OS X Daily:

With Power Reserve Mode enabled, Apple Watch will cease all functions except a limited onscreen clock display. While you can enter Power Reserve at any time, this is not something you’d probably use for fun since it disables basically all features of Apple Watch like fitness tracking and heart rate monitoring, messaging, notifications, etc. With that said, it’s an incredibly effective way to continue to use the Apple Watch as a time-telling device in situations when it would otherwise run out of battery.

Good to know how to do this.