When the school year starts, it can be difficult to get up early．After a few early mornings, extreme tiredness might make you feel like you’re going to fall over．The amazing thing is that you probably manage to stay awake all day long and into the night．But how? A chemical in the brain called dopamine（多巴胺）might be part of the answer．According to new research, dopamine is what keeps people who don’t get enough sleep from conking out．The chemical also has a complicated influence on your ability to think and learn when you don’t get enough sleep． To study sleep loss and its effect on the brain, scientists rounded up 15 healthy volunteers．The scientists tested each person’s memory and ability to pay attention twice: once after a good night’s sleep and once after being kept up all night long．During the tests, the scientists measured levels of dopamine in the brains of the volunteers． The results showed that when the volunteers stayed up all night, dopamine levels increased in the brain．Higher levels of dopamine kept the volunteers awake even though they felt tired．Some people are amazingly able to think clearly and react quickly, even when they haven’t had much sleep．Other people have a really hard time paying attention when tired, and their reactions are slow．All of these may be related to the dopamine levels．However, the researchers found that higher levels of dopamine don’t remove the trouble people have thinking and learning while sleep-deprived （缺乏睡眠的）． 60．The underlined phrase “conking out” in the second paragraph probably means     ． A．staying awake     B．falling asleep C．paying attention         D．falling over 61．With a higher level of dopamine, one     ． A．will not dream dreams at night              B．may have trouble in falling asleep C．can still stay awake after staying up          D．can do better in learning and working 62．We can infer from the last paragraph that        ． A．not all people can react quickly after they stay up late B．dopamine in our brain does not work when we are sleeping C．lack of sleep has no influence on one’s learning and thinking ability D．the higher the dopamine levels are, the more quickly one reacts 63．The passage is developed by        ． A．setting up a point and discussing it B．giving examples and drawing a conclusion C．pointing out similarities and differences D．showing a fact and explaining the cause