Biphasic sleep is defined by two distinct sleep periods within a 24-hour cycle. It typically includes a nighttime sleep episode and a daytime nap. Biphasic sleep holds historical, cultural, and biological significance. The Biphasic Sleep Scale (BiSS) is an innovative 16-item self-report instrument designed to evaluate perceptions, behaviors, and outcomes associated with biphasic sleep practices. The BiSS was developed through a comprehensive process that included literature reviews, expert consensus, and qualitative interviews. The BiSS assesses three primary dimensions: the likelihood of daytime napping, the consequences of napping, and nighttime sleep disturbances. Initial validation using a large international sample (n = 6965) revealed robust psychometric properties, including a 3-factor structure, good internal consistency, and convergent validity with established sleep measures such as the Pittsburgh Sleep Quality Index (PSQI), Epworth Sleepiness Scale (ESS), and Athens Insomnia Scale (AIS). This chapter details the development, psychometric evaluation, and potential applications of the BiSS, emphasizing its contribution to advancing research on biphasic sleep patterns. Future research directions include longitudinal studies and validation within clinical populations to enhance its applicability in sleep research and clinical practice.