Its detailed sleep metrics are what really sets Whoop 4.0 apart.

It provides robust recovery tracking as well, so you'll know if you're rested enough to workout at your best. The tracker provides you with an exertion goal, to make sure you train enough but don't over train. Where Whoop really shines is in its analytics. Whoop 4.0 can be charged while worn with the waterproof battery pack that slides into the Whoop 4.0 while in use. The insight-driven monitor can be detached from the strap, still tracking data even if slipped into your purse, pocket, backpack, or gym bag. Whoop 4.0 is a knit strap with a monitor attached. What's so great about Whoop? We thought you'd never ask.

Whoop 4.0 fitness tracker (1 year subscription), $239 It's designed solely to collect fitness and health data - and it does so at a more rapid rate than most other trackers. There's no screen on the Whoop 4.0, and it won't let you respond to text messages. We'll get into more deets below, but it's important to know that Whoop 4.0 does not replace an Apple Watch or Fitbit. It's on-the-go charging is a big hit with buyers, but it's sleep metrics are the star of the show.

Whoop 4.0 is a wearable that gives feedback on data including heart rate, blood oxygen levels, and progress over time. Understanding the magic of Whoop 4.0 means understanding what it's not, as much as what it is.

Amazon fit watch