Sleep，considered a luxury by many，is essential for a person's wellbeing．Researchers have found that insufficient sleep and tiredness increase a person's risk of developing severe medical conditions，such as obesity （being very overweight），high blood sugar levels，and heart disease．Now，a new study has found that getting sufficient sleep is also the key to improving academic performance．
   Jeffrey Gross，the university science professor who led the research，was not trying to find the relationship between sleep and grades when he handed out smart watches to the 100 students in his chemistry class．Instead，the professor hoped the wrist-worn devices，which track a person's physical activity，would show a connection between exercise and academic achievement．
   While Gross's data showed no relationship between these two factors，the study found something surprising．As the researchers were analyzing their data，they noticed that there was a straight-line relationship between the average amount of sleep a student got and their results in the course's 11 quizzes，three midterm tests，and the final exam．
   Even more interesting，it was not sufficient for students to just head to bed early the night before a test．Instead，it's the sleep you get during the days when learning is happening that matters most．
   The time students went to bed each night was similarly important．Those who went to bed in the early hours of the morning performed poorly，even if the total sleep time was the same as a higher-performing student． "When you go to bed matters，" Gross says． "If you go to bed at 10，or 12，or 1 at night，and sleep for seven hours，your performance is the same．But if you go to bed after 2，your performance starts to down even if you get the same seven hours．So，quantity isn't everything．"
   Perhaps most interesting was the huge impact that small differences in sleep patterns had on the students' grades．The overall course grades for students averaging six and a half hours of sleep each night were 25% lower than students who averaged just one hour more sleep．Similarly，students who varied their bedtime by even one hour each night had grades that dropped 45% below those with more regular bedtimes．
   Who knew getting A's just required some extra ZZZ's？

（1）Based on his original objectives，which best describes Professor Gross's research findings？

A

A

A．Accidental．
B．Complete．
C．Convincing．
D．Doubtful．
（2）Who were the people taking part in the study？

C

C

A．Middle school chemistry students．
B．Volunteers from different universities．
C．Professor Gross's own students．
D．University student athletes．
（3）How did Professor Gross's team measure academic performance？

D

D

A．Making the students wear a special watch．
B．Using students' university entrance test results．
C．Giving the students regular after class quizzes．
D．Using the students' normal test and quiz grades．
（4）Based on the study's findings，who is likely to perform best academically？

C

C

A．A person who has a good night's sleep the night before an important test．
B．A person whose normal bedtime varies between 9 p．m．and 12 p．m．
C．A person who sleeps from 11 p．m．to 6 a．m．each day．
D．A person who sleeps for a total of 7 hours each night．