Guide to Polysomnography (In-Lab Sleep Study)

Polysomnography (PSG) — commonly called an in-lab sleep study — is the most comprehensive diagnostic test available for sleep disorders. Conducted in a specialised sleep laboratory, it simultaneously records a wide range of physiological signals throughout the night, providing a detailed, objective map of your sleep. It is the gold standard for diagnosing sleep apnea, narcolepsy, REM sleep behaviour disorder, and other complex sleep conditions.

What Is Polysomnography?

The word "polysomnography" combines the Greek poly (many), the Latin somnus (sleep), and the Greek graphia (writing or recording). It is an apt description: PSG records many biological signals simultaneously across an entire night of sleep, building a comprehensive picture of what is actually happening in your body and brain when you sleep.

PSG is distinct from a home sleep test, which measures only a limited set of breathing parameters. PSG records brain activity, eye movements, muscle activity, heart rhythm, breathing effort and flow, and blood oxygen saturation — giving a complete picture of sleep stage architecture as well as any abnormal events occurring during sleep.

What Conditions Can Polysomnography Diagnose?

PSG is the required or recommended diagnostic test for a range of sleep disorders:

• Obstructive sleep apnea (OSA): Identifies apnea and hypopnea events, their severity (the Apnea-Hypopnea Index or AHI), associated oxygen desaturations, and sleep fragmentation. PSG is the gold standard for OSA diagnosis, though home tests are increasingly used for straightforward cases.
• Central sleep apnea (CSA): A home test cannot reliably identify CSA. PSG is necessary when CSA is suspected.
• Narcolepsy: PSG the night before is required before a Multiple Sleep Latency Test (MSLT) the following day — the combination is the diagnostic standard for narcolepsy.
• REM sleep behaviour disorder (RBD): PSG with video recording (video-PSG) is essential for RBD diagnosis, as it documents the loss of normal REM atonia and records any motor behaviour during REM sleep.
• Periodic limb movement disorder (PLMD): The repetitive leg movements of PLMD are detected and quantified by leg EMG recordings during PSG.
• Parasomnias: Complex night-time behaviours, seizures, and other parasomnias require video-PSG for characterisation and to distinguish between different causes.
• Unexplained excessive daytime sleepiness: When the cause of EDS is not apparent from history and home testing.

What Is Measured?

A standard PSG records the following channels:

• Electroencephalogram (EEG): Brain wave activity measured via scalp electrodes. This is the core measurement — it determines sleep stage (N1, N2, N3, and REM) and identifies arousals, seizure activity, and other brain patterns.
• Electrooculogram (EOG): Eye movement recordings that identify REM sleep (characterised by rapid eye movements) and sleep onset (slow, rolling eye movements).
• Electromyogram (EMG): Muscle activity. Chin EMG detects the muscle atonia (paralysis) of REM sleep and its absence in RBD. Leg EMG detects periodic limb movements.
• Electrocardiogram (ECG): Heart rate and rhythm. Identifies arrhythmias that occur during sleep or are triggered by apnea events.
• Pulse oximetry: Continuous measurement of blood oxygen saturation via a finger clip. Oxygen desaturations mark apnea events.
• Airflow: Measured by a thermocouple or thermistor at the nose/mouth and a nasal pressure transducer. Identifies airflow reduction (hypopneas) and cessation (apneas).
• Respiratory effort: Chest and abdominal belts (respiratory inductance plethysmography) measure breathing effort. This distinguishes obstructive apneas (breathing effort present but airflow blocked) from central apneas (no breathing effort).
• Body position: Records whether events occur specifically in the supine (back-lying) position.
• Video recording: Standard in most modern sleep labs, it allows review of any observed movements or behaviours correlated with physiological recordings.

What to Expect: A Step-by-Step Guide to the Night

Before the Study: Preparation

• Arrive at the sleep laboratory in the early evening, typically between 8–10pm
• Wash and dry your hair — clean hair is essential for good scalp electrode contact
• Avoid caffeine on the day of the study
• Take your regular medications as normal unless specifically instructed otherwise by the sleep team
• Bring comfortable sleepwear, toiletries, a change of clothes for the next morning, and any personal comfort items (your own pillow, a book to read before sleep)

Set-Up: Applying the Sensors

A sleep technologist will spend 45–90 minutes applying sensors before you sleep. This process, while somewhat laborious, is entirely painless and non-invasive. Scalp electrodes are applied with a small amount of conductive paste and secured with gauze or tape. Adhesive electrodes are placed on the face, chest, and legs. Belts are placed around the chest and abdomen. A pulse oximeter clips to the finger. A nasal cannula or thermistor rests near the nose and mouth.

All wires are gathered into a bundle that connects to a recording unit. The bundle is long enough to allow comfortable movement in bed, including turning over. If you need to use the bathroom during the night, the technologist can disconnect you from the system — you do not need to remove sensors yourself.

During the Night

The sleep laboratory room is designed to be quiet and comfortable — similar to a private hotel room with a hospital-grade bed. After lights out, the sleep technologist monitors all recordings from an adjacent room throughout the night. They will not disturb you unless a sensor has come loose or disconnected.

The study typically ends between 6–7am. The sensors are removed, and you can shower and leave. Most people are able to go directly to work or proceed with their normal day.

After the Study: Results and Follow-Up

The data recorded during the night undergoes a detailed review process: a trained sleep technologist manually scores every 30-second "epoch" of sleep, identifying sleep stages, arousals, apnea and hypopnea events, limb movements, and other features. This scoring takes several hours. The scored data is then reviewed and interpreted by a board-certified sleep physician, who prepares a formal report.

Results are typically discussed at a follow-up appointment 2–4 weeks after the study. The physician will explain the findings, any diagnosis made, and the recommended treatment plan. If sleep apnea is diagnosed, a CPAP titration study — either in the lab or at home — is usually the next step.

Key metrics you will likely hear discussed:

• Apnea-Hypopnea Index (AHI): Apneas + hypopneas per hour. Normal below 5; mild 5–14; moderate 15–29; severe 30 or above.
• Oxygen desaturation index (ODI): How many times per hour oxygen levels drop significantly.
• Sleep efficiency: Percentage of time in bed actually spent asleep (above 85% is normal).
• Sleep architecture: Proportions of N1, N2, N3 (deep), and REM sleep. Abnormalities in these proportions provide diagnostic information.

References

• Berry RB, et al. Rules for scoring respiratory events in sleep. Journal of Clinical Sleep Medicine. 2012;8(5):597–619.
• Iber C, et al. The AASM Manual for the Scoring of Sleep and Associated Events. American Academy of Sleep Medicine; 2007.
• Kushida CA, et al. Practice parameters for the indications for polysomnography. Sleep. 2005;28(4):499–521.
• American Academy of Sleep Medicine. Clinical guideline for the evaluation, management, and long-term care of obstructive sleep apnea. 2009.