Biometric assessments relevant to Apple Watch sleep tracking accuracy and differences in sleep tracking across Apple Watch models.

Hi everyone, I have three main questions: Is wrist temperature tracking incorporated into the apple watch's sleep tracking/sleep stage classification? What all are the biometric measurements being incorportated to classify sleep/sleep stages (heart rate, temp, movement, etc)? What are the differences in sleep tracking accuracy between S6 and S8 models and which features are responsible for these discrepancies (what biometrics does the newer watch track that the older one doesnt)? I am looking to export my health data and run my own analyses on them to see what/if any new health insights I can gain. Thank you in advance for any responses :)