Sleep efficiency (SE) is a central variable in both the delivery and titration of Cognitive Behavioral Therapy for Insomnia (CBT-I). Despite its critical role, there remains a surprising lack of consensus on how SE should be calculated. This paper examines the conceptual and operational inconsistencies surrounding SE, particularly the widespread use of "Time in Bed" (TIB) as the denominator in its computation. We argue that literal interpretations of TIB—often including pre-sleep or post-sleep activities—introduce clinically significant distortions in SE values. These distortions can lead to erroneous treatment decisions during CBT-I, such as inappropriate titration of the sleep window. We review the standard sleep diary variables, examine previously proposed alternatives like Reed and Sacco’s “Duration of the Sleep Episode” (DSE), and identify their limitations. Building on these insights, we propose a new formula for SE that more accurately reflects the patient’s intended sleep window, using “Sleep Opportunity” (SO) as a more valid denominator. We demonstrate through clinical examples how our proposed approach offers more accurate and actionable SE metrics, aligning with both patient experience and clinical judgment. Ultimately, we argue that close enough is not good enough, and a consensus definition is essential for clinical precision and research standardization in CBT-I.