How much sleep does a person need? We are all different. Some people need only three hours of sleep a day     1    others need ten hours or more. The amount of sleep     2     (suggest) is 8 hours for people aged 18 to 21 and 7 hours for people over 21. As people get older, they need less sleep time,    3     decreases to 6.5 hours a night after age 65.

Most people have     4     experience where they sometimes cannot fall asleep at night. About one third of Americans have difficulty in falling     5     (sleep).The name of this illness     6     (call) insomnia. This is not the modern disease. Many famous people in history had insomnia.    7     (actual), some of these people had special ideas     8     (make) themselves sleep. Benjamin Franklin, the famous statesman and     9     (invent), had four beds. He moved from one to another to make himself sleep. Mark Twain, the famous American humorist, had a different way. He     10     (lie)on his side across the end of the bed.

Many athletes have been told that smiling while     1    (sweat) will make our efforts feel easier. A marathon runner smiled widely through the final miles of his fastest-ever marathon; afterward, he said what he had hoped     2    (be) that the smiling would ease him to the finish line; But there has been little solid scientific evidence to support this idea. In recent years several past studies     3    (examine) whether deliberately smiling can make changes to how people feel psychologically during races,     4     few have looked at the physical effect     5     sports performance.

Researchers decided     6    (gather) a group of experienced recreational runners and have them alternatively smile broadly as they ran. Many of the runners found it difficult to smile throughout the whole session, though. Their smiles became     7    (increase) fixed and unnatural. Such false smiles make     8    (few) facial muscles (肌肉) start working than the sincere version and most likely result in less     9    (relax) and performance improvement. So the key to using a happy smile to make you     10     better runner may be to smile sincerely and often near a race’s end, but in 30-second bursts, rather than constantly.

Uh-oh, the new year's just begun and already you're finding it hard to keep those resolutions to junk the junk food, get off the couch or kick smoking. There's a biological reason why a lot of our bad habits are so hard to break – they get    1    (wire) into our brains.

"Why are bad habits stronger? You're fighting against the power of an immediate reward," says Dr. Nora Volkow, director of the National Institute on Drug Abuse and an authority on the brain's pleasure pathway.

"We all as creatures are behaving that way, to give greater value to an immediate reward as opposed to     2     is delayed," Volkow says.

How this bit of happiness turns into a habit involves a pleasure-sensing chemicalnamed dopamine. It causes the brain     3    (pursue) that reward again and again strengthening the connection each time – especially when it gets the right hint from your environment.

People tend to overestimate their ability to resist temptations around them, thus     4    (destroy) attempts to give up bad habits. Even scientists who recognize it     5     show weakness. "I know popcorns are not healthy. But every time I go to the cinema, I have to eat it," Volkow says， "It's fascinating."

A movement to pay people for behavior changes may exploit that connection, as some companies offer employees outright payments or insurance reduction for adopting better habits.

    6    well paying for behavior plays out, researchers say there are still some steps that may help fight your brain's hold     7     newly-established habits:

Repeat, repeat, repeat the new behavior – the same routine at the same time of day. You decide to exercise. Doing it at the same time of the morning, rather than fitting it in casually,     8     (make) the striatum(终脑皮层)recognize the habit.Therefore, if you don’t keep doing it, you will feel frustrated.

Exercise itself raises dopamine levels, so eventually your brain will get a feel-good hit     9     your muscles protest.

Besides, try to reward yourself with     10     that you really desire. For instance, if you exercise all week or stick to your diet, you could try a fancy restaurant- safer perhaps than a box of cookies because the price inhibits the quantity.