爲什麼向東飛比向西飛時差反應更嚴重

Jet lag may be the worst part of traveling. And it hits many people harder traveling east than west. Why they feel this way is unclear. But scientists recently developed a model that mimics special time-keeping cells in the body and offers a mathematical explanation for why traveling from west to east feels so much worse. It also offers insights on recovering from jet lag.

時差應該是旅行中最討人厭的事了。許多向東行的人又比往西走的更容易受時差影響。我們並不清楚爲何這些人會有這種感受，不過科學家近來設計出一套模型，來模仿人體內專門計時的細胞，用數學說明爲什麼從西向東走造成的時差更難捱。這項研究也讓我們更加了解該如何倒時差。

Deep inside the brain, in a region called the hypothalamus (right above where our optic nerves cross) the internal clock is ticking. And approximately every 24 hours, 20,000 special pacemaker cells that inhabit this area, known as the suprachiasmatic nucleus, synchronize, signaling to the rest of the body whether it’s night or day. These cells know which signal to send because they receive light input from our environments — bright says wake, dark says sleep.

在人腦中央深處有個叫做下丘腦的部位（就在視神經交叉處正上方），人體自帶的時鐘就在這裏運轉。大約每隔24小時，2萬個位於下丘腦“交叉上核”的特殊節律調節細胞就會同步一次，向身體其他部位發出信號，告知身體現在是白天還是黑夜。這些細胞知道該打什麼信號，因爲它們從環境中接收到光線刺激，明亮時就表示人該醒着，昏暗時就該睡覺。

But when you travel across multiple time zones, like flying from New York to Moscow, those little pacemaker cells that thought they knew the routine scramble around confused before they can put on their show. The whole body feels groggy because it’s looking for the time and can’t find it. The result: jet lag.

然而，當我們跨過好幾個時區，譬如從紐約飛到莫斯科的時候，這些自以爲通曉例行公事的節律細胞就被打亂了，不知該怎麼辦纔好。我們的身體也糊塗了，因爲它想要對時，卻不知道是什麼時間。結果就導致了時差。

Most of our internal clocks are a little bit slow, and in the absence of consistent light cues — like when you travel across time zones — the pacemaker cells in your body want to have a longer day, said Michelle Girvan, a physicist at the University of Maryland who worked on the model published in the journal Chaos on Tuesday.

馬里蘭大學的物理學家米歇爾‧葛文(Michelle Girvan)參與了建構這套模型的工作，研究成果週二時在《混沌》(Chaos)期刊上發表。葛文表示，我們體內大部分內建時鐘都稍微慢些，在沒有持續穩定的光源信號時，比如跨時區旅行的時候，體內的節律細胞就會想讓一天更長一些。

“This is all because the body’s internal clock has a natural period of slightly longer than 24 hours, which means that it has an easier time traveling west and lengthening the day than traveling east and shortening the day,” Dr. Girvan said.

“這全是因爲人體內建時鐘的自然週期比24小時稍長一些，也就是說，在向西旅行、導致一天的長度拉長時，感覺會比向東旅行導致一天縮短時更好受一些。”

Jet lag can be resolved by matching your internal clock to your destination’s clock as soon as possible. (There’s an app for that). So the researchers built a model that considers all of your pacemaker cells, how sensitive you are to light, the brightness of light, multiple time zones and people’s slightly off kilter internal clocks. They hope the model offers a simple way of explaining how a typical body might recover from jet lag with no intervention. That is, how its pacemaker cells try to synchronize in the presence of different light cues (like sunshine, artificial light or dim light from clouds) when arriving at various time zones three hours, six hours, nine hours and 12 hours away either to the east or west.

要解決時差問題，你可以讓體內時鐘儘快調整到與目的地的時間同步（有個智能應用可以幫忙）。所以研究人員建立了一個模型，將你的節律細胞、你對光線的敏感度、光線強度、跨越時區的多寡，以及我們稍稍失準的體內時鐘全都納入考慮。他們希望這個模型可以提供一個簡潔的方式，來解釋正常人的身體如何在沒有外在干預的情況下，把時差倒回來。也就是說，我們在抵達與出發地相差3小時、6小時、9小時、12小時的目的地之後，不論旅程是向東或向西，身體節律細胞是如何根據新地點不同的光線信號（比如陽光、人造光，或雲層篩落的黯淡光線）進行同步的。

The model confirms what was already known: Generally, westward recovery is easier than eastward. But it also helps us understand that flying across more time zones can sometimes be easier than traveling across fewer.

這個模型證實了我們已知的事實：一般來說，向西飛行導致的時差會比向東飛行的時差更容易克服。但它也讓我們瞭解到，有時跨度大的時差反而會比差距較小的時差更容易適應。

For example, it would take you about eight days to recover from a westward trip across nine time zones, if you did nothing to fight it. But if you cross the same number of time zones going east, recovery would take more than 13 days, according to the model. This recovery time is worse than if you flew smack across the globe, crossing 12 time zones, which is about the distance from New York to Japan.

比方說，根據這個模型，在沒有任何額外努力的情況下，要把向西跨越了9個時區的旅程造成的時差倒過來，需要大約8天的時間。不過要是你向東跨越了同樣多的時區，想倒時差就得花至少13天。這比你環繞半個地球、跨越12個時區（大約是紐約到日本的距離）所需的恢復時間更長。

Confusing? The model shows that your body is confused, too, as your cells try to adjust to new light cues in different places. It also shows that a trip less than 12 hours going east is going to feel worse than the same time going west.

看懵了嗎？從這個模型看得出來，你的身體在試着適應不同地方的新光線信號時也很懵。模型還顯示出，一趟短於12小時、向東行的旅程，會比花費相同時間向西行更令人難受。

It all comes back to whether you’re a lark — an early riser — or an owl, and “most people are a little owlish,” said David Welsh, a neuroscientist at the University of California, San Diego, who studies the body’s pacemaker cells and was not involved in the study.

加州大學聖地亞哥分校的神經科學家戴維‧威許(David Welsh)說，時差的影響有多大，最終還取決於你是隻早起的鳥兒還是夜貓子，“而大部分人都有點兒愛熬夜”。 威許研究身體節律細胞，不過他沒有參與前述研究。

If you’re traveling across several time zones, like from New York to Moscow, and you want to start feeling normal sooner, “you really want to experience that external stimuli appropriate to your new time zone as quickly as possible,” said Dr. Girvan. And that sometimes means owls succumbing to early nights.

如果你要進行跨越好幾個時區的旅行，比如從紐約到莫斯科，而你希望快點恢復正常感受，“你就要儘快去體驗與新時區作息相符的外在刺激”，葛文說。對於夜貓子來說，這有時就意味着早點上牀。