Sleep Study — Purpose, Procedure & Normal Values | MyMedicPlus

A sleep study, or polysomnography (PSG), is a comprehensive, non-invasive overnight diagnostic test that simultaneously records multiple physiological parameters during sleep to identify and characterise sleep disorders. PSG is conducted in a dedicated, soundproofed sleep laboratory room designed to replicate a comfortable bedroom environment. The parameters simultaneously monitored throughout the night include: brain wave activity (EEG — electroencephalogram — for accurate sleep staging into NREM stages N1, N2, N3, and REM sleep); eye movement patterns (EOG — electrooculogram, for identifying REM sleep onset and characteristics); muscle tone and activity (EMG — electromyogram — recorded from chin and bilateral anterior tibialis muscles, for detecting REM atonia and periodic limb movements); cardiac rhythm (ECG); nasal and oral airflow (measured by thermistor and pressure transducer); respiratory effort (piezoelectric belts around the chest and abdomen); blood oxygen saturation (pulse oximetry on the finger); body position sensor; and audio-video recording for parasomnias and nocturnal behaviours. Home sleep apnoea tests (HSATs) are simplified, portable devices that record breathing parameters and oxygen levels at home but lack the full EEG capability and accurate sleep staging of in-laboratory PSG.

A sleep study is most commonly ordered to diagnose obstructive sleep apnoea (OSA) — the most prevalent sleep disorder, affecting an estimated 10–30% of adults — where the upper airway repeatedly collapses during sleep, causing apnoeas (breathing cessations lasting over 10 seconds) and hypopnoeas (partial airflow reductions). Clinical indicators of suspected OSA include loud habitual snoring, witnessed breathing pauses or choking/gasping episodes reported by a bed partner, excessive daytime sleepiness (Epworth Sleepiness Scale score greater than 10), unrefreshing sleep despite adequate duration, morning headaches, nocturia, impaired concentration, mood changes, and hypertension refractory to treatment. PSG is also ordered to diagnose central sleep apnoea (absent respiratory effort), upper airway resistance syndrome (increased respiratory effort without frank apnoeas), narcolepsy (requires a multiple sleep latency test following overnight PSG), periodic limb movement disorder (PLMD), REM sleep behaviour disorder (RBD — acting out dreams due to loss of normal REM atonia), parasomnias (sleepwalking, sleep terrors, confusional arousals), and nocturnal epilepsy. PSG is essential before initiating CPAP therapy and for CPAP pressure titration in severe OSA.

Avoid alcohol on the day of the study and for the preceding 24 hours, as alcohol suppresses REM sleep, promotes upper airway muscle relaxation (worsening OSA), and alters sleep architecture in ways that compromise diagnostic accuracy. Avoid caffeine (coffee, tea, energy drinks, cola, chocolate) from midday onwards on the day of the study, as caffeine prolongs sleep latency and reduces total sleep time. Do not nap during the afternoon before the test — adequate sleep pressure (homeostatic drive) is necessary for falling asleep in the unfamiliar environment. Take all regular prescribed medications as usual unless your sleep specialist has provided specific instructions to hold any drug. Arrive at the sleep laboratory between 8–10 PM with freshly washed, dry hair free of styling products — oils, gels, wax, or hairspray prevent electrode adhesion and compromise EEG signal quality. Bring comfortable pyjamas or sleepwear, your usual pillow if desired, any CPAP device you already use, essential toiletries for the following morning, and a change of clothes for leaving in the morning.

On arrival at the sleep laboratory, a sleep technician conducts a brief intake assessment and reviews your sleep questionnaires. The electrode and sensor application process takes approximately 30–60 minutes. Approximately 20–25 sensors are applied: scalp EEG electrodes (typically six standard positions plus reference and ground) applied with conductive paste and affixed with collodin or tape; bilateral EOG electrodes at the outer canthi; chin and anterior tibialis EMG electrodes; ECG leads; nasal-oral airflow sensors; chest and abdominal respiratory effort belts; a pulse oximeter clip on a finger; and a body position sensor. All sensors are connected to a bedside amplifier and recording system. The room lights are dimmed and you are invited to relax and sleep as naturally as possible. The sleep technician monitors all signals in real time from a separate control room throughout the night, ensuring signal quality and intervening only if a sensor disconnects. In some facilities, supplemental oxygen or CPAP is applied during the study if significant desaturation or apnoea is detected (split-night study). In the morning (typically around 6–7 AM), all sensors are removed and you are free to leave. The recorded data is subsequently scored manually by a registered polysomnographic technologist before the sleep physician's review.

The polysomnography report includes a comprehensive set of scored sleep parameters. Total sleep time and sleep efficiency (percentage of time in bed actually asleep — normal >85%). Sleep staging: normal distribution in adults approximates N1 5%, N2 45–55%, N3 (slow-wave/deep sleep) 13–23%, REM 20–25%, with REM cycles increasing in duration through the night. Apnoea-Hypopnoea Index (AHI) — the primary severity measure, defined as the number of apnoeas (complete cessation of airflow ≥10 seconds) plus hypopnoeas (≥30% airflow reduction with associated arousal or ≥3% oxygen desaturation) per hour of total sleep time: Normal less than 5 events/hour; Mild OSA 5–14 events/hour; Moderate OSA 15–29 events/hour; Severe OSA 30 or more events/hour. Oxygen desaturation index (ODI) — number of oxygen desaturation events (≥3% or ≥4% drop from baseline) per hour. Minimum oxygen saturation during sleep. Arousal index (normally below 15 arousals/hour). Periodic limb movement index (PLMI — below 15/hour is normal). The full report is reviewed and signed by a sleep medicine physician within 5–10 working days.

Polysomnography is a non-invasive diagnostic test with an excellent safety profile and no significant medical risks. Minor inconveniences include the time and effort of spending a night away from home, mild discomfort from the adhesive electrode paste (which washes out easily), and the unfamiliar sleep environment. The most important diagnostic limitation is the first-night effect — lighter sleep, longer sleep latency, and reduced REM sleep compared to habitual home sleep, which is a recognised phenomenon in laboratory PSG and affects approximately 15–20% of patients. This can occasionally underestimate the severity of mild OSA by reducing total sleep time or REM-related apnoea events. Home sleep apnoea tests (HSATs) offer a more convenient and lower-cost alternative for diagnosing uncomplicated moderate-to-severe OSA in patients without significant comorbidities, but they systematically underestimate OSA severity (dividing events by total recording time rather than actual sleep time) and cannot diagnose non-respiratory sleep disorders such as narcolepsy, PLMD, RBD, or parasomnias. A technically adequate PSG requires a minimum of 4–6 hours of recorded sleep. Repeat testing is sometimes required if the initial night yields insufficient data.