A sleep study — formally called polysomnography (PSG) — records what happens in your body while you sleep. It's used primarily to diagnose sleep apnoea, but also to investigate other sleep disorders including restless legs syndrome, narcolepsy, parasomnias and unusual sleep behaviours.
Many patients find the experience strange and worry about sleeping in an unfamiliar environment attached to sensors. This guide explains what to expect from the test and how to make sense of the results. If you're monitoring your own health data between appointments, our article on what wearables actually track covers which consumer devices measure sleep reliably.
What a sleep study measures
- Brain activity (EEG) — records sleep stages (light, deep, REM)
- Eye movements (EOG) — confirms REM sleep
- Muscle activity (EMG) — particularly the chin and legs
- Heart rate and rhythm (ECG)
- Airflow at the nose and mouth
- Chest and abdominal movement — effort to breathe
- Blood oxygen level (SpO2 via pulse oximetry)
- Body position and leg movements
- Snoring — via a microphone or vibration sensor
The key metric: AHI (Apnoea-Hypopnoea Index)
The AHI is the number of times per hour that breathing stops (apnoea) or is significantly reduced (hypopnoea) during sleep. It is the primary measure used to diagnose and classify sleep apnoea.
| AHI | Severity |
|---|---|
| 0–4 events/hour | Normal |
| 5–14 events/hour | Mild sleep apnoea |
| 15–29 events/hour | Moderate sleep apnoea |
| 30+ events/hour | Severe sleep apnoea |
What sleep apnoea looks like and why it matters
In obstructive sleep apnoea (OSA) — the most common type — the muscles in the throat relax during sleep, narrowing or blocking the airway. Breathing stops; the brain detects low oxygen and wakes you just enough to restart breathing. This cycle can happen dozens or hundreds of times a night — without you ever remembering it.
The consequences go beyond tiredness: untreated severe sleep apnoea is associated with elevated blood pressure, increased risk of heart disease, stroke, type 2 diabetes, and depression.
Treatment options
- CPAP (continuous positive airway pressure) — gold standard; a machine delivers pressurised air through a mask to hold the airway open
- Mandibular advancement device (MAD) — a dental appliance that repositions the jaw; less effective than CPAP but better tolerated by some patients
- Weight loss — significant improvement in AHI with 10%+ weight reduction for overweight patients
- Positional therapy — for patients whose apnoea mainly occurs on their back
- Surgery — in selected patients where anatomy is contributing
Home sleep tests
A home sleep apnoea test (HSAT) is a simplified version of a full polysomnography. It typically measures airflow, chest effort and oxygen levels — enough to diagnose moderate-to-severe OSA, but not sufficient for diagnosing other sleep disorders or mild apnoea. If an HSAT is negative but clinical suspicion remains high, a full in-lab study is usually recommended.