[fredfoobar42]

I actually do sleep tracking with my Apple Watch using the Sleep++ app. The Apple Watch's battery is good enough, and charges fast enough, that all I have to do is give it, maybe, 30 minutes of charging when I wake up, and another 30 before I go to bed.

I just woke up after eight and a half hours of sleep, and had 88% battery left. Now, this was also with the Watch in Airplane Mode, but I've forgotten some nights, and still woke up with battery percentages in the same ballpark.

(Data point: I wear the 42mm model. The 38mm has a smaller battery.)