Tim Hardwick, MacRumors:

When enabled, Optimized Battery Charging learns from your personal device habits and waits to finish charging all the way up until you need your ‌Apple Watch.

For example, if you regularly charge up your watch at night while you’re asleep, the device may opt to limit its charging to the 80 percent mark, and then wait for an hour or so before you wake up to charge the remaining 20 percent.

Read the post to find this setting. Worth knowing about.