According to researchers, this is because of a hormone called melatonin (褪黑激素) that is secreted (分泌) by the pineal gland — a tiny structure deep inside the brain. Melatonin regulates the body's natural day­night rhythms. It causes a person to become sleepy by lowering his body's core temperature. In teens, melatonin is secreted much later in the evening and continues to increase throughout the night. This makes it difficult for teens to fall asleep earlier as they did in their younger years. Likewise, the effect of melatonin continues until much later in the morning, making it hard for them to wake up early.

　　A study found that at least one in four teens is tired and falls asleep in school at least once a week. During sleep, important body functions and brain activities happen. So how can teenagers get their sleep enough? The American Academy of Pediatrics (AAP) recommends that middle and high schools should start later — not before 8：30 am. This should give teens some more time to sleep at night.

　　语篇解读：本文是一篇说明文。早睡才能早起，晚睡会导致早上赖床，睡觉周期的改变实际上受制于大脑中一种被称为褪黑激素的荷尔蒙。

　　1．From Paragraph 2, we can learn that if you sleep late, the next day you should ________.

　　A．stay sharpB．skip breakfast

　　C．get up late

　　D．stay up late

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