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.
Would you please clarify some confusion I have about this? For example, say I normally go to bed around midnight or shortly afterward, but fell asleep earlier from 6pm to 9:30 pm. I wake up and stay awake until after midnight, then fall asleep again until 8am (say I start that 2nd sleep manually). Is the first sleep (6pm - 9:30pm) counted towards that calendar day or added to the next morning’s sleep duration ending at 8am?
Also, if I take a nap or naps during the daytime, is that added to that morning’s already recorded sleep duration? How does Sleep know to what day to attribute the duratation? It’s there an easy way to override the attributed day?
On default setup, all sleep entries from one day (before midnight) are counted towards one day, and all entries from another day (after midnight) are counted towards the next day.
This applies to all sleep entries, naps, and manually added sleep times.
If this does not suit your sleep schedule, you can change the time, which is considered the “day cut off” - in Settings > Personalize > Stat > Sleep day cutoff
If you track a nap during the day, it will create an individual sleep entry, but the duration for that day will sum up your night’s sleep plus the nap - so it is counted towards your sleep duration together, to the same day.