He was especially interested in the so-called period gene, which a decade earlier had been proposed by other scientists to play a key role in the regulation of biological rhythms. In 1984 Young successfully isolated and sequenced the period gene, and further showed that introducing a fragment of DNA from the period gene locus into the genome of period-mutant flies restored circadian rhythm, thereby demonstrating the functional significance of the gene. In the 1990s, Young helped elucidate the molecular mechanism by which period controls the circadian clock. He discovered a second key gene, timeless, RNA levels of which oscillate on a 24-hour cycle. Young’s later research included the investigation of molecular changes in circadian rhythm that underlie sleep disorders in humans.