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