[dariosalvi78]

Sleep cycles are very hard to measure reliably, basically no wrist worn device should be trusted for that.

[cj]

I find wrist sleep apps accurate enough for showing me trends over time and correlations between things that make sleep worse (e.g. drinking before bed shows different sleeping patterns than not drinking). Interesting and useful data but I agree it's not scientifically precise.

[dariosalvi78]

I agree, which is why I think that it would be more interesting, and probably more honest, to show some sort of "sleep quality" index instead.