Earlier quoted context omitted.
There's a device with app which can reliably determine if you are in REM: https://dreem.com/ . I have sleep apnea and bought it in order to keep track of the quality of my sleep.
What do you do with the data? After knowing the quality, how do you improve it?
Additionally, I can also quite reliably see when my CPAP needs adjusting by my doctor.
The Dreem 2 has two functions I'm rather fond of:
1) It can wake you up in light sleep within a specific timeframe you've set. That made me less groggy and grumpy in the mornings.
2) It can generate pink noise which is said to enhance the quality of your deep sleep (there are a handful of studies about it).
Both features work well for me but I think the pink noise function is not available in the US for some FDA-related terminology reason.