This article describes the adaptation of a cardiac Holter monitor to more conveniently collect and store infant sleep data. The cardiac monitor was first adapted to connect to a sleep mattress, then refined until readable infant sleep signal data were produced. System data were compared to data collected by a live observer of infant sleep (kappa = 0.84). The IRB-approved testing used newborn infants, conducted with parental consent. Development of a rule-based computer program was designed to categorize the physiologic data into infant sleep wake states. The cardiac monitor was found to be lightweight, portable, battery powered, nonintrusive, and safe for collecting infant sleep data. Preliminary assessment of the validity of the program in scoring infant sleep was compared to hand scored data (kappa = 0.76). Refinement of the system and software program is ongoing and expected to greatly facilitate the study of infant state behavior.