Total hours of sleep per 24-hour period, measured via polysomnography (gold standard), actigraphy (wearables), or self-report (ATUS, NHANES). Recommended 7-9 hours for adults (AASM). US average: 6.8 hours on workdays. Population-level data available from ATUS (N>200K), NHANES sleep module, and Fitbit aggregate studies (N>6M nights).