Polysomnography (PSG) is the comprehensive multi-channel recording of physiologic parameters during sleep — EEG, EOG, chin and limb EMG, ECG, respiratory effort and flow, oximetry, body position, and audio/video — that allows clinical sleep medicine. Sleep staging according to AASM rules transforms the raw signals into characterized epochs of wake, N1, N2, N3, and REM that allow diagnosis of sleep disorders. This page covers the PSG setup, AASM scoring rules, sleep stages, and the practical interpretation of a PSG report.

PSG Channels

EEG

• Standard montage includes frontal (F3, F4), central (C3, C4), occipital (O1, O2) referenced to mastoids (M1, M2).
• Frontal channels: best for N3 slow waves.
• Central channels: best for spindles, K complexes.
• Occipital channels: best for PDR and visual cortex activity.

EOG (Electrooculography)

• Electrodes near outer canthi of eyes.
• Detect eye movements: slow rolling, REM saccades, blinks.
• Essential for staging REM and N1.

Chin EMG

• Submental electrodes.
• High tone in wake and N1; lower in N2; lowest in REM (atonia).
• Loss of chin atonia in REM = REM sleep without atonia (RWA), suggesting RBD.

Limb EMG

• Bilateral anterior tibialis.
• Detects periodic limb movements (PLMs).

ECG

• Single lead.
• Detects arrhythmias, heart rate variability.

Respiratory Effort

• Chest and abdominal belts (inductance plethysmography).
• Distinguishes obstructive apnea (effort + no flow) from central apnea (no effort + no flow).

Airflow

• Nasal pressure transducer + thermistor.
• Detects apneas, hypopneas, snoring.

Pulse Oximetry

• Continuous SpO₂.
• Detects desaturations associated with respiratory events.
• Standard 3% and 4% drops from baseline used for scoring.

Body Position

• Sensors detect supine, lateral, prone.
• Important for positional sleep apnea.

Video and Audio

• Synchronized with all channels.
• Essential for parasomnias, abnormal movements, behavior during sleep.

AASM Sleep Stages

Wake

• Posterior dominant rhythm (alpha, 8–13 Hz) on occipital channels when eyes closed.
• Active beta when alert.
• Eye movements: rapid voluntary saccades, blinks.
• Chin EMG: high tone.

N1 (Stage 1 NREM, Light Sleep)

• Loss of PDR or PDR replaced by low-amplitude mixed-frequency.
• Vertex sharp transients at Cz.
• Slow rolling eye movements.
• Chin EMG: lower than wake.
• Sleep latency: time from lights out to first N1.
• ~5% of total sleep.

N2 (Stage 2 NREM, Light Sleep)

• Sleep spindles: 11–16 Hz bursts, 0.5–2 s duration, max at C3/C4.
• K complexes: large biphasic complex at Cz; spontaneous or evoked.
• Slow eye movements absent.
• Background mostly theta.
• 45–55% of total sleep.

N3 (Stage 3 NREM, Slow-Wave Sleep)

• Slow waves: ≥75 μV (peak-to-peak), 0.5–2 Hz frequency.
• Definition: ≥20% of epoch contains slow-wave activity.
• Maximum amplitude in frontal channels.
• 10–25% of total sleep in young adults; decreases with age.
• Concentrated in first half of night.

REM Sleep

• Low-amplitude mixed-frequency EEG (similar to N1 wake).
• Rapid eye movements visible in EOG.
• Chin EMG: lowest (atonia).
• “Saw-tooth waves” — triangular 4–7 Hz waves in central/frontal regions.
• 20–25% of total sleep.
• REM cycles every ~90 minutes.

Scoring Rules

Epoch Definition

• 30-second epochs.
• Each epoch assigned a single stage (the stage that predominates).
• Sequential epochs scored to create hypnogram.

Transition Rules

• From wake to N1: appearance of slow eye movements + loss of PDR + vertex waves.
• From N1 to N2: appearance of spindle or K complex.
• From N2 to N3: ≥20% slow waves.
• To REM: chin atonia + rapid eye movements + characteristic EEG.

Arousal Rules

• Arousal: abrupt change in EEG frequency lasting ≥3 seconds.
• In REM, requires concurrent EMG increase.
• Arousals interrupt sleep and reduce sleep quality.
• Arousal index = arousals per hour of sleep; normal <15; elevated suggests fragmented sleep.

Sleep Architecture

Hypnogram

• Plot of sleep stages across the night.
• Cycles: roughly 90-minute cycles of NREM (light → deep) then REM.
• First half of night: more N3.
• Second half of night: more REM.

Sleep Parameters

• Sleep latency: time from lights out to first N1.
• Sleep efficiency: total sleep time / time in bed × 100; normal >85%.
• REM latency: time from sleep onset to first REM; normal 70–110 minutes.
• WASO (wake after sleep onset): time spent awake after first sleep until final wake.
• TST (total sleep time): actual sleep time.

Normal Sleep Architecture by Age

Common Sleep Disorder Findings on PSG

Obstructive Sleep Apnea

• Apneas: complete cessation of airflow ≥10 seconds.
• Hypopneas: ≥30% reduction in airflow with 3% or 4% O₂ desaturation (depending on insurance/payer criteria).
• AHI (Apnea-Hypopnea Index): events per hour of sleep.
• Severity: mild 5–15, moderate 15–30, severe >30.

Central Sleep Apnea

• Apneas with no effort.
• Often in heart failure (Cheyne-Stokes), high altitude, opioid use.
• CAHI (central AHI) calculated separately.

REM Behavior Disorder (RBD)

• REM sleep without atonia.
• Patient may move during REM.
• Stronger association with α-synucleinopathies.

Restless Legs Syndrome / Periodic Limb Movements

• PLMs: periodic leg movements at intervals of 5–90 seconds.
• PLM index: PLMs per hour; elevated index suggests disease.

Insomnia Disorders

• Prolonged sleep latency, frequent arousals, reduced sleep efficiency.
• PSG less critical for diagnosis (clinical) but useful for excluding other causes.

Narcolepsy

• PSG must exclude OSA.
• MSLT next day (covered in dedicated page).

PSG vs Home Sleep Apnea Test (HSAT)

• HSAT: portable monitor with 4–7 channels (typically airflow, effort, oximetry, position).
• Suitable for evaluating obstructive sleep apnea in patients with high pre-test probability.
• Limitations:
  • Cannot stage sleep without EEG.
  • Underestimates AHI (no certainty of sleep).
  • Cannot diagnose other sleep disorders.
  • Not validated for severe medical comorbidities.
• PSG standard for: parasomnias, complex sleep disorders, treatment titration, suspected non-OSA conditions.

Practical PSG Reading

1. Verify technical adequacy: all channels recording, calibration signals correct.
2. Stage the recording: complete hypnogram epoch by epoch.
3. Calculate sleep parameters: TST, sleep efficiency, sleep latency, REM latency, WASO.
4. Score events: arousals, respiratory events, limb movements, abnormal behaviors.
5. Calculate indices: AHI, PLM index, arousal index.
6. Note specific findings: REM without atonia, periodic patterns, video-captured behaviors.
7. Synthesize: diagnostic conclusion based on findings.

Reporting

A PSG report should include:

• Indication.
• Sleep parameters: TST, sleep efficiency, sleep latency, REM latency, WASO, stages percentages.
• Respiratory events: AHI, CAHI, OAHI (obstructive AHI).
• Oxygen desaturation: nadir, time below 90%.
• Limb movement index.
• Arousal index.
• Specific abnormal findings.
• Sleep architecture characterization.
• Diagnostic conclusion and recommendations.

🔍 Did You Know?

The AASM scoring rules have evolved significantly over decades, with major changes in the 2007, 2014, and 2023 versions. The 2007 update replaced the older Rechtschaffen and Kales (R&K) staging — which divided NREM into Stages 1, 2, 3, and 4 — with the current N1, N2, N3 system, combining the old “Stages 3 and 4” into a single N3 category. This change reflects the recognition that the distinction between Stage 3 and Stage 4 was not clinically meaningful while adding complexity. The 2023 update (Version 3 of the AASM Manual) further refined criteria for hypopnea scoring and other areas. The most clinically important point: the hypopnea criterion now differs by payer — Medicare requires 4% desaturation; commercial insurance often accepts 3% desaturation or arousal. This means the same PSG can have a different AHI depending on which scoring rule is applied. For the practicing sleep medicine clinician, this has practical implications: a patient’s “moderate OSA” diagnosis using one set of rules may be “mild OSA” using another, affecting treatment access and reimbursement. The lesson generalizes: scoring criteria in clinical neurophysiology evolve and have practical clinical consequences, and the experienced clinician knows which rules apply to which patient and how to communicate the findings appropriately. The same principle applies to NCS/EMG normal values (lab-specific), EEG terminology (ACNS standardized), and other areas where standards have been refined over time.

Pitfalls and Pearls

• PSG comprehensive: EEG + EOG + chin EMG + limb EMG + ECG + respiratory + oximetry + position + video.
• 30-second epochs: standard scoring unit.
• AASM stages: Wake, N1, N2, N3, REM.
• N1: loss of PDR, vertex sharp transients, slow eye movements; ~5% of sleep.
• N2: sleep spindles, K complexes; 45–55% of sleep.
• N3: ≥20% slow waves (≥75 μV, 0.5–2 Hz); 10–25% young adults.
• REM: rapid eye movements + atonia + saw-tooth waves; 20–25%.
• REM sleep without atonia: marker of RBD; α-synucleinopathy risk.
• AHI: severity of OSA; mild 5–15, moderate 15–30, severe >30.
• Hypopnea: 30% airflow reduction with 3% or 4% desat (payer-specific).
• Sleep efficiency: TST/time in bed; >85% normal.
• REM latency: 70–110 min normal; shortened in depression, narcolepsy.
• Arousal index: <15 normal; elevated suggests fragmentation.
• PLM index: >15/hr in adults associated with disease.
• HSAT: suitable for high pre-test OSA; cannot stage sleep; underestimates AHI.
• PSG required: parasomnia diagnosis, complex sleep disorders, narcolepsy workup.
• Pediatric PSG: different staging; expert review.
• Scoring rules evolve: AASM updates; payer differences in hypopnea criteria.

References

1. Berry RB, Quan SF, Abreu AR, et al. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology, and Technical Specifications. Version 3. American Academy of Sleep Medicine; 2023.
2. Rechtschaffen A, Kales A, eds. A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects. National Institutes of Health; 1968.
3. Kushida CA, Littner MR, Morgenthaler T, et al. Practice parameters for the indications for polysomnography and related procedures. Sleep. 2005;28(4):499-521.
4. Collop NA, Anderson WM, Boehlecke B, et al. Clinical guidelines for the use of unattended portable monitors in the diagnosis of obstructive sleep apnea in adult patients. J Clin Sleep Med. 2007;3(7):737-747.
5. Iber C, Ancoli-Israel S, Chesson A, Quan SF. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology, and Technical Specifications. 1st ed. American Academy of Sleep Medicine; 2007.