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It" s a time when school， homework， active social lives and part-time jobs keep teenagers （少年 ） busy from early in the morning until late at night. They are likely to try to make up for a lack of sleep by "sleeping in" on the weekends. Unfortunately， this causes irregular sleep and actually makes the problem worse.
Most teenagers are not able to fall asleep until late at night. Since many teens aren"t sleepy until around 11 p.m.， but need to be at school by 7: 30 or 8: 00 A.m.， they cannot get enough sleep. During puberty（青春期）， the biological clock in the brain naturally re-sets to a later time， and this causes teens to fall asleep later. Then， when it" s time to get up， a teen" s body clock is likely to still be producing the night-time hormones（荷尔蒙）. This makes it hard for them to feel active and energetic in the morning.
A growing body of research suggests that starting high school later improves attendance， achievements and grades. A few years ago in an important study， test scores on the SAT college entrance exams in Edina， Minnesota jumped more than 100 points on average， when the morning school bell rang an hour later. Unfortunately， most schools are not set up to start later and fail to satisfy teen"s sleep needs.
What effect does "sleeping in" on the weekends have on teenagers？ 查看材料

A. Improving their health.
B. Weakening their energy.
C. Disturbing their biological clock.
D. Making up for their hormone loss.