题目内容：
A strange thing happens to nearly everybody at night. They turn off the lights, pull up the covers and close their eyes. Six or seven sleeping hours later, they wake up again. Strange, isn't it? Sleep puzzles science. Scientists and doctors would rather talk about why one can't fall asleep. They are not sure what causes sleep. The best conditions for sleep are good health and meal neither too big nor too small. No worries and a comfortable place are important, too. They advise against two in bed.
Strange things happen during sleep. For example, they move often. You would feel tired if you didn't. You also dream. Part of your brain is still awake when you dream. Dreaming happens when the memory and imagination in parts of your brain are still awake. Experiments have shown most of us dream in color. Bad dreams may be caused by stomachache. Don't worry if you dream. Some great stories and poems were begun while their writers slept.
根据以上内容,回答题.
Sleep puzzles scientists because they are not certain .( ) A．why one can't fall asleep
B．why one wakes up
C．both A and B
D．what causes sleep
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