A、Its habits. B、Its genes. C、Its mental states. D、Its physical conditions. B根据短文中提到的Sleep is genetically programmed in babies可知，

If you feel tired all the time, you don’t necessarily need to go to bed earlier. The solution could be as simple as taking a five-minute afternoon nap. Children under five have an abundance of energy and one of the reasons is because they nap once or twice a day. Many sleep experts think we are programmed to take a nap during the day, and getting back into this habit can be a solution for those who feel tired all the time. Professor Criss Ezeko-sky of the Sleep Advisory and Assessment Center thinks that one can feel refreshed after just five minutes’ sleep and research has shown that concentration and attention are improved after even a short nap. Sleep is genetically programmed in babies and it’s only as we get older that we learn to sleep about 8 hours at night and not at all during the day. But most research suggests that we are not physically designed to sleep for one long single block. Before the working day became 9 to 5, all western sleep patterns were all broken up. Research has shown that in the Middle Ages people’s sleep quite often occurred in 3 distinct parts. A nap in the afternoon, an early evening nap and another longer sleep until dawn. So if you find yourself feeling sleepy through the day, don’t feel your being lazy by giving in to sleep and having a nap. Your increased energy and alertness will make the rest of your day extra productive.
Questions 30 to 32 are based on the passage you have just heard.
30. Why do children under five have abundant energy according to the passage?
31. According to the speaker, what is the sleep pattern of a baby determined by?
32. According to the passage, what should you do if you feel sleepy through the day?