[1010G]Economist阅读帖--决战2010 by 磊子[F]

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How siestas help memory 午睡怎样增进记忆力

Feb 25th 2010 | SAN DIEGO | From The Economist printedition

Researchers say an afternoon nap prepares the brain tolearn

MAD dogs and Englishmen, so the song has it(像歌里唱的那样), go out in the midday sun.And the business practices of England’s lineal descendant, America, will haveyou in the office from nine in the morning to five in the evening, if not longer(甚至更长的时间). Much of the world, though, prefers to take a siesta.And research presented to the AAAS meeting in SanDiego suggests it may be right to do so. It has already been established thatthose who siesta are less likely to die of heart disease. Now, Matthew Walkerand his colleagues at the University of California, Berkeley, have found thatthey probably have better memory, too. Apost-prandial(post-为前缀意为after,午餐之后) snooze, Dr Walker has discovered, setsthe brain up for learning.

The role of sleep in consolidating memories that havealready been created has been understood for some time. Dr Walker has beentrying to extend this understanding by looking at sleep’s role in preparing thebrain for the formation of memories in the first place. He was particularlyinterested in a type of memory called episodic memory, which relates tospecific events, places and times. This contrasts with procedural memory, ofthe skills required to perform some sort of mechanical task, such as driving. The theory he and his team wanted to test was that theability to form new episodic memories deteriorates with accrued wakefulness,and that sleep thus restores the brain’s capacity for efficient learning.

They asked a group of 39 people to take part in twolearning sessions, one at noon and one at 6pm. On each occasion theparticipants tried to memorise and recall 100 combinations of pictures andnames. After the first session they were assigned randomly to either a controlgroup, which remained awake, or a nap group, which had 100 minutes of monitoredsleep.

Those who remained awake throughout the day became worseat learning. Those who napped, by contrast(相反的), actually improved their capacity to learn, doingbetter in the evening than they had at noon. These findings suggest that sleepis clearing the brain’s short-term memory and makingway for new information(为…铺路).

It is already well known that fact-based memories arestored temporarily in an area called the hippocampus, a structure in the centreof the brain. But they do not stay there long. Instead, they are sent to theprefrontal cortex for longer-term storage. Electroencephalograms, which measureelectrical activity in the brain, have shown that this memory-refreshingcapacity is related to a specific type of sleep called Stage 2 non-REM sleep.

The ideal nap, then, follows acycle of between 90 and 100 minutes(遵循90~100分钟的周期). The first 30 minutes is a light sleep that helps improve motorperformance. Then comes 30 minutes of stage 2 sleep, which refreshes thehippocampus. After this, between 60 and 90 minutes into the nap, comesrapid-eye-movement, or REM, sleep, during which dreaming happens. This,research suggests, is the time when the brain makes connections between the newmemories that have just been “downloaded” from the hippocampus and those thatalready exist—thus making new experiences relevant in a wider context.

The benefits to memory of a nap, says Dr Walker, are sogreat that they can equal(相当\相抵) an entire night’s sleep. Hewarns, however, that napping must not be done too late in the day or it willinterfere with night-time sleep. Moreover(此外), not everyone awakens refreshed from a siesta.

The grogginess that results from an unrefreshing siestais termed “sleep inertia”. This happens when the brain is woken from a deepsleep with its cells still firing at a slow rhythm and its temperature andblood flow decreased. Sara Mednick, from the University of California, SanDiego, suggests that non-habitual nappers suffer from this more often thanthose who siesta regularly. It may be that those who have a tendency to wake upgroggy are choosing not to siesta in the first place. Perhaps, though, as in so many things, it is practice that makes perfect.