John Gruber on one of two reasons Apple went with a dark UI:

Apple’s approach is more conservative energy-wise in both “on” and “off” states

Apple’s decision to have the screen display nothing while “off” was clearly a concession to battery life.

The battery-life advantages of this design are just a nice side effect.

Ok, pop quiz time. Before you take it, extra points if you read the whole of Gruber’s article. Which watch gets more battery life? The Apple Watch that is designed with a dark UI that is more ‘conservative energy-wise’ and makes ‘concessions to battery life’. Or an Android Wear watch like the LG G Watch R that has bright UIs and leaves its screen on the whole time?

If you answered the Apple Watch, you’d be wrong. The LG’s battery lasts far longer than the Apple Watch. Most days when I go to bed, there’s still 60% of it left. The quoted battery life on Google’s site is 24 hours (vs Apple’s 18) and I easily get more than that.

So the competitors have a watch that can keep the screen on with bright, vivid apps which run natively. It can connect to bluetooth devices and even WiFi connections and it still gets more battery life than an Apple Watch which has an all dark UI and turns its screen off all the time? What’s wrong with this picture?

Update: When is a mystery no longer a mystery? When iFixit teardowns the 38mm Apple Watch and finds a 205 mAh battery, compared to the 410 mAh in the LG. It all makes sense now. Thanks to Richard Gaywood for the tip.