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 　　Day and night existed long before the first primitive¹ cells coming into being. And that light/dark cycle left its mark on nearly every living thing, microbes, plants, insects, mammals---we all experience circadian rhythms, due to molecular² clocks in our cells. But that internal timer affects more than just bedtime. The body's cells may actually divide by the clock, too, in fruit flies at least. Researchers gave fruit flies a form of an inflammatory bowel³ disease. In response, the flies' intestinal⁴ stem cells got busy reparing the gut⁵, most of them dividing in sync around dawn. But when researchers blocked the action of two clock genes⁶ in the intestine⁷, the flies couldn't patch up the damage. Instead of a correlative repair, the cells stalled in various stages of division, indicating that the internal clock plays a role in healing too. Those results appear in the journal Cell Reports. The researchers say the circadian clock might inform healing in us too, since it's such an ancient trait. If so, they say, doctors might want to time surgeries or chemotherapy for when the body is primed to heal, helping⁸ patients clock a faster recovery.