Biphasic sleep, characterized by nighttime sleep plus daytime napping, has demonstrated some cognitive, health and performance impacts when compared with consolidated monophasic sleep. This motivated the development and validation of the Biphasic Sleep Scale, reported in this paper. Scale development involved a literature review, expert input and individual interviews. The 16-item Biphasic Sleep Scale was then administered to an international online sample (n = 6965) alongside well-established validated sleep scales. To ensure a robust evaluation of the Biphasic Sleep Scale, the sample was divided into two parts: with 15% (n = 1000) of the participants allocated to the exploratory analytic phase; and the remaining 85% (n = 5965) reserved for confirmatory analyses. Psychometric evaluation included both exploratory and confirmatory factor analysis, reliability analysis, correlations, network analysis, and item response theory. Exploratory factor analysis indicated a three-factor structure assessing daytime napping as to likelihood, consequences and effect on nighttime sleep. Confirmatory factor analysis largely confirmed this model with no sex invariance. The three-factor structure showed adequate fit. The Biphasic Sleep Scale demonstrated good internal consistency (alpha = 0.88, Omega = 0.89). Network analysis revealed varying centrality and connectivity of items. Item response theory found items covering a range of biphasic sleep levels. Significant positive correlations with sleep criteria provided evidence for convergent validity. Further testing is warranted to confirm the factor structure, refine model parsimony, and establish clinical utility. With additional validation, it is hoped that the Biphasic Sleep Scale will become a widely utilized tool.