They tell us when to get up or when we’re running late for work and whether we are in danger of missing our favourite TV programme. But while our daily routines rely on clocks that tell us the time, science is discovering that our well-being is influenced by a very different kind of timepiece. Circadian rhythms---the human body’s own internal clock---have a powerful influence on our health and behaviour. They are programmed from birth and control functions ranging from temperature and blood pressure to sleep patterns.
In recent years researchers have also discovered that this built-in mechanism can influence everything from the way we react to medicines to how well we learn music.
The latest example, from experts at Harvard University in the US, shows that the human body clock can even dictate whether or not we are likely to tell the truth. Researchers found it was easier for people to fib（撒小谎） in the afternoon because, as they were tired, the self control that would normally prevent them from lying started to break down. Tiredness made it harder to resist the temptation to tell lies---especially if it meant they got a financial reward at the end.
“The body clock has a great effect on us all,” says Professor Jim Horne from the Sleep Research Centre at Loughborough University. “Most people tend to feel good around late morning and then decline in the early afternoon. But the time most of us will feel our sharpest is between 6pm and 8pm. That’s because sleepiness tends to build up throughout the day. By early evening our body clock kicks in to wake us up. One reason may be to ensure we get home safely. When our ancestors were coming home after hunting all day their internal clocks kicked in to get them home in one piece.”
So what effects do circadian rhythms have and what’s the best time of day to take advantage of them?
The underlined word “They” in the first paragraph refers to            .

A．our clocks	B．scientists and experts
C．our daily routines	D．circadian rhythms

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