A summary, not the whole recording
A hypnogram plots the stage assigned over time. Real sleep scoring considers multiple signals, usually in 30-second epochs; brief arousals and breathing events require additional detail.
How a night of sleep unfolds
A night of sleep, then a closer look at the signals behind it.
Every night and signal below is a synthetic educational illustration, not a patient recording, study average or diagnostic test.
Follow an illustrative night through irregular cycles, with more deep sleep early and longer REM periods later.
The whole night
23:48 · N3 · deep sleepSwipe horizontally to explore the whole night
Hover to explore. Click to keep a moment selected. Use the slider, or focus the chart and press ← / →.
N3Selected · 23:48
Deep sleep, marked by slow waves
N3 is also called deep or slow-wave sleep. Longer stretches generally occur earlier in the night, and the amount changes with age.
- Brain
- Large, slow waves are the defining EEG feature.
- Eyes
- Prominent rapid eye movements are absent.
- Muscles
- Muscle tone is lower than wakefulness but is not normally suppressed as strongly as in REM.
What this can tell us
Slow-wave processes are studied in memory and other sleep functions. Increasing N3 alone is not proof that a medicine improves sleep or daytime function.
Inside a sleep study
30 seconds · synthetic signals · separate display scales
Swipe horizontally to follow the signals
Brain wavesEEG
Slow waves
Eye movementsEOG
Chin muscle toneEMG
0s5s10s15s20s25s30s
A slow rhythm is emphasized here; a real N3 epoch contains more varied activity. These drawings are not calibrated clinical traces.
What each sensor tells us
- Brain waves · EEG
- Scalp electrodes record electrical activity used to identify stages and brief arousals.
- Eye movements · EOG
- Sensors around the eyes help distinguish slow eye movements from REM bursts.
- Chin muscle tone · EMG
- Chin electrodes record muscle activity; very low tone helps identify REM.
Reading sleep architecture
A pattern, not a perfect schedule
The order and duration of stages vary between people and nights. Age and recording conditions matter. These examples contain no recommended stage percentages.
More deep sleep is not a health score
A randomized insomnia trial increased slow-wave sleep without improving the usual sleep measures or reported daytime function. Architecture, symptoms, function and safety answer different questions.
Illustration methods, sources & limits
Educational content reviewed 2026-10-03. This date does not represent a new trial result or a full sleep-topic research check.
- All overnight segments, clock times, waveforms and event durations were authored for education. None were extracted from a participant, trial or source figure.
- Signal heights use display units and separate channel scales. They are not calibrated voltages, airflow measurements, effort amplitudes or oxygen saturations, and cannot be used to score a clinical recording.
- The stage order on the vertical axis is conventional, not a numerical depth or health scale. REM is a separate phase, not an intermediate level of non-REM depth.
- The obstructive event is a 30-second teaching vignette, not a measured event or a simulation of every event in OSA. It does not calculate AHI, hypoxic burden, insomnia severity or treatment benefit.
- A standard laboratory polysomnogram includes more electrodes and channels than shown here. Some home apnea tests do not measure EEG stages; consumer stage estimates are not interchangeable with laboratory staging.
- No real deidentified polysomnography is included in this version. Adding one requires confirmed reuse permission, privacy review and signal-level provenance.
NHLBI · Sleep phases and stages ↗Official physiology explanation · 2022-03-24
NHLBI · What sleep studies measure ↗Official sleep-study explanation · 2022-03-24
NHLBI · Obstructive and central sleep apnea ↗Official disease explanation · 2025-01-09
American Epilepsy Society · Defining EEG features of sleep stages, Table 1 ↗Society-authored EEG atlas; indexed table reviewed · 2016
AASM · Sleep stages, arousals and respiratory-event scoring ↗Official scoring-manual overview; not the licensed full manual · 2023-02
AASM Sleep Education · Sleep cycles ↗Official educational explanation · source date not stated
Hertenstein et al. · PSG reference data in 206 healthy adults (2018) ↗Primary human research; indexed full text reviewed · 2018-04-15
Ruch et al. · N2 rhythms and memory retention in a human nap study (2012) ↗Primary human research; abstract reviewed · 2012-06-27
Walsh et al. · More slow-wave sleep without improved usual insomnia measures (2006) ↗Randomized human trial; abstract reviewed · 2006-01-15