That there is an association between energy and aging may seem commonsensical in modern society. Nonetheless, the question of how aging came to be associated with energy is less well known. This article explores how the 19^th century disease of neurasthenia became related to aging through contemporaneous ideas about productivity, energy surplus and energy dissipation based on an analysis of how a lack of energy was featured as a symptom of the disease. It examines the specific historical intersection where a lack of energy was related to a diagnosis, illustrates how aging and energy have become intrinsically tied to each other and how the focus on the productive uses of energy has antecedents in religion as well as moral economics. As aging continues to be considered a problem in modern society--in large part due to the inherent unproductivity associated with old age caused by a lack of energy--the discourses surrounding neurasthenia demonstrate how the concept of energy manifested itself in contemporaneous consciousness.