Classification of sleep stages

Sleep experts have been working since the 1960’s, mostly relying on polysmonography to determine your stages of sleep and wakefullness. Brain waves or EEG (electroencephalogram) represent the key components in this sleep staging.

Here are how the brain waves usually look like for each sleep stage:

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 headband detects during the night via its different sensors:

Capture_d_e_cran_2018-11-19_a__14.06.50.png

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

Hypnogram

The hypnogram is a review of your night, minute by minute where you can discover in which sleep stage you were from sleep onset until the morning. 

hypno_zoom_fr.png

Of course, those changes of sleep stages are not that brutal. For example, a consolidated light sleep stage (N2) can look a lot like a beginning of deep sleep stage (N3). Also, some sleepers might have a deep sleep (N3) with a lower amplitude or lower frequency, leading to an interpretation as a light sleep (N2). Age can for example be a factor of those patterns. Those edge cases can sometimes be subject to a different interpretation by sleep specialists.

It’s exactly to tackle this issue that we train our algorithms on the consensus of 5 scorers to decrease this interscorer variability. Because the definition of sleep stages is subject to interpretation that can evolve over time, our team update frequently the algorithm to include new nights scored and new scoring rules adopted by scientists.

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