http://www.usatoday.com/story/tech/2015/04/26/my-weekend-with-apple-watch—verdict/26429827/:

Saturday, I was 100% charged at 10 a.m. and dead by 7 p.m. That’s not something most watch users would expect from a wristwatch. Apple believes that because I used the workout app to monitor my activity for a four-hour stretch, that was the culprit that ate away at the battery. So Sunday, I took the company at its word and didn’t run any of the background apps. I checked email, texts and responded to some, made a few phone calls and, as I mentioned, monitored my steps. Fully charged at 10 a.m, by 5:30 p.m. I had 66% capacity. It’s not much of a surprise that the Apple Watch’s battery can last all day, as promised by Apple, or die at night if you heavily used it during the day.