We've all been there: staring at the ceiling at 2:43 a.m., unable to fall asleep while the world slumbers around us. How do our internal clocks stay synced to our environment? What exactly do circadian rhythms control? Might future research provide relief for late-night workers or the jet-lagged? Erik Herzog, professor of biology at Washington University in St. Louis, explains how the brain's "master clock" works and how genetics can influence our daily biological rhythms.