A data visualization tool, the PhenoFinder, is developed to help researchers find clinically meaningful or heritable patterns in brainwave activity during sleep in a cohort of 1836 nocturnal polysomnography studies. The interactive software lets researchers quickly view and explore various electroencephalography power spectral density patient profiles and select desired phenotypes for genome-wide association or sequencing. The design study addressed here evolved over an iterative process of informal studies conducted with end-users from Stanford's Center for Sleep Sciences and Behavioral Medicine and focused on highlighting the similarities and differences in patient data. Several new hypotheses were formed and new phenotypes considered during the design process. The software is primarily for domain experts; however, a lay user may find exploration of demographic changes in sleep insightful. It is available online with a small companion data-set at http://www.github.com/informaton/phenofinder .