This study explores the usage and the respective affordances of sleep-tracking with a smart ring and the associated app. Sleep-tracking is a growing area of self-tracking. While the use and influences of self-tracking have drawn considerable scholarly attention, the specific affordances of sleep-tracking are not well understood. Hence, this study aims to explore what affordances can be attributed to using a sleep-tracking device and the correspondence between affordances and features. We used the Gioia method to analyze and theorize on interview data from 14 Oura ring users. We identified four affordances of sleep-tracking executed through a smart ring, namely, dynamic goal setting, self-quantification, learning, and nudging. Drawing on the affordances, we further corresponded affordances with device features. The findings extend the body of knowledge on the affordances of self-tracking.