"It's been overlooked for a long time as a real public health problem. All of western society is a li..." — Michael Rosbash

It was always intriguing that flies had two peaks of activity, in the morning and evening, with a siesta during the day and not very much activity at night. There are several ways to explain that, but one possibility was that there were two clocks running - one governing the morning peak and one governing the evening peak.