Polysomnography

A polysomnography setup records several signals at once: brain activity by EEG, eye movement by EOG, chin and leg muscle tone by EMG, airflow and chest effort, blood oxygen, and heart rhythm. The night is then scored in 30 second epochs against standard criteria, and every epoch comes out labelled as wake, N1, N2, N3 or REM.

That scoring is where sleep stages come from, and nothing else produces them. Deep sleep is defined by slow waves on an EEG trace, and REM by the combination of an active brain, moving eyes and almost absent muscle tone. A device without electrodes on your head is inferring, not reading. The scored night becomes a hypnogram, plus the numbers a clinician actually looks at: sleep efficiency, wake after sleep onset, how long you took to fall asleep, and breathing events per hour.

What polysomnography is for

It is a diagnostic test, not a curiosity. Clinicians order one when they suspect something structural: sleep apnea, periodic limb movements, narcolepsy, unusual behaviour during sleep. Home sleep apnea tests are a stripped-down relative, usually airflow, effort, oxygen and heart rate without the EEG, which is why they can support an apnea diagnosis but cannot stage your night.

The tradeoff is obvious once you have been through one. You get one or two nights, in an unfamiliar bed, wired up, and the first night in a lab is often not a typical night for the person being recorded. That is not a flaw so much as the cost of measuring properly.

This is why actigraphy and phone tracking sit alongside polysomnography rather than in competition with it. One measures a single night accurately; the other watches thirty nights approximately and shows you the pattern. Tock is firmly in the second camp, its stages are estimates on a phone-only night, and it is not a medical device. If a doctor has suggested a sleep study, that is the measurement. If you just want to see your own trend, sleep tracking without a wearable covers what a phone on the nightstand can honestly tell you.