单选题
Making Light of Sleep
All we have a clock located inside our brains. Similar to your bedside alarm clock, your internal clock
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on a 24-hour cycle. This cycle,
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a circadian (昼夜节奏的，生理节奏的) rhythm, helps control when you wake, when you eat and when you sleep.
Somewhere around puberty (青春期),
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happens in the timing of the biological dock. The clock pushes forward, so adolescents (青少年) and teenagers are unable to fall asleep as early as they
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to. When your mother tells you it"s time
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bed, your body may be pushing you to stay up for several hours more. And the light coming from your computer screen or TV could be pushing you to
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even later.
This shift is
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for teenagers. But staying up very late and sleeping late can get your body"s clock out of sync (同步) with the cycle of light and dark. It can also make it hard to get out of bed in the
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and may bring other problems, too. Teenagers are put in a kind of gray cloud
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they don"t get enough sleep, says Mary Carskadon, a sleep researcher at Brown University in Providence, RI. It
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their mood and their ability to think and learn.
But just like your alarm clock, your internal clock can be reset. In fact, it
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resets itself every day. How? By using the light it gets through your
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.
Scientists have known for a long time that the light of day and the dark of night
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important roles in setting our internal clocks. For years, researchers thought that the signals that synchronize the body"s
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were handled through the same pathways that we use to see.
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recent discoveries show that the human eye has two separate light-sensing systems. One system allows us to see. The second system tells our body whether it"s day or night.