Every living thing has what scientists call a biological clock that controls behavior. It tells plants when to 62 flowers and insects when to leave protective cocoon(茧) and fly away. And it tells animals and human beings when to 63 , sleep and seek food. It controls our body temperatures, the 64 of some hormones and even dreams. Events 65 the plant and animal affect its actions. Scientists recently found that a tiny animal called Siberian hamster changes the color of its 66 because of the number of hours of 67 . In shorter days of winter its fur becomes white. The fur becomes gray-brown in longer 68 of daylight in summer. 69 signals control other biological clocks. German scientists found that some internal one seems to order birds to begin their 70 flights two times each year. Birds prevented from flying become 71 when it is time for the trip. 72 they become can 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 73 parts of the brain contain the biologicaI 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 74 of some of our actions. Probably there are other cells to control other body activities. He is studying 75 they affect the way we do our work. Most of us have great difficulty if we 76 often change to different work hours. 77 call take many days for a human body to 78 a major change in work hours. Industrial officials should have a better 79 of biological clocks and how they affect workers. He said such an understanding could 80 sickness and accidents at work, and would help increase 81 .