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 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 (73) 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 (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) can 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) .
A．days
B．lights
C．times
D．hours
