Here is the one I found. One study that finished in 2016, only 10 patients analyzed.
It’s from Stanford, Kushida is a global authority in #SleepApnea.
I initially thought this was the pilot study, to see if they could have something (it seems they did), but I can not find the study with the “unprecedented number” of patients registered nor referenced anywhere.
https://clinicaltrials.gov/study/NCT01364740?cond=Sleep%20Apnea&intr=Smart%20watch&rank=1&tab=results