Why Can’t We Remember Our Dreams?为何梦境易忘？

巴哈尔·戈利普尔

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时间都在睡觉，睡眠时经常做梦，可做过的梦往往忘个精光。即使碰巧有些日子，醒来时脑海中仍飘浮着对梦境的记忆，但很可能短短一分钟后，这缕记忆就会飘然而逝、遁回梦乡。

醒着的时候，如果对新近经历的事这么快就遗忘，那肯定得就医。但忘记梦境却很正常。这是为什么呢？

澳大利亚墨尔本莫纳什大学的神经科学家托马斯·安德里永指出：“我们总是很快遗忘梦境。极少述说梦境的人可能只不过是更容易忘却梦境。”如果你什么都不记得了，就难以让人相信你做过梦。不过多项研究一致表明，即使有人数十年甚至终生都未回想起哪怕一个梦，只要做梦后适时被唤醒，其实是能回想起梦境的。

科学家们对睡眠时的记忆过程已有一定的深入了解，由此形成若干见解，或可解释这种奇怪的遗忘梦境现象。

你醒了，但你的海马体也醒了吗？

我们入睡时，并非大脑的所有部位都同时休眠。研究人员发现，最后休眠的部位之一是海马体，这是左右大脑半球均有的沟回状结构，对于将信息从短时记忆转存为长时记忆至关重要。

如果海马体最后休眠，那它很可能也最后苏醒。安德里永表示：“如果你醒来时梦境仍留在短时记忆中，本应有一线机会记住梦境，可由于海马体尚未完全苏醒，结果大脑无法保有这段记忆。”

这可能就是梦境记忆稍纵即逝的原因，但并不意味着海马体整晚都在休眠。实际上，这一部位在睡眠时相当活跃，似乎是在存储和维护已有记忆，加以巩固，而非谛听新临体验。

大脑苏醒后，可能至少需要2分钟才能启动记忆编码能力。在发表于《人类神经科学前沿》杂志的一项于2017年进行的研究中，法国研究人员对受试者的睡眠模式进行了监测，其中18人自述几乎每天都记得做过的梦，另外18人则很少记得做过的梦。该研究团队发现，与忆起梦境几率较低的人相比，忆起梦境几率较高的人更频繁在夜间醒来。半夜醒来后保持醒着的平均时间，对后者来说是2分钟，对前者来说则是1分钟。

神经化学汤

我们睡眠时对新记忆进行编码的能力较弱，还与体内两种神经递质即乙酰胆碱和去甲肾上腺素的水平变化有关，这两种神经递质对留存记忆尤为重要。我们入睡时，乙酰胆碱和去甲肾上腺素会急剧减少。

随后，我们进入快速眼动睡眠阶段，这时会发生奇妙的事，脑海中出现栩栩如生的梦境。在这一阶段，乙酰胆碱恢复到清醒时的水平，而甲肾上腺素则保持低水平。

一些科学家认为，这一特殊的神经递质组合或许就是我们遗忘梦境的原因。乙酰胆碱增加使大脑皮层处于近似于不睡觉时的觉醒状态，而去甲肾上腺素水平较低，则让我们事后无法忆起这段时间内天马行空的神游。

有时梦境不值一记

你还记得今早刷牙时在想什么吗？我们无时无刻不在驰思遐想，但所思所想大多会作为无关紧要的信息而被抛诸脑后。有些梦境，尤其是平淡无奇的梦境，就像白日梦一样，会被大脑视为毫无意义、不值一记。

安德里永指出，那些更加栩栩如生、感情更为强烈、情节更为连贯的梦境似乎更易被记住。这可能是因为它们令人意兴更浓，而且情节富有条理，更易留存于记忆中。一些研究表明，上床后反复提醒自己要记住梦境，事后记梦境日记，都可提高梦境记忆力。刚醒来时，可以这样留住缥缈的梦境记忆：眼不睁身不动，脑海中重温梦境记忆，直至你的海马体苏醒并妥善存储记忆。

（译者为“《英语世界》杯”翻译大赛获奖选手）