A、People now go to sleep when it gets dark. B、More and more people like to get up early. C、The average sleeping hours have short

W: With us today is sleep scientist, Doctor Iris Saunders.
M: Good afternoon.
W: Dr. Saunders, how much of a problem is sleeping, do you think?
M: Well, (19)we know that tiredness can cause accidents. More than 50% of road accidents in the USA are because of people driving when they’re tired. And when you think that 30% of people in the UK have problems getting to sleeping or staving asleep and 10% of those suffer from serious insomnia—that’s a lot of accidents waiting to happen.
W: So how much sleep are people getting these days?
M: Well, a hundred years ago, before electricity, people went to sleep when it got dark and woke up when it got light.(20)But now in our 24-hour society we sleep about one and a half hours less than we did a century ago.
W: Well, we have two teenage kids and no one can say they’re sleeping less.
M: Ah well, that’s interesting. Scientists now think that teenagers really do need more sleep than adults. It’s because they’re still growing.
W: So why do we need sleep?
M: We don’t actually know. Scientist used to think that sleep was the only time you had completely rest, but in fact we use about the same amount of energy when we’re asleep as when we’re sitting in the sofa resting.
W: Really? That’s surprising.
M: Yes, (21)and our brains are very active for some of the time we’re asleep. Apparently that’s when our brains organize information they’ve collected during the day.
W: Really? That’s amazing. Thank you Doctor Saunders.
19. Q: Why do people think the problem of sleeping serious?
20. Q: According to the conversation, what has changed about the sleeping habits?
21. Q: What do we know about sleep?