解析　细节理解题。根据文章第一段列举的一系列数字可知，国家睡眠基金会对一定年龄段孩子的睡眠习惯进行了调查，这也呼应了本段第一句"Getting less sleep has become a bad habit for most American kids."，故选A项。

26．How many hours of sleep do 11­year­olds need every day?

A．7 hours. B．8 hours.

C．10 hours. D．18 hours.

答案　C

解析　细节理解题。根据文章第二段第四句"For most school­age children，ten hours is ideal(理想的)．"可知选C项。

27．Why do teenagers go to sleep late according to Carskadon?

A．They are affected by certain body chemicals.

B．They tend to do things that excite them.

C．They follow their parents' examples.

D．They don't need to go to school early.

答案　B

解析　细节理解题。根据文章第三段第一句"More children are going to bed with TVs on，and there are more opportunities(机会) to stay awake，with more homework，the Internet and the phone..."以及第二句中的"...these activities at bedtime can get kids all excited and make it hard for them to calm down and sleep."可知，孩子们在睡觉前看电视、写作业、上网或打电话都容易让他们产生兴奋感而不愿意睡觉，故选B项。