 You have probably felt that your body is in sync with the cycles of day and night. This is due to our inner circadian clock, a set of genes whose activity oscillates with daylight. For many years, scientists thought that this inner rhythm is progressively lost as we get older. But this seems to be wrong. Recent research performed in Barcelona and California and published in the journal Cell shows that in stem cells, those that divide to regenerate tissues, the circadian rhythms of old cells are just as active as in young ones. But the difference is that they don't activate the same genes. Whilst in young mice, the circadian machinery triggers the production of proteins involved in normal cell function. In aged mice, cells produce more proteins involved in stress situations, such as when DNA damage occurs. Researchers don't know how these changes happen, but they have identified a crucial factor, diet. They have studied different diets and found that, although the circadian profile of young mice doesn't change with a diet rich in fat, a calorie-restricted diet in old mice keeps the clock of their stem cells similar to their younger fellows. Thus, our circadian clock never stops and a diet low in calories would be very beneficial for delaying aging of our stem cells.