Hypnogram: how to read your sleep-stage chart, and why the rulebook moves the numbers
A hypnogram plots sleep stages against time in 30-second epochs. Re-scoring 72 adults under the 2007 AASM rules instead of the 1968 Rechtschaffen and Kales rules moved N1, N2 and N3 by 10.6, 20.5 and 9.1 minutes (Moser et al., 2009).
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A sleep-lab report prints the night as a staircase: wake along the top line, a long slab of N2 through the middle, and a few deep dips toward the bottom in the first hours, then a ragged climb back up toward morning. That chart is a hypnogram, a step plot of sleep stages against time. Each step is one 30-second epoch that a scorer, or an algorithm, has assigned to wake, REM, or one of three non-REM stages, N1, N2 and N3. The numbers on it depend on the rulebook used to assign them: when Moser and colleagues re-scored the same 72 recordings under two rulebooks, the minutes in N1, N2 and N3 changed by 10.6, 20.5 and 9.1.
What is on the chart
Think of a hypnogram as a lossy encoding of a continuous signal. Electrodes record brain waves (EEG), eye movements (EOG) and chin muscle tone (EMG). The recording is cut into 30-second windows, and each window gets one label. Whatever happens inside a window is flattened to that label, usually whichever stage fills most of it. Anything shorter than half a minute can be missed, and features such as sleep spindles and K-complexes do not appear on the chart even though they influenced the label.
Time sits on the horizontal axis. Stages sit on the vertical, conventionally with wake at the top and N3 at the bottom, so that deeper sleep draws as a lower line. Many charts file REM just under wake, where it belongs by brain-wave activity. How much REM a chart shows near the end of the night also depends on where that night finishes on the body clock, as the 28-hour-day experiment of Dijk and Czeisler found.
Which rulebook?
Until the mid-2000s, nearly every sleep lab scored by Rechtschaffen and Kales (R&K), the 1968 manual. It named five non-wake stages: four of non-REM and REM. In 2007 the American Academy of Sleep Medicine published its own manual. It relabeled the non-REM stages N1, N2 and N3, folding R&K’s stages 3 and 4 into one.
Critics of the 1968 manual had pointed to its room for subjective judgment and to its basis in young healthy sleepers, which fits older adults and patients less well. Moser and colleagues (Sleep, 2009) took recordings from 72 adults aged 21 to 86 (56 healthy, 16 patients) and scored each under both. Relative to R&K, AASM scoring produced these differences:
| Measure | Difference (AASM minus R&K) |
|---|---|
| N1 (R&K stage 1) | +10.6 min |
| N2 (R&K stage 2) | −20.5 min |
| N3 (R&K stages 3 + 4) | +9.1 min |
| Total sleep time | −2.5 min (not significant) |
| Sleep efficiency | −0.5% (not significant) |
| REM | −1.8 min (not significant) |
| Wake after sleep onset | +4.1 min (significant; mostly a change in the sleep-onset definition) |
Total sleep time, efficiency and REM barely moved. The split of light and deep sleep did. The authors concluded that new normative data have to be established for the AASM standard, so numbers from older papers cannot be dropped in beside new ones. Two caveats: the patient groups were small, and the study was funded by The Siesta Group, which several of the authors are affiliated with. The size of the shift also varied with the scorers.
Reading one in practice
- Check the axes. Confirm which direction is deep and where REM sits.
- Find the first long stretch of N3. In healthy young adults, slow-wave sleep is concentrated in the early part of the night. A chart with its deepest block near dawn is worth a second look at the labels.
- Look at wake. Brief wake bars are ordinary. A single bar may be one 30-second epoch.
- Compare like with like. Use the same device and version when you compare nights, and don’t hold a stage total up against a figure from a paper that used another rulebook.
- Ask what generated the chart. A lab chart is scored from brain, eye and muscle signals. A consumer chart is an estimate.
That last point carries weight. Validation work reviewed in this piece on tracker accuracy found consumer devices tended to overestimate REM and underestimate light sleep, which makes a fine-grained stage chart from a ring or watch a rougher object than it looks. The cycle-shaped bands on any hypnogram also invite the idea that you can time a wake-up to the end of one, a claim picked apart in the 90-minute cycle explainer. A chart this tidy also leaves out any arousal shorter than an epoch, and how you felt on waking.