Fore R, Hart JE, Choirat C, et al. - Given environmental exposures, including built and natural environments, may have an impact on certain behavioral risk factors for cancer, ie, physical activity and sleep, but many investigations in this field are limited by residence-based exposure evaluation and/or self-reported, time-aggregated measures of behavior, so, researchers undertook a pilot study, the Nurses' Health Study 3 Mobile Health Substudy, with 500 participants. To measure minute-level location, physical activity, heart rate, and sleep, the participants used a smartphone application and a Fitbit for seven-day periods, four times over a year. From 435 participants, data were obtained. During 819 sampling weeks, experts reported an average of 7,581 minutes of heart rate and step data per participant-week, and > 2 million minutes of sleep in over 5,700 sleep bouts. For 5,237 unique participant-days, location data were documented, averaging 104 location observations per participant-day. In this work, a protocol was defined to include mobile health technology into a nationwide prospective cohort to measure high-resolution objective data on environment and behavior.