Classification of sleep stages

Since the 1960s, sleep experts have been working with polysomnography to determine the different stages of sleep and wakefulness. Brain waves or EEG (electroencephalogram) represent the key components of this sleep staging. Here’s how the brain waves for each sleep stage typically look:

Sleep stage

Look of EEG (1 second)

Wave name

Frequency

Wake with eyes open)

beta_big.png

Beta wave

>13hz

Wake with eyes closed)

Alpha_big.png

Alpha

8-12hz

N1 (transition from wake to sleep)

theta_big.png

Theta wave

4-7hz

Light sleep (N2)

theta_with_K_complex_big.png

Theta wave with K-complex spindle*

4-7hz

Deep sleep (N3)

delta_big.png

Delta wave

<4hz

REM

REM_stage_big.png

Sawtooth wave

Low intensity and mixed frequency

*A K-complex spindle is a large, slow peak followed by a smaller valley. It lasts at least ½s.

Here’s what your Dreem Band detects during the night via its different sensors:

Capture_d_e_cran_2018-11-19_a__14.06.50.png


100 values are extracted every 30 seconds by the headband’s different sensors (electroencephalogram, accelerometer, pulse oximeter), our algorithm then reproduces the expertise of a consensus of 5 sleep experts. Specialists have been hired at Dreem to train this algorithm every day by scoring (pre-anonymized) nights spent with the headband.

Hypnogram

The hypnogram is a minute-by-minute review of your night. Here you can see which sleep stages you were in from sleep onset until the morning.

hypno_zoom_fr.png

Of course, the changes between sleep stages are not that abrupt in real-life. For example a consolidated light sleep stage (N2) can look a lot like the beginning of deep sleep stage (N3). Also, some sleepers might have a deep sleep (N3) with a lower amplitude or frequency leading to it being interpreted as light sleep (N2). Age, for example, can be a factor for these patterns. These edge cases can sometimes be subject to different interpretations by different sleep specialists.

This inter-scorer variability is the reason we train our algorithms on a consensus of 5 scorers.

Because the definition of sleep stages is subject to interpretations that can evolve over time, our team frequently updates the algorithm to include new scored nights and new scoring rules adopted by scientists.

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