Because the Deep Sleep values apple is reporting is points to pathological issues.
I read white papers of hypnograms from different resources and Deep sleep occurs more often than not in the beginning 50% of the sleep session and happens relatively quickly when you do sleep.
Apple fails to detect that. In fact apple picks it up for me after a while while Garmin picks it up as you'd expect.
Also referring to this white paper:
https://assets.firstbeat.com/firstbeat/uploads/2019/11/Firstbeat-Sleep-Solution_white-paper_short.pdf
You see the typical sleep hypnograms and note that in both instances, deep sleep is detected properly in the beginning half of sleep. I get that with Garmin devices yet Apple Watch ultra only picks up SLIVERs of deep sleep in time frames that are frankly wrong. My sleep profile correlates strong with the Quantified Scientists's latest vid of Theresa as I link here:
https://youtu.be/BQMIAkt4P48?t=893
Theresa sleep profile shows Apple Watch Ultra have very poor deep sleep detection.
Also, if you read online from many people, there is a consistent theme that Apple fails to detect enough Deep sleep when measured against other wearables and people are also questioning this.
I am just showing some data to go against but you are right, how can we trust Garmin. But one thing to note is that 10% of deep sleep is apples high end for deep sleep for many many users and i think the typical deep sleep percentage for a session is 16-33% of total sleep time.
Yes I did a ton of sleuthing reading and seeing what others / studies have shown.
Apple just claims to be gold standard tested but where do we see it besides the quantified scientist where he can be the N=1 where the algo works great. Yet even in his test Theresa shows us that it doesnt work well.
Where do you see the validation studies outside of Quantified Scientist youtube showing good correlation against PSG?
