Adult Sleep Staging

1. The AASM staging framework and the 30-second epoch

The AASM staging framework (from the AASM Manual for the Scoring of Sleep and Associated Events) is the standardized set of rules that govern how a PSG is scored. The basic scoring unit is the epoch, a fixed-length segment of the recording—in adults, the 30-second epoch. Each epoch is labeled W, N1, N2, N3, or R based on the predominant EEG, EOG, and EMG activity within it. Because the rules are updated periodically, the technologist must always verify against the current manual rather than relying on memory or older editions.

2. EEG, EOG, and EMG features of each stage

• Stage W (wake): with eyes closed, posterior (occipital) regions show a dominant alpha rhythm (8–13 Hz); with eyes open, alpha attenuates and the record shows eye blinks and reading eye movements (conjugate, slow-phase-then-fast-phase EOG deflections). Chin EMG is typically relatively high.
• Stage N1: alpha occupies less than 50% of the epoch (alpha attenuation), background shifts to low-amplitude theta (4–7 Hz), slow rolling eye movements appear in the EOG, and vertex sharp waves (brief, sharply contoured negative waves maximal at Cz) may occur. Spindles and K complexes are absent.
• Stage N2: defined by the presence of sleep spindles (bursts of ~11–16 Hz activity, often 12–14 Hz, lasting about 0.5 seconds or more) and/or K complexes (well-delineated negative sharp wave followed by a positive component, often >0.5 seconds). N2 continuation means N2 is maintained across epochs when spindles or K complexes persist and no criteria force a transition to N3 or R. Arousal effects on N2 are limited: an abrupt EEG frequency shift (arousal) can be superimposed on the stage without changing it.
• Stage N3: slow-wave activity—waves of 0.5–2 Hz (delta) meeting the amplitude criteria (a peak-to-peak amplitude of 75 µV or greater over frontal regions)—occupies 20% or more of the epoch (percentage-of-epoch criterion). Eye movements and EMG are typically absent or low.
• Stage R (REM): rapid eye movements in the EOG, a low-amplitude mixed-frequency EEG resembling N1 (without spindles or K complexes), low submental EMG (at or near the lowest level of the recording), and sometimes sawtooth waves (notched, 2–6 Hz waveforms maximal centrally that often precede bursts of rapid eye movements).

3. Continuation, transitions, and major body movement

Scoring is hierarchical and rule-driven. When an epoch shows no spindles or K complexes, no REM features, and a low-voltage mixed-frequency EEG, it defaults toward N1 unless it meets N3 criteria. An epoch that follows N3 but shows mixed features can continue to be scored N3 under specific continuation rules. A major body movement (movement and muscle artifact obscuring the EEG for more than half the epoch, with an accompanying body-movement signal) is handled by scoring the epoch based on the rules the manual specifies rather than guessing. Any genuine uncertainty is scoring ambiguity, and the technologist documents it and, where the manual requires, defers or flags it for review.

How it works

1. The technologist reviews the calibrated, artifact-annotated PSG epoch by epoch.
2. For each 30-second epoch, the dominant EEG frequency, EOG pattern, and submental EMG level are compared against the stage definitions.
3. The stage with the best rule match is assigned, following the manual's hierarchy and continuation rules.
4. Consecutive epochs are combined into a hypnogram for downstream calculations (TST, sleep efficiency, stage percentages).
5. The scored record is validated, with ambiguous epochs flagged for review by the interpreting provider.

Common confusions

Memory aids

"W-N1-N2-N3-R — Alpha Theta Spindles Delta Eyes." Walk down the night: Wake (alpha), N1 (theta + slow rolling eyes), N2 (spindles/K complexes), N3 (delta ≥20%), REM (rapid eyes + low chin EMG). This maps each stage to its single most diagnostic feature in order.

Quick review

Topic Recap

Adult sleep staging labels each 30-second epoch as W, N1, N2, N3, or R using EEG, EOG, and submental EMG. Wake is alpha with blinks/reading movements; N1 is theta, alpha attenuation, slow rolling eyes, and vertex sharp waves; N2 is defined by spindles and K complexes; N3 requires ≥20% slow-wave activity; and REM shows rapid eye movements, low-amplitude mixed-frequency EEG, low chin EMG, and sawtooth waves. Ambiguity is documented, not guessed away, and staging is always a scoring function verified against the current AASM manual—never a diagnosis.

Knowledge Check

1. Which EEG activity must occupy at least 20% of an epoch to score Stage N3?
2. What two waveforms define Stage N2?
3. What EOG finding distinguishes N1 from relaxed wakefulness?
4. In Stage R, what happens to the submental EMG?
5. Who is responsible for interpreting what the staged record means clinically—the technologist or the physician?

Answers and Rationales

1. Slow-wave activity (delta, 0.5–2 Hz, ≥75 µV) must occupy ≥20% of the epoch. Rationale: N3 is defined by the percentage of slow-wave activity, not merely its presence.
2. Sleep spindles and K complexes. Rationale: N2 is the stage defined by these two discrete waveforms.
3. Slow rolling eye movements (along with theta background and alpha attenuation). Rationale: slow rolling eyes mark the transition into N1, while wake shows blinks and reading movements.
4. It falls to its lowest level of the recording. Rationale: low submental EMG is a defining REM feature because muscle tone is suppressed.
5. The physician. Rationale: the technologist scores stages; only a physician interprets the record and renders a diagnosis.