Scientists say they now have proof to support the old-fashioned advice that it’s best to sleep on a problem. They say sleep strengthens the memory and helps the brain organize the masses of information we receive each day.Lead researcher Bob Stickgold at the Harvard Medical School said, "Sleep helps us draw rules from our experiences. It’s like knowing the difference between dogs and cats, even if it’s hard to explain. "The US research team studied how well students remembered connections between words and symbols (象征), reports New Scientist. They compared how the students performed if they had had a sleep between seeing the words and having the test, and if they had not slept. They found that people were better able to remember lists of related words after a night’s sleep than after the same time spent awake during the day. They also found it easier to remember themes (主题) that the words had in common. But they forgot around one in four more themes if they had been awake. Prof. John Groeger, of Surrey University’s Sleep Research Centre, said, "People have been trying for years to find out what the purpose of sleep is, as we know that only certain parts of it have a restorative (有助恢复的) value. ""We form and store huge numbers of experiences in the head every day, and sleep seems to be the way the brain deals with it all.\ The phrase "to sleep on a problem" in Paragraph 1 most likely means to ().

A. pay full attention to a problem
B. wait until later for a decision
C. sleep to forget a problem
D. have difficulty sleeping