Every watch on this page is rated for 8 to 15 days of typical use, which is the point where charging stops being a daily habit and becomes something you do on a weekend. That single change is what makes sleep tracking practical, because the watch is on your wrist at night instead of on a charger.

Long runtime usually comes from a bigger cell paired with a lighter operating system. Watches running a full app platform do more, but they trade most of the week away to do it.

The quoted figure assumes typical use, which means notifications and heart rate on, but the always-on display and continuous GPS off. Enabling either changes the number substantially, so match the claim to how you actually plan to wear it.