`n How to Read Your Sleep Study Results: Every Metric Explained by an RRT
You did the sleep study. The report arrived. Now you’re staring at a page full of numbers — AHI, RDI, SpO₂ nadir, NREM, REM, arousal index, sleep efficiency — and you have no idea what any of it means or how worried you should be. Your follow-up appointment is two weeks away and you need answers now.
As a licensed Registered Respiratory Therapist with ICU and critical care experience who reads sleep study data clinically, here is a complete guide to every metric on your sleep study report — what it measures, what the numbers mean, and what they tell you about your next steps.
The Two Types of Sleep Studies: What Each Measures
Before interpreting your results, know which study you had — because they measure different things and have different limitations.
| Study Type | Setting | What It Measures | Best For |
|---|---|---|---|
| Polysomnography (PSG) | In-lab, overnight | Full sleep architecture (brain waves, eye movements, muscle activity), respiratory events, SpO₂, heart rhythm, leg movements, body position | Complex cases; children; suspected central apnea; split-night titration |
| Home Sleep Apnea Test (HSAT) | Your own bed | Respiratory events, airflow, SpO₂, heart rate, body position (varies by device); does NOT measure sleep stages | Adults with straightforward suspected OSA; most common first-line test today |
This distinction matters for one key reason: home sleep tests calculate AHI based on recording time, not actual sleep time. This means home test AHI can be lower than in-lab AHI for the same patient — if you slept 5 of the 8 hours the device was recording, events during the 3 waking hours are included in the denominator, artificially lowering the calculated AHI. If your home test shows borderline or mild OSA but your symptoms are significant, this is worth knowing. For the complete comparison, see our guide on home sleep test vs in-lab sleep study.
AHI — The Most Important Number on Your Report
AHI stands for Apnea-Hypopnea Index — the total number of apneas and hypopneas divided by hours of sleep (or recording time on a home test). It is the primary diagnostic metric for sleep apnea severity.
What Counts as an Event
- Apnea: Complete cessation of airflow for ≥10 seconds. Obstructive apneas occur when the airway collapses; central apneas occur when the brain stops sending the breathing signal.
- Hypopnea: Partial reduction in airflow (typically ≥30% reduction) for ≥10 seconds, accompanied by either an oxygen desaturation of ≥3% or an arousal.
AHI Severity Scale — Adult Thresholds
| AHI | Classification | Clinical Significance |
|---|---|---|
| < 5 | Normal | No sleep apnea diagnosis; symptoms may have another cause |
| 5 – 14.9 | Mild OSA | Treatment recommended if symptomatic; lifestyle and positional therapy may be sufficient; CPAP or oral appliance both appropriate |
| 15 – 29.9 | Moderate OSA | Treatment strongly recommended; CPAP preferred; significant cardiovascular and neurocognitive risk if untreated |
| ≥ 30 | Severe OSA | Treatment urgently recommended; CPAP strongly preferred; high cardiovascular risk; driving safety implications |
RDI — Why It’s Sometimes Higher Than AHI
RDI stands for Respiratory Disturbance Index. It includes everything in the AHI plus respiratory effort-related arousals (RERAs) — brief arousals from partial upper airway obstruction that don’t meet the threshold for a scored hypopnea but still fragment sleep and cause symptoms.
RDI is always ≥ AHI. A patient with an AHI of 8 and an RDI of 18 has significant sub-hypopnea flow limitation causing frequent arousals that the AHI alone understates. This is particularly relevant in women, who have proportionally more RERAs and hypopneas relative to full apneas — and can have significant symptoms with relatively modest AHI values that don’t capture the full burden of sleep-disordered breathing. For the full picture of how OSA presents differently in women, see our guide on sleep apnea and CPAP therapy for women.
Oxygen Saturation Metrics
Your sleep study measures SpO₂ (blood oxygen saturation) continuously throughout the night. The key metrics reported:
| Metric | What It Means | Normal / Concerning Values |
|---|---|---|
| Baseline SpO₂ | Your oxygen saturation at rest before sleep | Normal: 95–100%; Below 95% warrants attention |
| SpO₂ nadir | The lowest oxygen saturation recorded during the entire night | Above 90%: mild desaturation; 80–90%: moderate; Below 80%: severe — significant hypoxic burden |
| % time below 90% (T90) | Percentage of total sleep time spent with SpO₂ below 90% | Under 1%: minimal; 1–5%: moderate; Above 5%: significant nocturnal hypoxemia |
| Oxygen desaturation index (ODI) | Number of times SpO₂ drops by ≥4% per hour | Closely tracks AHI; high ODI with low AHI may indicate HSAT underestimation |
| Mean SpO₂ during sleep | Average saturation across the sleep period | Should be above 95%; persistently below 92% during sleep suggests significant gas exchange impairment |
The SpO₂ nadir is clinically important beyond AHI alone — a patient with moderate AHI but a nadir of 72% is experiencing significantly more cardiovascular stress per event than a patient with the same AHI and a nadir of 88%. Deep desaturations drive the cardiovascular consequences of OSA more directly than event count alone. For a patient-focused guide to understanding oxygen saturation numbers, see our guide on SpO₂ levels — what’s normal and when to call your doctor.
Sleep Architecture Metrics (PSG Only)
In-lab polysomnography measures your brain waves (EEG) and categorizes every moment of the night into sleep stages. Home sleep tests do not provide this data.
| Metric | What It Measures | Normal Range | Clinical Significance |
|---|---|---|---|
| Sleep efficiency | % of time in bed actually spent asleep | 85–90%+ is normal | Below 80% suggests insomnia, frequent arousals, or poor sleep consolidation |
| Total sleep time (TST) | Total minutes of actual sleep | 6–8.5 hours typical for adults | Low TST in a lab may reflect first-night effect or severe sleep fragmentation |
| Sleep latency | Minutes from lights out to first sleep onset | 10–20 minutes normal | Under 5 minutes suggests severe sleep deprivation; over 30 minutes suggests insomnia |
| REM latency | Minutes from sleep onset to first REM period | 70–120 minutes typical | Very short REM latency (<20 min) may suggest narcolepsy; very long may indicate REM suppression |
| N1 sleep % | % time in light, transitional sleep | 2–5% | Elevated N1 reflects excessive sleep fragmentation and frequent arousals |
| N2 sleep % | % time in baseline sleep stage | 45–60% | The largest sleep stage in most adults |
| N3 (slow-wave/deep) sleep % | % time in deep restorative sleep | 15–25% in younger adults; decreases with age | Reduced N3 correlates with non-restorative sleep; OSA disproportionately disrupts this stage |
| REM sleep % | % time in REM (dreaming) sleep | 20–25% | OSA events cluster in REM; reduced REM % from fragmentation impairs emotional and memory processing |
| Arousal index | Number of brief arousals per hour of sleep | Under 15/hr normal | High arousal index confirms sleep fragmentation driving non-restorative symptoms even if AHI is borderline |
Obstructive vs. Central Events: Why the Distinction Matters
Your report will categorize apnea events by type. This distinction is clinically critical — the treatments are different:
Obstructive Apnea (OA)
Airway physically collapses during the event. Respiratory effort continues — you’re trying to breathe but the airway is blocked. Treated with CPAP, oral appliance, or airway surgery.
Central Apnea (CA)
Brainstem fails to send the breathing signal. No respiratory effort during the event — airway is open but breathing stops. NOT treated by standard CPAP; requires adaptive servo-ventilation (ASV) or BiPAP with backup rate. For the complete comparison, see our guide on central vs obstructive sleep apnea.
Most patients have predominantly obstructive events. A central apnea index (CAI) above 5 or central events comprising more than 50% of total events warrants specific clinical attention and changes the treatment approach entirely. If your report shows significant central events, ensure your sleep physician has specifically addressed this before prescribing a CPAP device.
Positional Data: The Supine Effect
Most sleep study reports include a breakdown of AHI by body position — supine (back) versus non-supine (side or prone). This is clinically useful because many patients have predominantly positional OSA.
| Pattern | Definition | Clinical Implication |
|---|---|---|
| Positional OSA | Supine AHI ≥2x non-supine AHI; both are abnormal | Positional therapy (side sleeping) can meaningfully reduce AHI; CPAP pressure may be lower in non-supine positions |
| Isolated positional OSA | Supine AHI ≥5 with normal non-supine AHI (<5) | Positional therapy may be sufficient treatment without CPAP; discuss with physician |
| Non-positional OSA | AHI elevated in all positions | Positional measures are helpful adjuncts but CPAP or appliance therapy is needed |
Other Metrics You May See
| Metric | What It Means |
|---|---|
| PLMS / PLMI | Periodic Limb Movements during Sleep / Index — repetitive leg movements that fragment sleep; index above 15 with arousals is clinically significant; treated separately from OSA |
| REM AHI vs NREM AHI | OSA is almost always worse in REM sleep; a very high REM AHI with lower NREM AHI is typical; REM-predominant OSA may require higher CPAP pressure to control the worst events |
| Flow limitation | Partial airflow reduction without meeting hypopnea criteria; contributes to RDI and arousal index; relevant for UARS (upper airway resistance syndrome) |
| Snoring index | Frequency and intensity of snoring events; high snoring index with borderline AHI may support treatment decision |
| Heart rate data | Nocturnal heart rate pattern; arrhythmias occurring during apnea events are noted; atrial fibrillation in OSA context is a significant cardiovascular finding |
Reading Your Results: A Practical Decision Framework
What Your Results Mean for Next Steps
| Your AHI | Typical Next Step |
|---|---|
| AHI < 5 | No OSA diagnosis; follow up with physician about alternative causes for your symptoms |
| AHI 5–14 with no symptoms | Watchful waiting may be appropriate; lifestyle modifications; discuss with physician |
| AHI 5–14 with symptoms (fatigue, headaches, cognitive effects) | Treatment recommended; CPAP or oral appliance both appropriate; see our guide on oral appliance vs CPAP |
| AHI 15–29 | CPAP strongly recommended as first-line therapy; oral appliance acceptable alternative if CPAP not tolerated |
| AHI ≥ 30 | CPAP urgently recommended; oral appliance generally insufficient at this severity |
| Significant central events | Standard CPAP is not appropriate; ASV or bilevel with backup rate required; must be specifically addressed by sleep physician |
Frequently Asked Questions
My AHI is 6 but I feel terrible. Should I be treated?
Yes, in most cases. OSA treatment guidelines consider both AHI and symptom burden. An AHI of 6 with significant daytime fatigue, unrefreshing sleep, morning headaches, and cognitive difficulty warrants treatment — particularly if your RDI is higher than your AHI, suggesting RERAs are contributing to the arousal burden. Don’t let a “mild” AHI label dismiss symptoms that are meaningfully affecting your quality of life. Discuss your specific symptom profile with your physician; symptom burden is explicitly part of treatment decision guidelines.
My home test AHI was 9. My doctor ordered an in-lab study. Why?
Several legitimate reasons: home tests underestimate AHI in some patients (recording time vs. sleep time denominator); your symptoms may suggest more severe disease than the home test captured; your doctor may suspect central apnea components that home tests cannot characterize; or your results are borderline and an in-lab study provides more complete data. In-lab PSG remains the diagnostic gold standard and is appropriate in any case where the clinical picture doesn’t fully match the home test results.
My report mentions “mixed apneas.” What does that mean?
A mixed apnea begins as a central apnea (no respiratory effort) and ends as an obstructive apnea (effort resumes but airway is obstructed). They are common in patients with moderate-to-severe OSA and generally treated as obstructive events with CPAP. A high proportion of mixed events may warrant attention to whether pressure-induced central events are contributing — discuss the event breakdown with your sleep physician.
What is a good AHI on CPAP therapy?
Below 5 on therapy is the clinical target. Below 2 is excellent. AHI 5–10 on therapy warrants investigation — typically mask leak, pressure inadequacy, or positional/REM-related events exceeding the machine’s upper limit. AHI above 10 on therapy despite good compliance suggests therapy optimization is needed. For the complete guide to therapy data interpretation, see our CPAP AHI guide and myAir data guide.
My sleep efficiency was 72%. Is that a problem?
Sleep efficiency below 80% is clinically reduced and suggests meaningful sleep fragmentation. In a sleep lab, a first-night effect (sleeping less well in an unfamiliar environment) contributes to lower efficiency for many patients — this is a documented phenomenon in sleep research. However, severely reduced efficiency (<70%) even accounting for first-night effect, or efficiency below 80% combined with high arousal index, reflects true sleep quality impairment worth addressing. Discuss the combination of your efficiency, arousal index, and symptom profile with your physician.
The Bottom Line
Your sleep study report is a detailed physiological snapshot of what happens to your body every night. AHI is the headline number but not the only one that matters — SpO₂ nadir, RDI, arousal index, sleep efficiency, event type breakdown, and positional data all contribute to the full clinical picture that determines whether, how, and how urgently you should be treated.
Mild AHI with significant symptoms warrants treatment. Significant desaturations warrant attention even with moderate AHI. Central events change the treatment entirely. Positional data may simplify your management. The report gives you all of this if you know how to read it.
For next steps after your diagnosis, see our guides on PAP therapy types, setting up your CPAP, and how to get a CPAP prescription. Browse our CPAP machines and accessories, or book a $49.99 RT Consultation for a clinical walkthrough of your sleep study results and a personalized therapy recommendation from a licensed Respiratory Therapist.
Written by Yashil Bhatt, RRT — Licensed Registered Respiratory Therapist with ICU and critical care experience and owner of My Respiratory Company.