Samsung's Gear S2 smartwatch has a circular display as well, and it also has an ambient light sensor for automatic brightness adjustment — the sensor is in the middle of the display. (androidcentral.com)
The new light used with the Kindle Voyage includes automatic brightness adjustment based on ambient light levels, slowly dimming or increasing to avoid drastic changes. (slashgear.com)
In most cases, the Huawei Watch 2 can handle automatic brightness adjustment on its own, but particularly if you're in a dark room, you might benefit from manually dimming the display. (androidcentral.com)