A growing body of evidence indicates significant sex differences in the circadian system, but little is known about how temporal organization of physiology and behavior differs between sexes. Drosophila is a powerful model system in which to study the neuronal basis of sexual dimorphism in timekeeping thanks to its well-characterized circadian clock neuron network and a highly conserved molecular oscillator. Our work is focused on using genetic, anatomical, and behavioral tools to study how sex differences in molecular, neuronal, and physiological pathways control sexual dimorphism in sleep/wake cycles and circadian entrainment.