Over the past decade, we've seen a boom in gadgets that claim to track all sorts of health metrics, from heart rate to calories burned. Many of these wearables can also be used to measure the quantity and quality of your sleep. But how well do they really work? Michael Breus, Ph.D. (aka “The Sleep Doctor”, shares his honest review of four popular products in the following excerpt from his book, Sleep Drink Breathe. To improve at anything, it’s best to have metrics to measure your progress.