Circadian rhythms are physical, mental, and behavioral changes that follow a 24-hour cycle. Some people's biological cycles are more receptive to waking up early and going to bed early; others do best if they can sleep late. Ignoring circadian rhythms is a disaster: More than half the population in industrialized societies have out of whack circadian rhythms. "I want to build a startup to help people find and obey their circadian rhythm," he says.