Why Can’t We Remember Our Dreams?為何夢境易忘？

巴哈爾·戈利普爾

You spend a third of your life asleep， a good chunk1 of which involves dreaming. But most often， you don’t remember any of your dreams. And even on those lucky days when you wake up with a memory of the dream still floating in your mind， there’s a good chance that in just a minute the memory will vanish into thin air2 and back to dreamland.

In waking life， such a case of quickly forgetting recent experiences would surely land you in a doctor’s office. With dreams， however， forgetting is normal. Why？

“We have a tendency to immediately forget dreams， and it’s likely that people who rarely report dreams are just forgetting them more easily，” said Thomas Andrillon， a neuroscientist3 at Monash University in Melbourne， Australia. It might be hard to believe that you had a dream if you don’t remember anything， but studies consistently show that even people who haven’t recalled a single dream in decades or even their entire lifetime， do， in fact， recall them if they are awakened at the right moment.

Scientists have gained some insight into memory processes during sleep， leading to several ideas that may explain our peculiar forgetfulness.

You are awake， but is your hippocampus4？

When we fall asleep， not all the brain’s regions go offline at the same time. Researchers have found one of the last regions to go to sleep is the hippocampus， a curved structure that sits inside each brain hemisphere and is critical for moving information from short-term memory into long-term memory.

If the hippocampus is the last to go to sleep， it could very well be the last to wake up. Andrillon said： “You could have this window5 where you wake up with a dream in your short-term memory， but since the hippocampus is not fully awake yet， your brain is not able to keep that memory.”

While this might explain why dream memories are so fleeting6， it doesn’t mean that your hippocampus has been inactive throughout the night. In fact， this region is quite active during sleep， and appears to be storing and caring for existing memories to consolidate them， instead of listening for incoming new experiences.

Upon awakening， the brain may need at least 2 minutes to jump-start its memory-encoding abilities. In a 2017 study published in the journal Frontiers in Human Neuroscience， researchers in France monitored sleep patterns in 18 people who reported remembering their dreams almost every day， and 18 others who rarely remembered their dreams. The team found that compared with low-dream recallers， high recallers woke up more frequently during the night. These middle-of-the-night awakenings lasted an average of 2 minutes for high recallers， whereas low-recallers’ awakenings lasted for an average of 1 minute.

Neurochemical7 soup

Our poor ability to encode new memories during sleep is also linked to changes in the levels of two neurotransmitters8， acetylcholine9 and noradrenaline10， which are especially important for retaining memories. When we fall asleep， acetylcholine and noradrenaline drop dramatically.

Then， something strange happens as we enter the rapid eye movement （REM） stage of sleep， where the most vivid dreams occur. In this stage， acetylcholine returns to wakefulness levels， but noradrenaline stays low.

Some scientists suggest that this particular combination of neurotransmitters might be the reason we forget our dreams. The boost in acetylcholine puts the cortex11 in an aroused state similar to wakefulness， while low noradrenaline reduces our ability to recall our mental escapades12 during this time.

Sometimes your dreams are just not memorable

Do you remember what you were thinking about this morning when brushing your teeth？ Our minds wander all the time， but we discard most of those thoughts as nonessential information. Dreams， especially mundane13 ones， may be just like daydreaming thoughts and deemed by the brain to be too useless to remember.

But dreams that are more vivid， emotional and coherent14 seem to be better remembered—perhaps because they trigger more awakening15， and their organized narrative16 makes them easier to store， Andrillon said. Once in bed， repeatedly reminding yourself that you want to remember your dreams may improve your dream recall， and so does keeping a dream journal， some studies have suggested. Upon waking up， hang on to17 that fragile dream memory： Keep your eyes closed， stay still and replay the dream memory， until your hippocampus catches up and properly stores the memory.

我們一生中三分之一的時間都在睡覺，睡眠時經(jīng)常做夢，可做過的夢往往忘個精光。即使碰巧有些日子，醒來時腦海中仍飄浮著對夢境的記憶，但很可能短短一分鐘后，這縷記憶就會飄然而逝、遁回夢鄉(xiāng)。

醒著的時候，如果對新近經(jīng)歷的事這么快就遺忘，那肯定得就醫(yī)。但忘記夢境卻很正常。這是為什么呢？

澳大利亞墨爾本莫納什大學(xué)的神經(jīng)科學(xué)家托馬斯·安德里永指出：“我們總是很快遺忘夢境。極少述說夢境的人可能只不過是更容易忘卻夢境?！比绻闶裁炊疾挥浀昧?，就難以讓人相信你做過夢。不過多項研究一致表明，即使有人數(shù)十年甚至終生都未回想起哪怕一個夢，只要做夢后適時被喚醒，其實是能回想起夢境的。

科學(xué)家們對睡眠時的記憶過程已有一定的深入了解，由此形成若干見解，或可解釋這種奇怪的遺忘夢境現(xiàn)象。

你醒了，但你的海馬體也醒了嗎？

我們?nèi)胨瘯r，并非大腦的所有部位都同時休眠。研究人員發(fā)現(xiàn)，最后休眠的部位之一是海馬體，這是左右大腦半球均有的溝回狀結(jié)構(gòu)，對于將信息從短時記憶轉(zhuǎn)存為長時記憶至關(guān)重要。

如果海馬體最后休眠，那它很可能也最后蘇醒。安德里永表示：“如果你醒來時夢境仍留在短時記憶中，本應(yīng)有一線機(jī)會記住夢境，可由于海馬體尚未完全蘇醒，結(jié)果大腦無法保有這段記憶?！?/p>

這可能就是夢境記憶稍縱即逝的原因，但并不意味著海馬體整晚都在休眠。實際上，這一部位在睡眠時相當(dāng)活躍，似乎是在存儲和維護(hù)已有記憶，加以鞏固，而非諦聽新臨體驗。

大腦蘇醒后，可能至少需要2分鐘才能啟動記憶編碼能力。在發(fā)表于《人類神經(jīng)科學(xué)前沿》雜志的一項于2017年進(jìn)行的研究中，法國研究人員對受試者的睡眠模式進(jìn)行了監(jiān)測，其中18人自述幾乎每天都記得做過的夢，另外18人則很少記得做過的夢。該研究團(tuán)隊發(fā)現(xiàn)，與憶起夢境幾率較低的人相比，憶起夢境幾率較高的人更頻繁在夜間醒來。半夜醒來后保持醒著的平均時間，對后者來說是2分鐘，對前者來說則是1分鐘。

神經(jīng)化學(xué)湯

我們睡眠時對新記憶進(jìn)行編碼的能力較弱，還與體內(nèi)兩種神經(jīng)遞質(zhì)即乙酰膽堿和去甲腎上腺素的水平變化有關(guān)，這兩種神經(jīng)遞質(zhì)對留存記憶尤為重要。我們?nèi)胨瘯r，乙酰膽堿和去甲腎上腺素會急劇減少。

隨后，我們進(jìn)入快速眼動睡眠階段，這時會發(fā)生奇妙的事，腦海中出現(xiàn)栩栩如生的夢境。在這一階段，乙酰膽堿恢復(fù)到清醒時的水平，而甲腎上腺素則保持低水平。

一些科學(xué)家認(rèn)為，這一特殊的神經(jīng)遞質(zhì)組合或許就是我們遺忘夢境的原因。乙酰膽堿增加使大腦皮層處于近似于不睡覺時的覺醒狀態(tài)，而去甲腎上腺素水平較低，則讓我們事后無法憶起這段時間內(nèi)天馬行空的神游。

有時夢境不值一記

你還記得今早刷牙時在想什么嗎？我們無時無刻不在馳思遐想，但所思所想大多會作為無關(guān)緊要的信息而被拋諸腦后。有些夢境，尤其是平淡無奇的夢境，就像白日夢一樣，會被大腦視為毫無意義、不值一記。

安德里永指出，那些更加栩栩如生、感情更為強(qiáng)烈、情節(jié)更為連貫的夢境似乎更易被記住。這可能是因為它們令人意興更濃，而且情節(jié)富有條理，更易留存于記憶中。一些研究表明，上床后反復(fù)提醒自己要記住夢境，事后記夢境日記，都可提高夢境記憶力。剛醒來時，可以這樣留住縹緲的夢境記憶：眼不睜身不動，腦海中重溫夢境記憶，直至你的海馬體蘇醒并妥善存儲記憶。

（譯者為“《英語世界》杯”翻譯大賽獲獎選手）