Ultrahuman Ring AIR - No App Subscription, Smart Ring, Size First with Sizing Kit, Sleep Tracker, Track Recovery, Fitness TrackerA sizing kit will be shipped to you: Choose from the available 10 sizes. Ensure a snug fit of the Ring AIR. Lock in your size: Log in to the Ultrahuman App with the same email as given during your purchase and submit your ring sizes. Sleep Index is derived to help you understand the various sleep stages such as Deep sleep, REM sleep, Light sleep and being Awake. During different stages of sleep, different metabolic processes of the body occur for