As fitness monitors get more and more complex, Amazon's heading in the other direction with the Halo. The lightweight, screen-free device tracks activity, body composition, sleep, and your tone of voice, leading to a more holistic, laid-back view of overall health. For body composition, it combines a body fat reading from the band with photos from your phone's camera, creating a 3D model of your body. To do all this, it packs in an accelerometer, a temperature sensor, a heart rate monitor, and two microphones, and can run for up to two days with tone of voice tracking on, or up to six with it off. It's available now via an Early Access program with a wide release yet to be announced.