Teens aren ’ t Getting Enough Sleep Most teens need about 8 to more than 9 hours of sleep each night . The right amount of sleep is essential ( necessary ) for anyone who wants to do well in test s or sports . And lack of sleep impacts (influences) everything from a person ability to pay attention in class to his or her mood. It also damages teens ability to do their best in athletics. Unfortunately , research shows many teens don ’ t get enough sleep , and more than 20% of high school students fall asleep in class. Until recently, teens were often blamed for staying up late, oversleeping for school and falling asleep in class. But recent studies show that adolescent sleep patterns actually differ from those of adults or kids. These studies show that during the teen years, the body ’ s circadian rhythm ( 昼夜节奏), which is controlled by a biological clock in the brain, is temporarily reset, telling a person to fall asleep later and wake up later. This change might arise from the fact that the brain hormone melatonin (褪黑色素) is produced later at night for teens than it is for kids and adults. This can make it harder for teens to fall asleep early. These changes in the body ’ s circadian rhythm are in line with a time when we ’ re busier than ever. For most teens, the pressure to do well in school is more intense than when they were kids, and it ’ s harder to get by without studying hard. And teens also have other time demands---everything from sports and other extracurricular activities (课外活动) to fitting in a part-time job to save money for college. Early start time in some schools may also play a role in this sleep deficit. Teens who fall asleep after midnight may still have to get up early for school, meaning they they may only squeeze in 6 to 7 hours of sleep a night. A couple hours of missed sleep a night may not seem like a big deal but can create a noticeable sleep deficit over time. 1 . What is the main idea of this passage?