It's also compatible with just about every model of Apple Watch, with a 40mm size for the Series 4, 5, 6, and SE, 41mm and 45mm sizes for the Series 7 and 8, and a 49mm size for the Apple Watch Ultra.

If you’re interested in using Apple Watch Series 8 or 7 for sleep tracking, you also get 8 hours worth of use from an 8-minute charge (Ultra too). Apple Watch Series 8 vs 7 and more – Key features ...