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第1段
1 .<-NARRATOR:-> Listen to part of a lecture from a biology class.
请听一个生物学课程的讲座片段。
第2段
1 .<-FEMALE PROFESSOR:-> Most animals, including humans, follow biological cycles that are 24-hours long.
绝大部分动物，包括人类，遵循24小时的生物周期。
2 .What determines these 24-hour cycles?
是什么因素决定这些24小时的周期呢？
3 .Do animals have something like a 24-hour clock inside them?
是动物体内有一个差不多24小时的钟表吗？
4 .Or are biological cycles determined by external factors—like the alternation of daylight and darkness?
还是生物周期决定于外部因素——例如白天和黑夜的交替？
第3段
1 .Well, research suggests that the answer’s somewhere in between.
研究者认为，答案介于这两者之间。
2 .Animals do have an internal clock, but external cues are important too.
动物确实有内部生物钟，但外部线索也很重要。
第4段
1 .Take flying squirrels.
就以飞鼠为例。
2 .Flying squirrels are nocturnal—they are active during the night and sleep during the day.
飞鼠是夜行动物——它们在夜间活跃，在白天睡觉。
第5段
1 .But in an experiment, some of these squirrels were kept in constant darkness—they were not exposed to any daylight for about a month.
但在一个实验中，一些飞鼠被置于持续的黑暗中——它们在约一个月的时间内没有见到任何阳光。
2 .Did the squirrels continue to follow regular cycles of activity?
这些飞鼠继续遵循惯常的活动周期吗？
3 .Actually, they did.
实际上，它们仍然遵循。
第6段
1 .They continued to have regular patterns of sleeping and waking.
它们仍然保持睡眠，清醒的常规模式。
2 .This indicates that animals do have an internal clock which regulates their activity cycles.
这表明动物确实有规定活动周期的内部生物钟。
第7段
1 .But the internal clock is not precisely 24 hours long.
但内部生物钟并非是精确的24小时长度。
2 .Instead of following 24-hour cycles, the squirrels followed cycles that were about half an hour shorter than that.
飞鼠所遵循的不是24小时的周期，而是比24小时短半个小时。
3 .So every day, they woke up a little bit earlier.
所以，它们每天醒来更早一点点。
第8段
1 .Without external cues—without sunlight to fine tune their internal clocks—the squirrels’ biological cycles drifted.
没有外部线索——没有阳光对内部生物钟作微妙的调整，飞鼠的生物周期会发生偏移。
2 .So, what happened when the squirrels were exposed to daylight again?
所以，当飞鼠重新暴露于日光会发生什么？
3 .Well, after a month of darkness—a month of waking up half an hour earlier each day—the squirrels’ activity cycle had shifted a lot.
在一个月的黑暗之后——每天的觉醒时间会提前半个小时——飞鼠的活动周期已经偏离了很多。
第9段
1 .So at first, their schedules didn’t match up with the normal day.
所以，最初，它们的周期已经不能与正常的白天黑夜相匹配。
2 .They weren’t active during the nighttime, as flying squirrels usually are.
它们不能在夜晚变得活跃，不能像正常的飞鼠一样。
3 .Instead, they were waking up in the middle of the day.
相反，它们会在白天的正午醒来。
第10段
1 .But after a while, the squirrels’ cycles began to change.
但过了一段时间，这些飞鼠的周期开始变化。
第11段
1 .Gradually, external cues—the cycle of light and darkness—adjusted their internal clocks.
逐渐地，外部线索——白天和黑夜的周期——调节它们的内部周期。
2 .So eventually, they were brought back onto a normal, 24-hour schedule.
所以，最后，这些飞鼠会被调整回到正常的24小时周期。