阅读理解（十八）
（一）
It is not unusual to see your desk-mate yawn (打呵欠) as he complains that he didn't have a good sleep the night before. Many teens don't get enough sleep because they have too much homework, which keeps them up at night. Some teens cannot sleep at night even when they want to.
Studies show that during your teenage years, the body's biological clock is reset, telling you to sleep later at night and wake up later in the morning.
But changes in the body clock aren't the only r eason teens lose sleep. Lots of people have insomnia (失眠). The most common cause of insomnia is stress. But all sorts of things can lead to insomnia. These include physical discomfort, emotional troubles and even sleeping environment. It's common for everyone to have insomnia from time to time. But if it lasts for a month or more go and see a doctor.
Some people have a condition that causes stomach acid (酸) to move backward towards the chest. This produces the uncomfortable, burning feeling called heartburn. So even if the person doesn't notice the feelings of heartburn because he is sleeping, he may feel tired the next day.
Most teens have nightmares sometimes. But, if they are frequent, a person's sleep pattern can be seriousl
