How do smart bands measure heart rate and activity without a dedicated chest strap?

29 Eki 21:45

I’m trying to understand the core technology behind smart bands. Specifically, how do the optical sensors and accelerometers work together to estimate heart rate and steps, and what are the limitations compared to traditional fitness devices? Also, how reliable is sleep tracking when it’s based solely on movement and heart rate data? Would you say the current algorithms are sufficient for casual users, or is there still a noticeable gap? Looking for a general overview.