Context: - Apple's bedtime isn't good enough. Need a better solution. - Must integrate with Apple HealthKit. - I travel a lot, so the ideal solution is compact. - Optimizing for data accuracy 1st, data points 2nd. - Wearables like Oura ring are fine, but I prefer non-wearable.
Out of curiosity: What makes it not good enough?
I use a Fitbit, and I really have no idea how good the data is: I'm asleep at the time it's being collected.
Disclosure: #FitbitEmployee but I don't speak for Fitbit.
Any Fitbit with heart rate will give you maximum acccuracy according to https://www.wareable.com/fitbit/fitbit-sleep-tracking-defini...
Heart rate is key to getting good sleep stages data, so a non-wearable won't do. https://blog.fitbit.com/heart-rate-during-sleep-stages/
To integrate with Apple HealthKit you need a 3rd party app.