People who enjoy a dream-filled sleep are better at recalling information and making links between facts when they wake, scientists found. But recharging with a shallow nap   offers no such mental improvement, the research suggests. The results of the study add to the evidence that Rapid Eye Movement (REM) sleep is very important to the brain's ability to strengthen memories.  An average night's sleep includes four or five periods of REM sleep, but these tend to be longer towards the end of the night. This means that adults who get less than the recommended seven to eight hours a night -- and therefore inadequate REM sleep -- may be damaging their mind's ability to form strong memories. 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 most recent study, participants were shown groups of three words -- such as "cookie", "heart", and "sixteen" -- and asked to find another word that can be associated with all three words. In this example, the answer would have been "sweet". 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 showed no improvement in their test results, but the REM sleep group improved on their morning performance by an average of almost 40 percent.    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 -- which leads to creative problem-solving -- is encouraged by mental changes which occur during REM sleep. People woken while their closed eyes made rapid movements recalled much more color1ful and detailed dreams than people woken during other stages of sleep. According to the passage, to have a good memory, a person had better   _   at night.
sleep four to five hours at most sleep as much as possible have a shallow sleep sleep with four or five periods of REM