Xiaomi will make Your data opensource.

If there is product like this (foss based, no cloud) i want to know too

Pine64 pinetime supports custom OSs but I don’t know of any with sleep tracking. It is only like 30 bucks though.

Edit: GitHub repo for a wasp-os sleep tracker for pinetime: https://github.com/thiswillbeyourgithub/sleep_tracker_pinetime_wasp-os

The best way I know of is finding a device that's supported by gadgetbridge: https://codeberg.org/Freeyourgadget/Gadgetbridge#supported-devices (be sure to check the warnings, some devices require you to pair with the OEM app first or possibly even keep the OEM app around)

Though, I used this with an Amazfit Bip and didn't find the data particularity insightful. It often was just straight up wrong about whether I was sleeping or not. So, I too am curious to see what other answers come in.

I have a PineTime and a Xiaomi Smart Band 7. There has been little work done on sleep tracking for the PineTime using InfiniTime.

I'm tracking my sleep using the Smart Band 7 and Gadgetbridge not Xiaomi's app. You do have to use Xiaomi's app to set up the band the first time, but that's been the case since the Mi Band 4.

Xiaomi's bands combined with FOSS Gadgetbridge is an excellent combination.

After going with Xiaomi devices for some years (with Gadgetbridge) I revived my trusty Pebble Time Steel only yesterday. This thing is 8 years old and doesn't do heartrate but it still tracks my sleep more precise than any of Xiaomi's devices ever did. Maybe you can find a device in good condition on eBay? Be sure to ask sellers for daylight pictures of the screen with settings menu or a bright watchface. If the battery is dying the screen looks like nacre cause it's being lifted by the battery.

I like my Fitbit Charge 5. Honestly, really the only reason that I went with a health/fitness tracker instead of a smartwatch is battery life. As much as the extra features of a smartwatch would be nice. The problem with smartwatches is that you have to charge them daily (something I already do with my phone, and I'd rather not add another device that I have to do that for). Whereas, with a health/fitness tracker (like the Fitbit Charge 5) I only have to charge it like... every couple of days (I haven't kept track, sadly, but I'd estimate at least a minimum of 5 days)

I keep hoping that smartwatches will get good enough battery life like health trackers that they'll be viable (the pixel smartwatch looks nice), but we haven't reached that day yet.