Every living thing has what scientists call a biological clock that controls behavior.

The biological clock tells plants when to form flowers and when the flowers should open. It tells insects when to leave the protective cocoon and fly away. And it tells animals and human beings when to eat， sleep， and wake. It controls our body temperature， the release of some hormones， and even dreams.

Events outside the plants and animals affect the actions of some biological clock. Scientists recently found， for example， that a tiny animal called the Siberian hamster changes the color of its fur because of the number of hours of daylight. In the short days of winter its fur becomes white. The fur becomes gray-brown in color in the longer hours of daylight in summer.

Inner signals control other biological clocks. West German scientists found that some kind of internal clock seems to order birds to begin their long migration flight twice each year. Birds prevented from flying become restless when it is time for the trip， but they become calm again when the time of the flight has ended.

每一種生物都有科學(xué)家稱(chēng)之為生物鐘的東西來(lái)支配其行動(dòng)。

生物鐘告訴植物何時(shí)形成花苞，何時(shí)開(kāi)花；它告訴昆蟲(chóng)何時(shí)離開(kāi)保護(hù)自己的繭子飛走；它告訴動(dòng)物和人類(lèi)何時(shí)吃，何時(shí)睡，何時(shí)醒來(lái)；它控制著我們的體溫，某些荷爾蒙的釋放，乃至于做夢(mèng)。

發(fā)生在動(dòng)植物體外的情況影響著某些生物鐘的作用。例如，科學(xué)家最近發(fā)現(xiàn)，一種名叫西伯利亞倉(cāng)鼠的小動(dòng)物會(huì)根據(jù)日照時(shí)間改變自己的毛色。其毛在晝短的冬季變成白色，而在晝長(zhǎng)的夏季變成灰褐色。

身體內(nèi)部的信號(hào)控制著另外一些生物鐘。西德的科學(xué)家們發(fā)現(xiàn)，某種內(nèi)部的生物鐘似乎安排著鳥(niǎo)類(lèi)開(kāi)始它們一年兩次的長(zhǎng)途遷徙。如果遷徙的季節(jié)到來(lái)時(shí)不讓它們飛走，它們會(huì)煩躁不安，但是遷徙的季節(jié)一過(guò)，它們就又安靜下來(lái)。