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People who enjoy a dream-filled sleep are better at recalling information and making links between facts when waking. But recharging with a shallow nap（小睡）offers no such mental improvement, the research suggests. The results of the study add to the growing body of evidence that Rapid Eye Movement (REM) sleep is very important to the brain’s ability to from strong memories.
An average night’s rest includes four or five spells of REM sleep, but these bursts tend to be lengthier towards the end of the night. This means adults getting less than the recommended seven to eight hours a night – and therefore adequate REM sleep—may be damaging their mind’s ability to memorize. Although REM sleep is not classified as a type of deep sleep, it is only reached after the brain has passed through deep sleep stages.
In the recent study, participants were shown groups of three words and asked to find another word that can be associated with all three words. They were tested once in the morning and then given the same task again in the afternoon. In between, some were allowed a nap with REM sleep, some a nap without REM sleep, while the others had a quiet rest period. The quiet rest and non-REM sleep groups didn’t improve, but the REM sleep group improved on their morning performance by an average of almost 40 per cent. Dr Sara Mednick, a sleep researcher at the University of California who led the study, believes the formation of connections between previously unassociated information in the brain occurs during REM sleep.       
It is believed that sleep acts as a sort of mental filing system, enabling the brain to sort relevant information from useless information. This process may take place as memories are removed from one part of the brain to others.

Title	REM sleep        to memory
The result of the study	Rapid Eye Movement sleep is of great importance to the brain’s ability to     memories
The      of the study	◆      were required to think of another word associated with all three words. ◆The same task was given to participants          a day. ◆The quiet rest and non-REM sleep groups showed no         . ◆The REM sleep group        well by an average of almost 40 per cent.
Dr Sara Mednick’s	Connections between previously unassociated information in the brain are         during REM sleep.
Conclusion	Sleep acts as a sort of mental filing system, enabling the brain to abandon useless information and keep the         information.