American high school students who slept less than seven hours each night during the pandemic were more likely to struggle in school. That finding comes from a recent study from the U.S. Centers for Disease Control and Prevention, or CDC.

For the study, the CDC questioned 7,705 students about their sleep schedules and success in school. More than two-thirds of students said they experienced more difficulty in their schoolwork during the pandemic than before the pandemic. About 75 percent of students did not sleep enough during the pandemic, the study found. Students who reported poor mental health during the pandemic were also more likely to get less sleep. Overall, 37 percent of students said they struggled with their mental health during the pandemic. But about 50 percent of students who slept five hours or less each night said they had poor mental health.

As students recover from learning loss due to the pandemic, schools “can consider including policies and practices known to improve sleep duration (持续时间),” the CDC said. Teenagers experience many physical changes when they go through puberty (青春期).     ▲                                                     

As children get older, there is a change in their body’s natural clock. Before puberty, children may feel sleepy around 8 or 9 at night. But when they become teenagers, the body’s clock changes to make them tired much later — around 10 or 11 at night. Because of this, many teenagers may experience an inability to sleep.

Some school systems around the U.S. are pushing back start times for high schools so students can get more sleep. Usually, high schools start the earliest compared to middle and primary schools. The National Center for Education Statistics says the average start time for U.S. high schools is 8 a.m. But the American Academy of Pediatrics recommends a start time of 8:30. The organization Start School Later supports pushing back school start times. It says at least 13,000 school systems in the United States have pushed back start times.

1.What can we know from Paragraph 2?

A 7,075 students are questioned in the study.

B 2,300 students had difficulty studying due to the pandemic.

C The sleep duration can affect mental health.

D Half of of students did not sleep enough in 2020.

解析：选C。C推理判断题。根据第二段最后一句的“But about 50 percent of students who slept five hours or less each night said they had poor mental health.”可知，大约50%每晚睡眠时间不超过5小时的学生表示，他们的心理健康状况不佳，因此睡眠时间和心理健康状况可能有一定联系。故选C。

2.Which can be put back into the “ ▲ ”?

A Puberty can also cause changes in sleep patterns.

B A good sleep schedule can improve test performance.

C Teenagers should sleep 8 to 10 hours a night.

D The results could be applied to younger students as well.

解析：选A。A推理判断题。根据空白部分后面的“As children get older, there is a change in their body’s natural clock.”可知，青少年在青春期会经历许多身体变化，随着年龄的增长，他们身体的自然时钟发生了变化。在青春期之前，孩子们可能会在晚上8点或9点左右感到困倦。但当他们成为青少年时，身体的时钟会发生变化，使他们在晚上10点或11点左右才感到困倦。因此，青春期也会导致睡眠模式的变化。故选A。

3.What is the last paragraph mainly about?

A Why students who sleep well do better on tests.

B When is the best time for students to go to bed.

C How pushing back school start times affects parents.

D What school systems did to improve students’ sleep.

解析：选D。D段落大意题。本段主要讲述了美国各地的一些高中学校系统正在推迟每天早上的上课时间，以便学生能够获得更多的睡眠。故选D。

4.Where can the text be found?

A In a history book.

B In a novel.

C In a health magazine.

D In a biography.

解析：选C。C推理判断题。本文主要讲了一项最新的研究发现：学生的睡眠时间越少，学习成绩可能越差。睡眠与健康相关，因此，本文最有可能摘自一本健康杂志。故选C。