Circadian rhythms are 24-hour cycles that regulate our daily activity throughout the day, otherwise known as a “molecular clock”. This internal biological clock governs the timing of our behavior, such as when do we eat, when we are most attentive, and when do we sleep. We study the genetic basis of natural variation in circadian rhythm phenotypes to better understand the molecular components of the clock machinery in humans, how environmental inputs synchronize the clock, and how the clock regulates behavior and ultimately may link with human health and disease. We also use genetics to explore the cellular and physiological consequences of timing of molecular processes.