The last 35 years has seen a sea change in the field of circadian rhythms. The molecular era began with work in Drosophila (fruit flies), which has been a leading genetic system for more than 100 years. My colleagues and I discovered the mechanism that underlies circadian timing, and it turns out that this mechanism is conserved in all animals. Moreover, the circadian clock governs a large fraction of all gene expression, once again in fruit flies as well as humans. This explains why such a large fraction of animal physiology (biochemistry, metabolism, endocrinology, behavior, sleep, etc.) is governed by the circadian clock. This control indicates that circadian biology will continue to be important to many aspects of human well-being, from jet lag to metabolic disorders, and that it will become increasingly relevant to medicine as more knowledge and applications accrue.