Hello. I realize that, as the documentation says, it makes sense to count our sleep as belonging to the day we wake up, since that day is affected by the sleep we got. And this IS how I prefer it.
However, there are other people who would like it to count on the day they actually FALL asleep (or if past midnight, the previous day). Because their sleep will be affected by how that day had been. Or because that’s just the way their minds naturally process it.
Everyone has different minds, and time is a very ethereal thing. Each of us has our own in-born way to quantify it. And for those who process sleep times differently than provided by the app, they have to do mental gymnastics every day to comprehend it. Amounting to confusion and frustration being at the start of every morning. 
So, I propose that in our Settings, we should be able to choose “Date your sleep period as…”
“The beginning of the day of awakening (default).”
Or
“The end of the previous day”
Or something like that. A simple date offset flag should be able to make the addition pretty simple, though of course I’m not a programmer.
User experience with the app should be positive, not negative. So giving us an option for the smoothest way to read our results would really help those who think differently. 