The first article I read was called Fitness Trackers & Sleep: How Accurate Are They? Many trackers measure not only the number of steps you take every day and the calories you burn, but also your sleeping habits. Although from first hand experience I have seen the fitbits count people raising and lowering their hands as a lap. This makes the count inaccurate.
Another article I looked at was Study Confirms All Suspicions: Fitness Trackers Arent Magic Bullets. This states that if you want to be successful with wearables, make sure that you also understand the factors behind creating fitness transformations. People often think that if you are wearing a fitness tracker that you can get motivated but the average time people wear it is 6 months before they throw their fitbit away.