The relationship between sleep and cognitive performance is one of the most rigorously quantified cause-and-effect relationships in neuroscience. We have precise data on how 6 hours of sleep for 14 nights equals 48 hours without sleep (Van Dongen 2003), how one poor night increases amygdala reactivity 60% (Yoo 2007), and how sleep extension produces 9% athletic performance gains (Mah 2011). This article covers the quantified cognitive cost and the performance uplift evidence — framed as the practical case for treating sleep as the highest-leverage cognitive intervention available.