There is no clearcut answer to which day the sleep between them belongs.
We have decided to attach the sleep to the day after, because how you slept will largely determine how your day will be.
This is helpful to know but it would be more helpful to have it seen as the night connected to the day we went to bed.
If you know what country we live in and have the set hour cut offs (which are currently pointless) is there no way you can trial it in part of an upgrade?
I know when I posted a question on it someone else agreed that this is confusing and would be preferable and more helpful to have it connected to that day.
I enjoy this app as it’s easy to use but it’s actually a reason I would change apps as I have to started to have to put a tag stating the day I slept to the night to remind me and it’s incredibly frustrating and needlessly time consuming but I have memory problems and cannot leave it how it is.
While I myself prefer how they have it for their stated reason, I support folks who would like it the other way around. Everyone has different needs when it comes to time comprehension, and it’s not at all easy to force yourself to think in a pattern that’s unnatural to you.
I’ll go make a Feature Suggestion in the relevant subforum, to create an option for the user to select which way works best for them. Feel free to vote for it and comment on it to show your voice included in the idea (Unless such a suggestion is already there, then I’ll of course vote for it. 
)
I don’t know anyone in the USA (where there is a huge base of SAA users) who thinks of Saturday night as Sunday night. Maybe in other parts of the world there is a different conception of time perception,.but given the number of users in the USA, please add a user switch to be able to change what day a sleep record is assigned to. It is just so confusing now especially if looking back at the sleep graphs trying to figure out what day/night belongs to each record. I can’t find the feature request that ZaCloud was going to create, but I found another one on the topic, so I will vote for it. I hope others will find it too.
I would vote for a user switch so both options are available.
But contrary to the opinions above, I am happy with and find it helpful the data is labeled with the end of sleep day.
Sleeping in a single period starting one day and ending the next doesn’t apply to a large number of SAA users - especially those with sleep disorders… or night shift workers.
Having been in both groups and seeking out help for my disordered sleep, I know we are greater in number than most people realize.
Bottom line is there are too many different use-cases to please everyone. Yet, the reasoning that sleep in preparation for the ‘end of sleep day’ determines how that day goes is (nearly) universally true.
@MMcF more over we have recently fixed a bug in the Settings > Personalization > Stats > Sleep day cutoff which makes it IMHo more useful… so you can change the grouping time for dame day sleep from midnight to earlier up to 12:00pm So even when you sleep in the afternoon, then do a short gap and then continue the next day both sleeps will be part of the same day…