I had a couple of days of 100 percent sleep quality where I felt groggy the next day, tired, unrested etc. Not sure how they work out that.
I don't think they'd know if you knocked yourself out drinking and slept the whole night through, and woke up with a headache. It is a computer program working on a limited sets of inputs - sound and motion - and their sleep quality scoring system is based on assumptions that may or may not apply to you at any given night.
I've been testing a different sleep app, Sleep as Android. I thought it would work well as it should be able to hear when I wake up and go to the bathroom, and my Galaxy S5's accelerometer should confirm what the mic hears. Last night, I woke up to pee. I didn't try to soften my motion, yet it didn't notice that I woke up. Maybe the app was also asleep at that time, who knows? Still, it was satisfactory as it was the first night it did that in a week. I wonder if a pillow snuffed the smartphone that time and made it deaf.
Overall though, I think the app helped. It kept track of when I wake up for the most part. It detected my snoring and recorded it. It kept track of how long I slept. Without it, I wouldn't know that I snored. Funny thing though, last night I had my mouth taped snuggly. I thought I slept very well. I was disappointed though when it said I snored 3% of the time, and that was the highest snore score I got. And I heard and verified my snoring as recorded.
That left me wondering how I could have snored with my mouth taped all that time. Maybe it was my cat? Do cats snore? It sure sounded human.