A、It can be found in both plants and animals. B、It can be found only in human beings. C、It cannot be found in one-cell creatures

Good morning, everyone. Our topic for today’s talk is about sleep and time. Let’s start with day and night. Life evolved under conditions of light and darkness. [20]And so plants and animals developed their own internal clocks, so that they would be ready for these changes in light. These are chemical clocks, and they’re found in every known being that has two or more cells and in some that only have one cell. So, the chemical clocks in our humans are called the "body clock."
    So fine, we have a body clock, and it turns out that it’s incredibly important in our lives. It’s a huge driver for culture and I think that it’s the most underrated force on our behavior. We evolved as a species near the equator, and so we’re very well-equipped to deal with 12 hours of daylight and 12 hours of darkness. But of course, we’ve spread to every corner of the globe and in Arctic Canada, where I live. We have daylight in summer and 24 hours of darkness in winter. So the culture, the northern aboriginal culture, traditionally has been highly seasonal. [21]In winter, there’s a lot of sleeping going on; you enjoy your family life inside. And in summer, it’s hunting and working activity very long hours, very active.
    So, what would our sleeping patterns be in the sort of ideal sense? Well, it turns out that when people are living without any sort of artificial light at all, they sleep twice every night. [22]They go to bed around 8:00 p.m. until midnight and then again, they sleep from about 2:00 a.m. until sunrise. And in-between, they have a couple of hours of sort of meditative quiet in bed. The people in these studies report feeling so awake during the daytime, that they realize they’re experiencing true wakefulness for the first time in their lives.
    So, cut to the modem day. We’re living in a culture of jet lag, global travel, 24-hour business, shift work. And you know, our modern ways of doing things have their advantages, but I believe we should understand the costs. Thank you.
20. What does the speaker say about internal clocks?
21. What do we learn about the people living in Arctic Canada?
22. What does the speaker say about the ideal sleeping patterns?