Circadian rhythms regulate how much energy a cell consumes at different times of the day. In young animals and humans, this mechanism works well, but with the onset of old age, it virtually shuts down completely. Calorie restriction resets these biological clocks and slows down ageing,” says Paolo Sassone-Corsi of the University of California, Irvine (USA). Scientists identified the genes that determine the daily rhythms of the human body back in the 1990s. Such genes are found in the genomes of all plants and animals. These DNA segments control various periodic processes — sleep and wakefulness, flower opening, food search, and others. When studying these genes, biologists discovered that not only sleep and wake cycles depend on them, but also many processes in cells (for example, cleaning them of ‘garbage’), which greatly affect health and longevity. By investigating how these ‘biological clocks’ work, Sassone-Corsi and his colleagues have revealed their unusual connection to low-calorie diets, which explains why calorie restriction prolongs the life of monkeys and some other animals that scientists have been actively observing over the past 10-20 years. According to the American biologist, his team observed the life and work of several dozen mice, studying the influence of circadian rhythms on cell behaviour as the body ages. To do this, the scientists extracted small portions of cells from the rodents’ livers and monitored gene activity at different times of the day. This led them to discover an interesting phenomenon: it turned out that circadian rhythms control not only sleep and wakefulness, but also the energy efficiency of cells, increasing it when the animal is asleep or not very physically active. This discovery led scientists to believe that such changes in cell energy efficiency could be prevented by restricting calorie intake. They tested this idea by raising a new group of mice that were fed food containing 30% fewer calories than normal feed. These experiments showed that calorie restriction ‘repaired’ the mice’s biological clocks, causing liver cells to conserve energy in old age as efficiently as in youth. Similarly, experiments by another team of scientists showed changes in the functioning of stem cells in the skin and some other body tissues. As a result, the rodents’ lives were extended by several dozen weeks, and their activity in old age increased significantly thanks to improved cell cleansing of ‘debris’ and a reduction in the rate of cell ageing. Scientists suggest that similar processes may also occur in the cells of other animals, so artificial ‘rejuvenation’ of the cellular clock may also be possible for humans.