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Timestamps:
00:00 Intro
00:52 How Apple Tracks Sleep
02:24 Test Design
03:49 Deep Sleep Test
06:21 Light Sleep Test
06:48 REM Sleep Test
09:05 Sleep Cycles Test
09:53 Awake Detection
11:53 37 Watches Tested
13:25 Sleep Timing Test
14:34 Limitations
15:29 Missing Features
16:06 Am I Biased?
16:52 Conclusions

Intro: When Apple first announced their new sleep tracking functionality, they did not provide much detail as to how it actually works, but let’s briefly summarize what they did tell us, and what this means.The best way to measuring sleep stages, what scientists call the gold standard, is called polysomnography. This basically means measuring the electrical signals coming from the brain while you are sleeping, and in addition also measuring the movements of the eyes and of some muscles. This way scientists can get an accurate picture of what sleep stages you went through. So, to make sleep tracking more comfortable and readily available, Apple trained a computer algorithm that takes in the heart rate and movement data from the Apple Watch, and uses that to predict your sleep stages. To create this algorithm, they collected both polysomnography data and heart rate and movement data with the Apple Watch, on what they say is “the largest and most diverse study population for a wearable”. That algorithm then uses the sleep stages measured with polysomnography, and tries to predict the same sleep stages with just heart rate and movement data from the Apple Watch. If Apple has enough data and good machine learning tools, they could have potentially created a very good sleep tracking algorithm, making sleep tracking available to the general public.

So, how did I test this? Well, I evaluated the sleep stage tracking of the Apple Watch for 18 nights by comparing it to the Dreem 2 EEG headband, a device that can actually measure my brainwaves, and was specifically designed for sleep stage tracking. The EEG device should therefore give us an impression if the Apple Watch is good enough for sleep stage tracking. In this case I used the Apple Watch 7 as the test device, but I think both older and newer Apple Watches will give more or less the same result, since the heart rate and movement measurements are roughly equally reliable between generations, as we saw in previous videos. Measuring heart rate and movement is especially easy during the night, when you are generally relatively still with a low heart rate. I should mention that I am using the beta version of WatchOS 9, but I suspect nothing will change with regards to sleep tracking in the official release, since the algorithm was already changed.