Jeffrey Hall and Michael Rosbash, at Brandeis University, together with Michael Young, at Rockefeller University, isolate in 1984 the "period" gene in the fruit fly Drosophila melanogaster, a gene whose mutation alters the roughly twenty-four-hour internal biological rhythm that regulates sleep, body temperature, and numerous metabolic processes in virtually all living beings. Hall and Rosbash demonstrate in 1990 the underlying molecular mechanism: the protein produced by the period gene progressively accumulates during the night inside the cell, then inhibits the activity of its own gene through a negative feedback loop, and degrades during the day, thereby generating a self-sustaining cycle of approximately twenty-four hours with no need for any continuous external stimulus. Michael Young identifies additional genes — including "timeless" and "doubletime" — that stabilize and precisely adjust the length of this molecular cycle, as well as the mechanism by which sunlight daily resets the internal clock to keep it synchronized with the planet's actual day-night cycle. The molecular mechanism discovered in the fly turns out to be extraordinarily similar to the one operating in human cells, explaining everyday phenomena such as jet lag and the health effects of shift work, and opening lines of research into the relationship between circadian rhythm disruption and metabolic diseases, sleep disorders, and certain types of cancer.