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1 .<-NARRATOR:-> Listen to part of a lecture from a biology class.
1 .<-FEMALE PROFESSOR:-> Most animals, including humans, follow biological cycles that are 24-hours long.
2 .What determines these 24-hour cycles?
3 .Do animals have something like a 24-hour clock inside them?
4 .Or are biological cycles determined by external factors—like the alternation of daylight and darkness?
1 .Well, research suggests that the answer’s somewhere in between.
2 .Animals do have an internal clock, but external cues are important too.
1 .Take flying squirrels.
2 .Flying squirrels are nocturnal—they are active during the night and sleep during the day.
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.
2 .Did the squirrels continue to follow regular cycles of activity?
3 .Actually, they did.
1 .They continued to have regular patterns of sleeping and waking.
2 .This indicates that animals do have an internal clock which regulates their activity cycles.
1 .But the internal clock is not precisely 24 hours long.
2 .Instead of following 24-hour cycles, the squirrels followed cycles that were about half an hour shorter than that.
3 .So every day, they woke up a little bit earlier.
1 .Without external cues—without sunlight to fine tune their internal clocks—the squirrels’ biological cycles drifted.
2 .So, what happened when the squirrels were exposed to daylight again?
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.
1 .So at first, their schedules didn’t match up with the normal day.
2 .They weren’t active during the nighttime, as flying squirrels usually are.
3 .Instead, they were waking up in the middle of the day.
1 .But after a while, the squirrels’ cycles began to change.
1 .Gradually, external cues—the cycle of light and darkness—adjusted their internal clocks.
2 .So eventually, they were brought back onto a normal, 24-hour schedule.