All the features you expect in a small, stylish package. That's what the Fitbit Alta HR Fitness Tracker promises. The world's slimmest wrist-based heart rate tracker, it required the development of a one-of-a-kind chip to pack in all its functionality. In addition to keeping tabs on your heart rate, calories burned, steps, and distance, it also uses data from the accelerometer and your heart rate variability to track how long you spend in the various stages of sleep, then gives you feedback and tips on how to maximize your time in bed. Available in a wide variety of colors and styles, paired with rubber, leather, or stainless steel wristbands and bracelets.