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旁白：听一听生物课上讲座的一部分。

Narrator: Listen to part of a lecture from a Biology class.

教授：大多数动物，包括人类，

Professor: Most animals, including humans,

follow biological cycles that are 24 hours long.

那么，什么决定了这些24小时周期呢？

Now what determines these 24-hour cycles?

动物体内有类似24小时钟的东西吗？

Do animals have something like a 24-hour clock inside them?

或者我们的生物周期是由外部因素决定，

Or our biological cycles determined by external factors,

like the alternation of daylight and darkness?

嗯，研究表明答案介于两者之间。

Well, research suggests that the answer is somewhere in between.

就拿飞鼠来说，飞鼠是夜行性的。

Take flying squirrels, flying squirrels are nocturnal.

They are active during the night and sleep during the day.

有些松鼠被关在持续的黑暗中。

some of these squirrels were kept in constant darkness.

它们在一个月的时间里没有暴露在任何阳光下。

They were not exposed to any daylight for about a month.

那么，松鼠们是否继续遵循常规的活动周期呢？

Now did the squirrels continue to follow regular cycles of activity?

它们继续有规律的睡眠和清醒的模式。

They continued to have regular patterns of sleeping and waking.

这表明动物确实有一个内部时钟来调节它们的活动周期。

This indicates that animals do have an internal clock which regulates their activity cycles.

但是生物钟并不是精准的24小时长。

But the internal clock is not precisely 24 hours long.

松鼠们遵循的周期比这短了半个小时。

the squirrels followed cycles that were about half an hour shorter than that.

So every day they woke up a little bit earlier.

without sunlight to fine-tune their internal clocks,

the squirrels' biological cycles drifted.

那么，当松鼠再次暴露在日光之下时会发生什么呢？

So what happened when the squirrels were exposed to daylight again?

a month of waking up half an hour earlier each day,

松鼠的活动周期发生了很大的变化。

the squirrels' activity cycle had shifted a lot.

所以一开始，它们的时间和正常的一天不匹配。

So at first, their schedules didn't match up with a normal day.

它们在夜间不像其他飞鼠那样晚上活跃，

They weren't active during the nighttime as flying squirrels usually are, instead,

they were waking up in the middle of the day.

但过了一阵，松鼠的周期开始发生变化。

But after a while, the squirrel cycles began to change.

光明和黑暗的循环调整了它们的内部时钟。

the cycle of light and darkness adjusted their internal clocks.

所以最终它们回到了正常的24小时的日程中。

So eventually they were brought back into a normal 24-hour schedule.

<img src="/upload/ti/2427000-2428000/2427072/0c4a0f825c2346218f04f895f4f3ac9f.png">
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<p>Using points and examples from the lecture, explain what determines animals' 24-hour activity cycle.</p>