All human beings possess a body clock.Along with other alerting(警报)systems，this governs the sleep/ wake cycle and is therefore one of the main processes which govern sleep behaviour.Typically，the preferred sleep/wake cycle is delayed in adolescents，which leads to many students not feeling sleepy until much later in the evenings.This typical sleep pattern is usually referred to as the“night owl”schedule of sleep.

　　This is opposed to the“early bird”schedule，and is a kind of disorder where the individual tends to stay up much past midnight.Such a person has great difficulty in waking up in the mornings.Research suggests that night owls feel most alert and function best in the evenings and at night.Research findings have shown that about 20 percent of people can be classified as“night owls”and only 10 percent can be classified as“early birds”－the other 70 percent are in the middle.Although this is clearly not true for all students，for the ones who are true night owls this gives them an excellent excuse for missing their lectures which unfortunately fall before midday.

　　【语篇解读】　这是一篇议论文。每个人都有属于自己的生物钟。研究发现：由于某些人的睡眠周期不能与日常事务很好地协调，从而造成睡眠紊乱。有相当一部分人成了半夜不睡觉早晨起不了床的“夜猫子”。

　　17．What does the author stress in Paragraph 1?

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