As sleep is considered an important aspect of our health, a range of products that would benefit our sleep is brought to market. Like many of these products, smart wristbands or fitness trackers make promises to improve the user’s quality of life by improving sleep quality. We performed a sensitizing diary study followed by a user experience evaluation comparing sleep-tracking features of the Fitbit Flex, Jawbone Up, Misfit Shine, and Polar Loop products with six users. We summarize their findings in three recommendations for sleep-tracking functionalities: find the right balance between automation and control, make data intelligible for users, and acknowledge the role of emotions. These design recommendations should make sleep trackers more transparent, and thus more usable and useful to the users in their endeavor to sleep well.