As far as my understanding goes the bottleneck for what you are talking about is hardware not software, so open source won't help that much for the foreseeable future.
Since this talks about sleep quality I want to add something that was almost unknown to me for over 30 years. Smartwatches AFAIK don't have CO2 sensors. It seems to me though, that that would be extremely useful for sleep quality tracking. I have just a couple of years ago found out that CO2 levels are having the highest impact on my sleep quality besides temperature. I can highly recommend getting a CO2 sensor to get a good feeling for that effect. Also understand how much air is available in your room and how much CO2 your body produces per hour. I totally underestimated this until I was able to measure it. In a small sleeping room with closed doors the amount of CO2 will reach unhealthy levels at the end of the night.