Sleep experts recommend teenagers get 8 to 10 hours of sleep every night. Many college students experience     1     (regular) and insufficient sleep.

David Creswell, professor of MIT, led a team of researchers to evaluate     2     relationship between sleep and grade point average (GPA).

"Animal studies so far     3     (show) how critical sleep is for learning and memory," said Creswell, "Here we show how this work translates to humans. The     4     (little) nightly sleep a first year college student gets at the beginning of the school term predicts lower GPA at the end of the term. Lack of sleep may be hurting students'     5     (able) to learn in their college classrooms."

The study evaluated more than 600 first-year students across five studies at three universities. The students wore wrist Fitbit devices     6     (monitor) and record their sleep patterns. The researchers found that students in the study sleep on average 6. 5 hours a night.

More     7     (surprise), the researchers found that students who receive less than six hours of sleep experienced a pronounced decline in academic performance.

"Once you start dipping below six hours, you are starting to accumulate massive sleep debt     8     can harm a student's health and study habits,    9     (damage) the whole system," said Creswell. "Most surprising to me was that no matter     10     we did to make the effect go away, it continued."

Are you sometimes a little tired     1     sleepy in the early afternoon? Many people feel this way after lunch. They may think that eating lunch is the cause of the sleepiness. Or, in summer, they may think it is the heat. However, the real reason lies inside their bodies. At that time——about eight hours after     2     (wake) up——your body temperature goes down. This is what makes you slow down and feel sleepy. Scientists have tested sleep habits in experiments in     3     there was no night or day. The people in these experiments almost always     4     (follow) a similar sleeping pattern. They slept for one long period and then for one short period about eight hours     5    .

In many parts of the world, people take naps     6     the middle of the day. This is     7     (especial) true in warmer climates, where the heat makes it difficult     8     (work) in the early afternoon. Researchers are now saying that naps are good for everyone in any climate. A     9     (day) nap gives one a more rested body and mind and therefore is good for health in general. In countries where naps are traditional, people often suffer less from     10     (problem) such as heart disease.