Sleep science

Deep sleep is defined in microvolts, measured at the forehead. Your ring is on your finger.

Sleep stages are scored from brainwaves. Rings and watches read pulse, motion and temperature, then predict the label a scorer would have written.

By the Muse editorial team · 8 min read · August 20, 2026
Every figure below is sourced at the end

You wake up, you reach for the phone, and the number is already waiting. Eighty-two. One hour and twelve minutes of deep sleep. You feel like you were run over, but the ring says the night was fine, so you file the feeling away and start the day.

Here is the part that is not on the box. That number was not measured. It was predicted.

Sleep stages are brain states. They are named, defined and scored from brainwaves. A device that never touches your head cannot read them. It can only estimate them from what your body does downstream.

This is not a scandal and it is not a scam. The estimate is often a good one, and the companies that make it publish how good. But an estimate has a property a measurement does not have: when it is wrong, nothing on your screen tells you.

What deep sleep actually means

There is an official answer, and it is unusually specific.

Sleep clinics score a night in thirty second blocks called epochs, following the manual published by the American Academy of Sleep Medicine. For the deepest stage, N3, the rule reads like an engineering spec: slow waves between 0.5 and 2 hertz, more than 75 microvolts from peak to peak, filling at least twenty percent of the epoch. Twenty percent of thirty seconds is six seconds.

Two things follow, and they matter more than they look.

First, deep sleep is not a feeling or a duration. It is a shape in a voltage trace, counted six seconds at a time. The hour and twelve minutes on your screen is a tally of epochs that met that shape.

Second, the manual says where to look. The rule for deep sleep is written for the frontal derivations, the electrodes on the forehead. Deep sleep is scored, by definition, from the front of your head.

What each device is physically touching

Your ring is a good instrument. It is simply not pointed at your brain.

Oura publishes what is inside it, and the peer reviewed paper written by its own researchers is explicit: the signals are collected via infrared photoplethysmography, a negative temperature coefficient sensor, and a 3-D accelerometer. In plain language, light shone through your finger to watch the blood volume rise and fall with each beat, a thermometer against your skin, and a motion sensor. From the pulse the algorithm derives heart rate and heart rate variability. From those, plus movement, temperature and where you are in the night, a machine learning model predicts which stage a human scorer would have written down.

Every one of those measurements is real. The last step is not a measurement. It is a prediction.

At the finger

blood volume pulse, read with light

At the forehead

cortical voltage, read with an electrode

eight seconds of the same night

Put the two routes side by side and the argument stops being a matter of opinion. Both end in the same word printed on your screen. Only one of them starts at the organ that word describes.

Ring or watch

EEG headband

Light through the fingerphotoplethysmography

Voltage at the foreheadelectroencephalography

Pulse, variability, motion, skin temperatureall real, all downstream of the brain

Wave shape, frequency, amplitudethe exact quantities the rule names

A model predicts the stagetrained to guess what a scorer would have written

The scoring rule appliesthe criteria a technologist uses on a lab recording

“Deep sleep: 1h 12m”an inference

“Deep sleep: 1h 12m”a reading

Same words on the screen. One route passes through a prediction.

That length has a cost you can feel. Your heart rate answers to more than your sleep: the temperature of the room, a late glass of wine, a cold coming on, the fact that your resting rate has drifted since the model met people like you. Every one of those lands in the input of a stage prediction. None of them lands in the definition of a stage.

Both approaches sit the same exam

Fairness needs a common yardstick, and there is one. It is called polysomnography: the overnight lab study where a technologist attaches electrodes to your scalp, face and chin, and a trained human scores the night epoch by epoch. It is the reference every sleep wearable is graded against, and both of the devices on this page have been through it. Both results are published. Here they are, unedited.

Where the difference shows up in an ordinary week

What is actually in the band

Seven sensors, and it is worth reading the list slowly, because the interesting part is not only what is there.

• EEGFour channels: Fp1 and Fp2 on the forehead, T9 and T10 behind the ears, with a reference at Fpz. Dry sensors, no gel.
• fNIRSNear infrared light that follows blood oxygenation in the frontal cortex. Muse describes Athena as the first device to combine EEG and fNIRS.
• PPGA pulse sensor, for heart rate.
• MotionAccelerometer and gyroscope, for breathing rhythm and body movement.
• BatteryUp to 10 hours, which is what an all night recording costs.
• BandSoft knit fabric with no rigid puck on the forehead, made to be slept in, in Carbon or Opal.

Notice the third line. The band carries a pulse sensor too. That is the point, and it is worth being straight about it: wrist and finger signals are not worthless, and Muse reads them as well. The argument was never that heart rate is useless. It is that heart rate should not be the only thing in the room when a sleep stage gets its name.

What a live read makes possible

Staging the night for the morning is one thing. Knowing the stage as it happens is another, and it is what the sleep features are built on. Three of them, each an optional toggle.

But can you actually sleep in it

It is the first question every side sleeper asks and it deserves a straight answer rather than a paragraph of adjectives. It is soft knit fabric with no rigid housing over the forehead, it is built to be worn all night, and plenty of side sleepers wear it without trouble. It will not suit everyone, and that is what the thirty day window is for.

What owners say

Muse holds a 4.0 rating across 2,332 reviews on Trustpilot as of August 20, 2026, which includes the people it has not suited. The reviews above are quoted in full and belong to their authors.

More than 200 peer reviewed studies have been run with Muse hardware, according to choosemuse.com, which is an unusual sentence to be able to write about a consumer device.

What this is not

Muse is a wellness product. It is not a medical device, it does not diagnose, treat or prevent anything, and nothing here is medical advice. If you snore heavily, if you stop breathing at night, if you fall asleep during the day, a consumer device of any kind is not the answer and a clinician is. The claim on this page is narrow on purpose: it is about where a measurement is taken, and what the published research says about how well it matches the laboratory.

Sleep on it for a month. If you would rather go back to a number you cannot check, the window is there for that too.