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


Wake with eyes open)


Beta wave


Wake with eyes closed)




N1 (transition from wake to sleep)


Theta wave


Light sleep (N2)


Theta wave with K-complex spindle*


Deep sleep (N3)


Delta wave




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:


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).


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. 


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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