Even brainless jellyfish enter sleep-like states where they pulse less and respond more slowly to food and movement.

即使是無腦的水母也有似睡狀態： 牠們收縮的次數會變少， 對食物和周圍動靜的反應也會變慢。

But all of the threats and demands animals face don't just go away when it's time to doze.

但這些動物必須面對的威脅和需求不會因為正值睡眠時間就自動消失。

That's why a range of birds and mammals experiences some degree of asymmetrical sleep where parts of their brain are asleep and other areas are more active.

這也就是為什麼某些鳥類和哺乳動物會有某種程度兩側大腦睡眠不對稱的狀態，也就是一部分大腦進入睡眠，而另一部分還在運轉。

This is even true for humans.

就連人類也會這樣。

So how does it work?

這是怎麼一回事？

All vertebrate brains consist of two hemispheres: the right and left.

所有脊椎動物的大腦都包含兩個半球：右腦和左腦。

Brain activity is usually similar across both during sleep.

睡眠中左右腦的活動相似，

But during asymmetrical sleep, one brain hemisphere can be in deep sleep while the other is in lighter sleep.

但是在睡眠不對稱的期間， 一邊的腦半球可能在沉睡，而另一邊則在淺眠狀態。

And in an extreme version called "unihemispheric sleep," one hemisphere may appear completely awake while the other is in deep sleep.

在稱為「單側腦半球睡眠」的極端情況中， 一邊的腦半球可能完全清醒，而另一邊則還在沉睡。

Take bottlenose dolphins.

以瓶鼻海豚為例：

Their breathing is consciously controlled, and they must surface for air every few minutes or they'll drown.

牠們有意識地控制呼吸，而且每隔幾分鐘就得浮出水面換氣， 否則就會溺斃。

When they have a newborn calf, they must actually swim nonstop for weeks in order to keep it safe.

牠們剛有新生兒的時候得要連續幾週不斷游來游去，才能讓寶寶存活下來。

So dolphins sleep unihemispherically, with just one hemisphere at a time.

海豚採用單側腦半球睡眠，一次只讓一邊的腦睡覺。

This allows them to continue swimming and breathing while snoozing.

這樣一來牠們就可以在打盹時 一邊游泳一邊呼吸。

Other marine mammals also need asymmetrical sleep.

其他海洋哺乳類也需要兩腦不對稱的睡眠。

Fur seals might spend weeks on end migrating at sea.

海狗可能會連續數週在海洋中遷徙，

They slip into unihemispheric sleep while floating horizontally,

牠們水平漂浮的時候陷入單側腦半球睡眠，

holding their nostrils above the surface, closing their upward-facing eye, and keeping their downward-facing eye open.

將鼻孔保持在水平面之上，闔起朝上的眼睛， 睜著向下的眼睛。

This may help them stay alert to threats from the depths.

此舉可以幫助牠們保持對海底威脅的警戒。

Similar pressures keep birds partially awake.

類似的壓力則讓鳥類呈現半醒狀態。

Mallard ducks sleep in groups, but some must inevitably be on the peripheries.

綠頭鴨會結伴成群一起睡，但免不了有些會在外圍。

Those ducks spend more time in unihemispheric sleep, with their outward-facing eyes open and their corresponding brain hemispheres more active.

那些鴨子較常有單側腦半球睡眠，會睜著向外的眼睛，同側的腦半球會較靈敏。

Other birds have been shown to catch z's in midair migration.

還有些鳥會在半空中遷徙時打盹。

While undertaking non-stop transoceanic flights of up to ten days, frigatebirds either sleep with one or both hemispheres at a time.

在長達十天跨洋飛行中，軍艦鳥會讓一邊的腦或是兩邊腦同時入睡。牠們會短暫入睡幾秒， 通常是在乘著氣流飛行之際。

But the frigatebirds still sleep less than 8% of what they would on land, suggesting a great tolerance for sleep deprivation.

但飛著睡的量依然比平常在陸地上時還少 8%，這也代表了牠們很能忍受睡眠剝奪。

It's currently unclear whether asymmetrical sleep packs the same benefits as sleep in both hemispheres and how this varies across species.

目前還不確定的是： 兩腦不對稱的睡眠和雙側腦半球睡眠是否有同樣的好處，是否也因物種而有所不同？

In one experiment, fur seals relied on asymmetrical sleep while being constantly stimulated.

實驗中，海狗靠兩腦不對稱的睡眠持續讓身體運作。

But in recovery, they showed a strong preference for sleep across both hemispheres, suggesting that it was more restorative for them.

但牠們在康復期間會偏好讓兩側的腦半球都進入睡眠，也就是說這樣能讓牠們修復功能更強。

Dolphins, on the other hand, have been observed to maintain high levels of alertness for at least five days.

海豚反而會保持至少五天高度警戒。

By switching which hemisphere is awake, they get several hours of deep sleep in each hemisphere throughout a 24-hour period.

牠們透過轉換清醒的半腦， 讓每一側的腦都有幾小時的深度睡眠分布在 24 小時之間，

This may be why unihemispheric sleep alone meets their needs.

也許是因為單側腦半球睡眠就能滿足牠們的需求。

So, what about humans?

那人類呢？

Have you ever woken up groggy after your first night in a new place?

你是否曾在外宿第一晚時半夜頭腦昏沉地醒來？

Part of your brain might've spent the night only somewhat asleep.

你的腦可能有部分沒有完全入睡。

For decades, scientists have recognized that participants sleep poorly their first night in the lab.

數十年來，科學家發現受試者在實驗室第一晚都睡不好，

It's actually customary to toss out that night's data.

根據慣例，當晚的數據通常會被扔掉。

In 2016, scientists discovered that this "first night effect" is a very subtle version of asymmetrical sleep in humans.

科學家在 2016 年發現 「第一夜效應」這種非常不易察覺的人類兩腦不對稱睡眠。

They saw that, during the first night, participants experience deeper sleep in their right hemisphere and lighter sleep in their left.

他們發現第一晚，受試者的右腦有較深層的睡眠，左腦則較淺眠。

When exposed to infrequent sounds, that lighter sleeping left hemisphere showed greater bumps in activity.

偶爾有聲響時，那個淺眠的左腦在活動中呈現較大的波動。

Participants also woke up and responded to infrequent sounds faster during the first night than when experiencing deep sleep in both hemispheres during nights following.

受試者第一晚醒來並對那些聲響做出反應的時間也會比接下來幾晚深層睡眠時更加快速。

This suggests that, like other animals, humans use asymmetrical sleep for vigilance, specifically in unfamiliar environments.

也就是說，就像其他動物，人類用兩腦不對稱睡眠來提高警戒，尤其是身在不熟悉的環境。

So, while your hotel room is obviously not trying to eat you and you're not going to die if you don't continue moving, your brain is still keeping you alert.

所以雖然你的旅館客房顯然不是要吃掉你，你也不會因為不動而喪命，但你的大腦仍然處在警戒之中，

Just in case.

以防萬一。