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    When US student Olivia Priedeman, 17, woke up one morning, she thought she had had a dream about making plans with a friend.

    But it wasn't a dream. Her phone showed that during the night, Priedeman had read a text message from her friend. She did it while she was fast asleep.

    Reading and sending text messages while asleep called─ "sleep texting"─ is an unusual sleep behaviour, similar to sleep walking. It's also a growing problem among doctors: young people can't live without their cell phones.

    One in three teenagers sends more than 100 text messages a day, according to the Pew Internet & American Life Project. And at least four out of five teenagers said they sleep with their phone on or near their bed.

    Elizabeth Dowdell, a professor at Villanova University in Pennsylvania, has studied sleep texting. She said that having a phone nearby all night is a big part of the problem.

    Andrew Stiehm, a sleep medicine researcher with Allina Health in Minnesota, agrees. It's possible for the part of the brain that controls skills to wake up, while the part of the brain that controls memory and judgment(判断) may be still asleep. That's why some people can perform basic movements-such as walking, talking, texting or even driving while they're sleeping. Some of Dowdell's students said that they're disturbed by their nighttime texting behaviour. But because sleep texting is unconscious, it's a difficult habit to break. Dowdell said she knows of some students who wear socks on their hands to keep themselves from texting.

    Marjorie Hogan, a doctor at Hennepin County Medical Centre in Minneapolis, Minnesota, suggests keeping all electronic products outside the bedroom.