A、They eat too much for lunch. B、They sleep too little at night. C、Their body temperatures become lower. D、The weather becomes a

　　Are you sometimes a little tired and sleepy in the early afternoon? Many people feel this way after lunch. [20]At that time—about eight hours after you wake up—your body temperature goes down. This is what makes you slow down and feel sleepy. Scientists have tested sleep habits in experiments where there was no night or day. The people in these experiments almost always followed a similar sleeping pattern. They slept for one long period and then for one short period about eight hours later.
    [21]In many parts of the world, people take naps in the middle of the day. This is especially true in warmer climates, where the heat makes work difficult in the early afternoon. Researchers are now saying that naps are good for everyone in any climate. A daily nap gives one a more rested body and mind and therefore is good for health in general. In countries where naps are traditional, people often suffer less from problems such as heart disease.
    Remember the best time to take a nap is about eight hours after you get up. [22]A short sleep too late in the day may only make you feel more tired and sleepy afterward. This can also happen if you sleep for too long. If you do not have enough time, try a short nap—even ten minutes of sleep can be helpful.
    20.Why do people feel sleepy in the early afternoon?
    21.What is especially true for people in warmer climates according to the passage?
    22.How will you feel if you sleep for too long?