Every living thing has what scientists call a biological clock that controls behavior. It tells plants when to【C1】______ flowers and insects when to leave protective cocoon (茧)and fly away. And it tells animals and human beings when to【C2】______ , sleep and seek food. It controls our body temperatures, the【C3】______ of some hormones and even dreams. Events 【C4】______ the plant and animal affect its actions. Scientists recently found that a tiny animal called Siberian hamster changes the color of its【C5】______ because of the number of hours of【C6】______ In shorter days of winter its fur becomes white. The fur becomes gray-brown in longer【C7】______ of daylight in summer. 【C8】______ signals control other biological clocks. German scientists found that some internal one seems to order birds to begin their【C9】______ flights two times each year. Birds prevented from flying become【C10】______ when it is time for the trip.【C11】______ they become calm again when the time of the flight has ended. A mix of outside and internal events controls some biological clocks such things as heartbeat and the daily change from sleep to walking take place because of both external and internal signals. Scientists are beginning to learn【C12】______ parts of the brain contain the biological clocks. A researcher at Harvard University, Dr. Martin Moore Ede, said a small group of cells near the front of the brain seemed to control the【C13】______ of some of our actions. Probably there are other cells to control other body activities. He is studying【C14】______ they affect the way we do our work. Most of us have great difficulty if we【C15】______ often change to different work hours.【C16】______ can take many days for a human body to【C17】______ a major change in work hours. Industrial officials should have a better【C18】______ of biological clocks and how they affect workers. He said such an understanding could【C19】______ sickness and accidents at work, and would help increase【C20】______ . 【C1】