make it into your AHI. RERAs and flow-limited breaths fragment sleep while flying under the radar of standard scoring — see RERA and flow limitation: the events that don't show in your AHI. When those subtle events dominate without big oxygen drops, the diagnosis may be UARS (Upper Airway Resistance Syndrome), the condition your AHI is most likely to miss entirely.

A note on how hypopneas are scored

Not all AHIs are calculated the same way. Under AASM scoring, Rule 1A (the recommended rule) counts a hypopnea when there's a 30% airflow drop accompanied by either a 3% oxygen desaturation or an arousal. Rule 1B (an acceptable alternative, and the one CMS uses for insurance) requires a 4% desaturation. That difference means the same night can produce a different AHI depending on which rule was applied.

It also explains why your machine's reported AHI won't perfectly match a lab study: your CPAP estimates events from airflow and pressure signals alone — it has no EEG to detect arousals and no pulse oximeter to score desaturations. Device numbers are a useful trend, not a clinical scoring of your brain waves. To learn to read those device numbers confidently, see how to read your CPAP data and CPAP event types decoded.

Symptoms: What Sleep Apnea Feels Like

Many people with sleep apnea never connect their symptoms to their breathing, because the most telling signs happen while they're asleep. Common warning signs include:

• Loud, habitual snoring
• Witnessed pauses in breathing, gasping, or choking during sleep
• Waking unrefreshed no matter how long you slept
• Morning headaches and dry mouth
• Daytime sleepiness, brain fog, and trouble concentrating
• Irritability, mood changes, and waking frequently to urinate

Snoring alone isn't apnea, but snoring with pauses or gasping is a red flag. Our full rundown of the signs you might need a CPAP covers the symptom picture in detail, and when to see a doctor about snoring or suspected sleep apnea helps you decide whether it's time to get evaluated.

Who Gets Sleep Apnea?

Sleep apnea doesn't strike at random. Some risk factors you can influence, and some you can't.

Risk factors you can't change:

• Male sex (though risk in women rises after menopause)
• Older age
• Family history and inherited airway/jaw anatomy
• A naturally narrow airway or large tonsils

Risk factors you can address:

• Excess body weight (the single most modifiable factor)
• Nasal congestion and chronic stuffiness
• Alcohol and sedative use near bedtime
• Smoking

Our guide to sleep apnea risk factors you can and can't change digs into each one and what, realistically, you can do about it.

Why Untreated Sleep Apnea Matters: The Heart Connection

This is the part that turns a nuisance into a medical priority. Sleep apnea is strongly associated with — and in several cases an independent risk factor for — serious cardiovascular and metabolic conditions, including hypertension, coronary artery disease, atrial fibrillation, heart failure, pulmonary hypertension, and stroke. It is also linked to type 2 diabetes.

The link runs deep enough that the 2021 American Heart Association Scientific Statement (Yeghiazarians et al., Circulation) recommends screening for OSA in patients with:

• Resistant or poorly controlled high blood pressure
• Pulmonary hypertension
• Recurrent atrial fibrillation after cardioversion or ablation

The metabolic side matters too. Observational cohort studies indicate that coexisting OSA and type 2 diabetes is associated with a greater (cumulative) risk of adverse cardiovascular outcomes and all-cause mortality than either condition alone — a reason to take both diagnoses seriously together rather than separately.

For the full picture, see untreated sleep apnea risks and the deeper cardiovascular guide, sleep apnea and your heart.

One specific breathing pattern deserves its own flag: Cheyne-Stokes respiration, a rhythmic crescendo-decrescendo central pattern strongly tied to heart failure. If your data shows it, that's a "see your doctor" signal, not a setting to fine-tune — read Cheyne-Stokes respiration and the heart connection, and to recognize it on a chart, periodic breathing and Cheyne-Stokes on your CPAP.

How Sleep Apnea Is Diagnosed

Sleep apnea is diagnosed with a sleep study — and there are two main kinds.

Per the AASM Clinical Practice Guideline for Diagnostic Testing for Adult OSA (Kapur et al., 2017), a home sleep apnea test (HSAT) with a technically adequate device may be used to diagnose OSA in uncomplicated adults who show signs of an increased risk of moderate-to-severe OSA. It's convenient and done in your own bed.

But the guideline recommends in-lab polysomnography (PSG) instead of a home test for patients with:

• Significant cardiorespiratory disease
• Potential respiratory muscle weakness from a neuromuscular condition
• Awake hypoventilation or suspected sleep-related hypoventilation
• Chronic opioid medication use
• A history of stroke
• Severe insomnia

PSG (not a home test) is also required to diagnose non-obstructive sleep-disordered breathing such as central sleep apnea and hypoventilation. And if a home test comes back negative, inconclusive, or technically inadequate, the next step is a lab study. The full comparison — including how an in-lab titration sets your CPAP pressure — lives in sleep study: in-lab PSG vs home test.

After Diagnosis: What "Working" Looks Like

If you start CPAP therapy, the goal is to bring your residual breathing events back into the normal range. A residual AHI below 5 events per hour is the widely used benchmark for effective therapy; the AASM defines an "optimal" titration as reducing the AHI to fewer than 5. Many clinicians aim lower for fuller symptom control when it's comfortably achievable, but there's no formal guideline establishing "below 2" or "below 1" as a recognized standard — your target is individualized and set with your provider.

A few principles will save you a lot of worry as you start out:

• Single nights are noise; trends over weeks matter. One bad night doesn't mean therapy is failing, and one perfect night doesn't prove it's solved. Watch the trend.
• A normal AHI doesn't guarantee good therapy. Large mask leaks can invalidate and under-report your AHI, so a low number with high leaks may be hiding the real picture. Learn the threshold in CPAP leak rate: what's acceptable.
• Use your data to have an informed conversation with your provider — never to self-adjust prescribed pressure. If central events rise on therapy, that calls for clinician evaluation, not more pressure on your own.

If you're still tired despite good numbers, or your AHI looks high, those are common and explainable — see still tired on CPAP with good numbers and why is my AHI high on CPAP.

Where SomniCharts Fits

Once you're tracking therapy, the raw numbers from your machine become a lot more useful when something reads them for you. SomniCharts imports data from ResMed, Philips Respironics (including the encrypted DreamStation 2), and Löwenstein prisma devices, then explains your AHI, leak rate, pressure, and event types in plain language automatically — across vendors, in the cloud, with no desktop software to install. It's built to turn your nightly data into the kind of clear, trend-aware summary you can actually bring to your sleep clinician. When you're ready to read your own night, start with the Reading Your CPAP Data pillar.

Frequently Asked Questions

Is snoring the same as sleep apnea?

No. Snoring is the sound of turbulent airflow through a partly narrowed airway, and many people who snore don't have apnea. But snoring combined with breathing pauses, gasping, or choking — plus daytime sleepiness — is a strong reason to get evaluated.

What's a "good" AHI on CPAP?

A residual AHI below 5 events per hour is the standard benchmark for effective therapy. Some clinicians aim lower when it's comfortably achievable, but goals are individualized with your provider, and a low AHI only counts if your mask leak is also under control.

Can a home sleep test diagnose all types of sleep apnea?

No. Home tests are appropriate for uncomplicated adults at increased risk of moderate-to-severe obstructive sleep apnea. Diagnosing central sleep apnea or hypoventilation — and evaluating patients with significant heart, lung, neuromuscular, or other conditions — requires an in-lab polysomnogram.

Why does my CPAP's AHI differ from my sleep lab's number?

Your machine estimates events from airflow and pressure alone — it has no brain-wave (EEG) or blood-oxygen monitoring, and it can't score arousals or desaturations the way a lab does. Treat your device AHI as a useful trend, not a clinical re-diagnosis.