C RIDING in school buses in the early morning，then sitting in poorly lit classrooms，are the main reasons students have trouble getting to sleep at night，according to new research． Teenagers，like everyone else，need bright light in the morning to allow their circadian rhythms(生理节奏)to get into step with nature's cycles of day and night． If they don't get blue 1ight in the morning，they get to sleep an average of six minutes later at night，until their bodies are completely out of sync(同步) with the school day，researchers from Rensselaer Polytechnic Institute in New York said last month． The finding was made by fitting goggles(护目镜)that block blue light from the sun to a group of students．The researchers discovered that their circadian rhythms were greatly affected． Parents and teachers have been complaining in recent years that teens stay up too late at night，then fall asleep in class the next morning and do poorly in school．The new findings provide a possible explanation for the problem． At the root of the research is the circadian rhythm, the body's natural sleep and waking cycle．Even though the Earth makes a full rotation(旋转)in 24 hours，the body's circadian cycle is about 24 hours and six minutes 1ong．The cycle is mediated(调节)by a chemical called melatonin(褪黑激素)．The body starts to produce it about two hours before it is time to sleep and, in the absence of blue 1ight，the body produces about six minutes later each day. In the study, the researchers studied 11 students at Smith Middle School in New York, which was designed so that a lot of sunlight  reaches classrooms． On a Friday night，the researchers measured what time the 11 students’ bodies began releasing melatonin．On Monday morning，the students were sent to school with orange goggles that blocked most blue light from their eyes to mimic(模拟) the conditions found in many---if not most--schools． By the end of the week，the students were releasing melatonin 30 minutes 1ater in the evening---an average of six minutes a day--and going to sleep correspondingly(相应的) later． “This is our first field study，”said lead author Mariana G．Figueiro．He said they would like to repeat it in larger studies and for longer periods of time。 If the findings are repeated，a variety of solutions are available．Ideally, new schools would be built to allow more natural sunlight into the classrooms．Students could also be exposed to more sunlight outside． 68. According to the findings related in this article, many teenagers stay up late because_______ A. they lack melatonin in their bodies B. they have to get up so early to catch the school bus C. their circadian rhythms are in disorder D. they do not get enough blue light in the morning 69. According to the findings, a student who normally slept at 10:00 pm, but who spent 10 days in a poorly lit classroom, would probably be falling asleep at___________after the period. A. 12:00pm   B. 11:30pm   C. 11:00pm   D. 10:00pm 70. What does the article tell us? A. Unlike adults, teenagers tend to feel sleepy during daytime. B. The sunlight is the only thing that can affect our circadian rhythms. C. If the findings prove correct, solutions could be found to teen sleeping problems. D. Most schools have small windows and the classrooms are poorly lit. 71. The main point of the article is to___________ A. warn teenagers not to stay up late or sleep in class B. report on some new findings related to teenagers' sleeping problems C. give suggestions on how to build schools D. advertise goggles which can protect the eyes from the sun