A、Because the time of a period is too long. B、Because they are lacking in sleep. C、Because they are not interested in the class.

Surveys of American teenagers find that about half of them do not get enough sleep on school nights. They get an average of 60-90 minutes less than experts say they need. One reason for this deficit is biology. Experts say teens are biologically programmed to go to sleep later and wake up later than other age groups.(16)Yet, many schools start classes as early as 7:00 in the morning. As a result, many students go to class feeling they are out of it and tired, and through first and second period, they can barely stay awake. Michael Breus, a clinical psychologist with a specialty in sleep disorders says teens need to sleep eight to nine hours or even nine to ten hours a night.(17)He also says sleepy teens can experience a form of depression that could have big effects on their general well-being. It can affect not just their ability in the classroom but also on the sports field and on the road. Michael Breus says any tired driver is dangerous, especially a teenager with a lack of experience.(18)So what can schools do about sleepy students? The psychologist says one thing they can do is to start classes later in the morning. He points to the studies showing that students can improve by a full letter grade in their first- and second-period classes. Eric Peterson is the head of St. George’s School in the northeastern state of Rhode Island. He wanted to see if a 30-minute delay would make a difference. It did.
16. Why do students feel tired during the first- and second-period classes?
17. How will lack of sleep influence teens according to the passage?
18. What is the probable solution to the sleepy students?