How animals keep time through their internal circadian rhythms could help us understand why we sleep and how we cope with jet lag. Using the fruit fly Drosophila melanogaster as an experimental model, researchers have discovered that the molecular and cellular ‘clock’ mechanisms of insects closely resemble those of mammals, including humans. As these biological clock systems not only control sleep, but also influence functions such as blood pressure and metabolic rate, they could give us greater insight into many medical conditions.