Understand Your Sleep Study Results
Sleep studies produce dense, jargon-heavy reports. This guide explains every term in plain English — and lets you enter your own numbers to see what they indicate.
Enter Your Results
AHI is required — everything else is optional.
Events per hour — the main number in your report.
Optional
Lowest oxygen saturation during the study.
% of sleep time with oxygen below 90%.
Time asleep ÷ time in bed × 100.
Enter your AHI above
Your interpretation will appear here
The AHI Severity Scale
Based on AASM (American Academy of Sleep Medicine) clinical guidelines.
No significant sleep-disordered breathing
Mild obstructive sleep apnea
Moderate obstructive sleep apnea — CPAP typically recommended
Severe obstructive sleep apnea — treatment required
Severity classification also considers symptoms and oxygen desaturation — AHI alone does not determine the final diagnosis.
Glossary of Sleep Study Terms
Every term you might see in your sleep study report, explained in plain English.
Respiratory Events
AHI
Apnea-Hypopnea Index
The number of apneas and hypopneas (partial breathing stoppages) per hour of sleep.
Why it matters: The single most important number in your sleep study. Used to classify severity of obstructive sleep apnea.
RDI
Respiratory Disturbance Index
Like AHI, but also includes RERAs — partial airway obstructions that disrupt sleep without meeting the full threshold for a hypopnea.
Why it matters: RDI is always ≥ AHI. A much higher RDI than AHI suggests upper airway resistance may be fragmenting your sleep even without full apnea events.
Apnea
A complete cessation of airflow for 10 seconds or more.
Why it matters: Apneas are the most severe breathing events — your airway fully closes or your brain stops sending breathing signals.
Hypopnea
A partial reduction in airflow (≥30%) for 10+ seconds, associated with oxygen desaturation or an arousal.
Why it matters: Less severe than apneas but still fragment sleep and reduce oxygen levels.
Obstructive Apnea
An apnea caused by the physical collapse of the airway — throat muscles relax and block airflow despite continued breathing effort.
Why it matters: The most common type. Obstructive events are what CPAP therapy directly treats by maintaining airway pressure.
Central Apnea
An apnea where the airway is open but the brain temporarily stops sending the signal to breathe. No breathing effort is made.
Why it matters: Different cause to obstructive events and may require different treatment — e.g. ASV therapy rather than standard CPAP.
Mixed Apnea
An apnea that begins as a central event (no effort) and ends as an obstructive event (effort resumes but airway is blocked).
Why it matters: Often responds to CPAP, similar to obstructive events.
RERA
RERA
Respiratory Effort-Related Arousal. Increasing respiratory effort that leads to an arousal from sleep without meeting full criteria for an apnea or hypopnea.
Why it matters: Can significantly fragment sleep without appearing in the AHI count. High RERA rate with low AHI may explain feeling unrefreshed despite a "normal" result.
Snore Index
The number of snoring events per hour, or the percentage of sleep time spent snoring.
Why it matters: Heavy snoring often indicates partial airway obstruction. High snore index with low AHI may suggest upper airway resistance syndrome.
Oxygen Levels
SpO2
SpO2 (Oxygen Saturation)
The percentage of haemoglobin in your blood carrying oxygen, measured by a pulse oximeter on your finger.
Why it matters: Normal SpO2 during sleep is 95–100%. Repeated drops below 90% indicate insufficient oxygen during apnea events.
SpO2 Nadir
The lowest single oxygen saturation reading recorded during the entire sleep study.
Why it matters: Shows the worst-case oxygen dip. Values below 80% are severe and associated with cardiovascular risk.
Time Below 90% SpO2
The percentage of total sleep time during which oxygen saturation was below 90%.
Why it matters: Even a moderate nadir can be significant if you spend substantial time below 90%. More than 1–2% of sleep time indicates clinically meaningful oxygen deprivation.
ODI
Oxygen Desaturation Index
The number of times per hour that SpO2 drops by 3% or 4% from baseline.
Why it matters: A companion to AHI — a high ODI confirms breathing events are causing actual oxygen drops, not just brief arousals.
Sleep Architecture
Sleep Efficiency
The percentage of time in bed actually spent asleep. Calculated as (total sleep time ÷ time in bed) × 100.
Why it matters: Normal is above 85%. Lower values suggest difficulty falling or staying asleep — commonly caused by sleep apnea, anxiety, or poor sleep hygiene.
Sleep Latency
How long it took to fall asleep from lights out.
Why it matters: Normal is 10–20 minutes. Under 5 minutes suggests sleep deprivation or a sleep disorder. Over 30 minutes may indicate insomnia.
REM Latency
How long after falling asleep before reaching the first REM (dreaming) stage.
Why it matters: Normal is 70–120 minutes. Very short REM latency (<20 min) can indicate narcolepsy or severe sleep deprivation. Sleep apnea often reduces overall REM time.
N1 Sleep
Stage 1 non-REM sleep — the lightest stage, the transition between wakefulness and sleep. Normally 2–5% of sleep time.
Why it matters: High N1 percentages indicate fragmented sleep, often caused by frequent arousals from breathing events.
N2 Sleep
Stage 2 non-REM sleep — the largest portion of a normal night (45–55%). Characterised by sleep spindles and K-complexes.
Why it matters: Considered restorative. Sleep apnea often increases N2 at the expense of deeper N3 and REM sleep.
N3
N3 — Deep Sleep
The deepest stage of non-REM sleep, also called slow-wave sleep. Normally 15–20% of sleep time. Physical restoration occurs here.
Why it matters: Reduced N3 is common in sleep apnea and associated with fatigue, poor memory, and impaired immune function. CPAP typically increases N3 over time.
REM Sleep
Rapid Eye Movement sleep — the dreaming stage. Normally 20–25% of sleep. Brain activity is high but muscles are largely paralysed.
Why it matters: Sleep apnea is often worst during REM. Low REM percentages are associated with mood problems and memory impairment.
Arousal Index
The number of arousals (brief wake-ups, even if not remembered) per hour of sleep.
Why it matters: Normal is under 10/hour. Sleep apnea causes elevated arousal indices — the brain wakes you briefly to resume breathing.
WASO
WASO
Wake After Sleep Onset — total time spent awake after first falling asleep.
Why it matters: High WASO contributes to low sleep efficiency and is associated with insomnia and fragmented sleep from breathing events.
Other Metrics & Test Types
PLMI
Periodic Limb Movement Index
The number of periodic leg or arm movements per hour of sleep.
Why it matters: PLMI above 15 in adults suggests Periodic Limb Movement Disorder, which can fragment sleep independently of breathing problems.
PSG
Polysomnography
A comprehensive in-lab sleep study monitoring brain waves, eye movements, muscle activity, heart rhythm, breathing, oxygen, and limb movements simultaneously.
Why it matters: The gold standard for diagnosing sleep disorders. Most complete picture — requires an overnight stay in a sleep lab.
HST
Home Sleep Test
A simplified sleep study done at home, monitoring breathing effort, airflow, oxygen, and heart rate — but not brain waves or sleep stages.
Why it matters: Cheaper and more convenient but may underestimate AHI (measures events per recording time, not per sleep time). Cannot diagnose central apnea or PLMD.
MSLT
Multiple Sleep Latency Test
A daytime test measuring how quickly you fall asleep across 4–5 nap opportunities spaced 2 hours apart.
Why it matters: Used to diagnose narcolepsy and idiopathic hypersomnia. Mean sleep latency under 8 minutes indicates excessive daytime sleepiness.
What Happens Next?
If your sleep study indicates obstructive sleep apnea, your doctor will discuss treatment options at your follow-up appointment. For most people with an AHI above 15, the first-line treatment is CPAP therapy — a machine that delivers continuous positive air pressure through a mask while you sleep, keeping your airway open throughout the night.
CPAP is highly effective. Most people notice significant improvements in sleep quality, daytime energy, and mood within the first week. The challenge is often compliance — getting used to wearing a mask and finding a setup that works for your lifestyle.
If you travel frequently, a travel CPAP setup is worth thinking about early. Devices like the ResMed AirMini are designed specifically for travel, and accessories like USB-C travel cables let you power your machine from a standard power bank — no bulky AC adapter required.
Frequently Asked Questions
Common questions about sleep study results and sleep apnea diagnosis.
The information on this page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. AHI severity thresholds are based on published AASM guidelines. Always consult your doctor or sleep specialist regarding your specific results and treatment options.