Not good at sports? Maybe you’re just playing during the wrong time of day. In a new study, researchers analyzed the lifestyles of field hockey and squash players using detailed surveys and diary entries. Based on the athletes’ natural circadian rhythms—or internal clocks—the researchers classified those who naturally rise and sleep early as the “larks,” those who do the opposite as the “owls,” and the rest as intermediates. The scientists then conducted a cardiovascular fitness test six times a day on the players, who had to run between two lines 20 meters apart with increasing speed. As expected, the larks performed best around noon, the intermediates in late afternoon, and the owls in the evening. More intriguingly, when the scientists tracked the players’ performances according to their internal biological time instead of real clock time, they found that the larks and the intermediates shared the same pattern: Both peaked about 6 hours after they woke up. The owls, on the other hand, hit their sweet spot 11 hours after their day started, and their performances also fluctuated more in the course of a day—by as much as 26%, the researchers report online today in Current Biology. That’s because sleeping late may have delayed and reduced the production of cortisol—a hormone essential for muscle function—in the owls, the researchers suggest. Because recruiting of adolescent athletes usually occurs during the school hours, owl players may miss their chance to shine simply because the timing isn’t right, the researchers suggest. The good news is that you can reset your internal clock through lifestyle adjustments—for example, by sticking to a regular wake and sleep schedule. So if you are a marathon runner, it pays to train yourself to be a lark, whereas football and soccer players may want to go the way of the owl.