Studies using electrodes attached to the heads of sleepers have shown that when we sleep, we do so in two ways that alternate throughout the night. The first is rapid eye movement (REM) or active sleep. During this stage our eyes move, even though our eyelids are closed. Our muscles also twitch slightly, though they are largely paralysed so we don't hurt ourselves. In contrast, we also engage in non-REM sleep, during which we barely move at all. Most dream states, and certainly those with the most vivid dreams, happen during REM sleep.

研究显示，当人们入睡时，睡眠会以两种方式交替进行。第一种是快速眼动（REM）睡眠，即活跃睡眠阶段。在此期间，尽管眼皮闭合，但眼睛仍会活动；肌肉也会轻微抽动，不过大部分时间处于瘫痪状态，以免造成伤害。相比之下，我们还会进入非快速眼动（non-REM）睡眠，此时几乎没有任何运动。大多数梦境以及最生动的梦境都发生在快速眼动睡眠阶段。

There's some evidence that other mammals may also dream. For example, researchers compared the brain patterns of rats running through a maze when awake with their brain patterns during REM sleep. They found the patterns were very similar and concluded that the sleeping rats were dreaming about going through the maze.

有证据表明，其他哺乳动物也可能做梦。例如，研究人员比较了老鼠在清醒状态下通过迷宫时的大脑活动模式与它们在快速眼动睡眠期间的大脑活动模式。他们发现两种模式非常相似，并据此得出结论：这些正在睡觉的老鼠是在梦到自己正在穿越迷宫。

But finding evidence of dreaming in non-mammals has proved more difficult. Their brains are very different from those of humans, and it can often be difficult to record their activity while they are sleeping. Recently, however, researchers succeeded in recording brain activity in sleeping pigeons. As in mammals, the recordings revealed both REM and non-REM sleep. Intriguingly, REM sleep activity was high in brain regions involved in processing visual information, especially images related to physical activities such as flying, which suggests that this may possibly be what the pigeons were dreaming about.

但要找到非哺乳动物做梦的证据却更加困难。它们的大脑与人类大不相同，而且在睡眠期间记录其大脑活动常常十分困难。然而最近，研究人员成功记录了正在睡觉的鸽子的大脑活动。与哺乳动物类似，这些记录显示了快速眼动睡眠（REM）和非快速眼动睡眠（non-REM）。有趣的是，与处理视觉信息相关的脑区在快速眼动睡眠期间表现出较高活动，尤其是与飞行等身体活动相关的图像处理区域，这表明鸽子可能正在做着与这些内容有关的梦。

That said, dreaming and REM sleep are unlikely to be universal in the animal kingdom. For example, sponges don't have brains, so they lack the machinery for dreaming. There are also some animals with unusual sleep patterns. These include whales and dolphins, which do not shut down their entire brain when they sleep, but only half of it, keeping the rest awake. They also show no sign of REM sleep, suggesting that they may only experience non-REM dreams, which are less vivid. This is surprising because we tend to think of whales and dolphins as having complex inner lives. It's thought that they don't experience REM sleep because during REM sleep animals are more vulnerable to extremes of temperature.

尽管如此，梦和快速眼动睡眠（REM）在动物界可能并不普遍存在。例如，海绵没有大脑，因此缺乏做梦所需的生理机制。还有一些动物的睡眠模式较为特殊，比如鲸鱼和海豚。它们睡觉时并非完全关闭整个大脑，而只有一半的大脑进入休眠状态，其余部分保持清醒。此外，它们也未表现出快速眼动睡眠的迹象，这表明它们可能仅经历非快速眼动阶段的梦境，且梦境内容较不生动。这一点令人惊讶，因为我们通常认为鲸鱼和海豚拥有复杂的内在生活。人们普遍认为它们不会经历快速眼动睡眠，因为在此期间动物更容易受到极端温度的影响。

Nevertheless, in many cases REM sleep does seem to have benefits. Growing evidence from birds and mammals suggests that REM sleep and dreaming are important for forming memories and learning. It is believed that when events are replayed in dreams, this helps to integrate memories into longer-term storage. As soon as animals evolved moderately complex lifestyles, they would have needed to dream in order to manage these lifestyles.

然而，在许多情况下，快速眼动睡眠似乎确实具有益处。来自鸟类和哺乳动物的越来越多的证据表明，快速眼动睡眠和做梦对记忆形成和学习至关重要。人们认为，当事件在梦中被重演时，有助于将记忆整合到长期存储中。一旦动物进化出较为复杂的生活方式，它们就需要做梦来适应这些生活方式。

However, we still don't understand how this outward behaviour relates to internal experience. It seems impossible to know what it is like to be a rat or a pigeon, let alone imagine their dreamscapes. We are quick to interpret the twitching limbs and quiet barks of sleeping dogs, but the truth is that we don't know if there is an internal experience of chasing rabbits that comes along with that.

然而，我们仍然不明白这种外在行为与内在体验之间有何关联。似乎很难知道老鼠或鸽子是什么样的存在，更不用说想象它们的梦境世界了。我们很容易去解读睡着的狗四肢抽搐、发出低沉吠叫的样子，但事实上，我们并不知道，伴随这些行为是否存在一种追逐兔子的内在体验。

Another non-human dreamer offers insight here. In 2019, while making a documentary, David Scheel of Alaska Pacific University in the USA housed an octopus named Heidi in a tank in his living room. At one point, in the middle of the night, Heidi seemed to dream: her limbs and head moved, and her skin rapidly changed colour, as though she was pursuing a crab.

另一位非人类的梦者在此提供了见解。2019年，美国阿拉斯加太平洋大学的戴维·谢尔在客厅里用一个水箱饲养了一只名叫海蒂的章鱼。深夜时分，海蒂似乎做了一个梦：她的四肢和头部开始移动，皮肤迅速变色，仿佛正在追逐一只螃蟹。

Similarly, a report recently emerged of a sleeping octopus apparently having a nightmare. Costello, as the octopus was called, thrashed around, extended his mantle as if trying to make himself look bigger, and squirted ink as though he were being attacked by a predator. The nightmare study is intriguing, says Scheel, but is only based on one animal. He argues that as well as outward behaviour, brain imaging is needed to show that the octopuses are replaying sequences of activities from their waking lives in dreams.

同样，最近有一份报告称，一只正在睡觉的章鱼似乎做了一个噩梦。这只章鱼被称作科斯托洛，它四处扭动身体，伸展触手仿佛试图让自己看起来更大，还喷出墨汁，好像正遭到捕食者的攻击。谢尔表示，这项关于噩梦的研究很有趣，但仅基于一只动物。他认为，除了观察其外在行为之外，还需要通过脑部成像技术来证明，这些章鱼在梦中会重复其清醒时经历过的活动序列。

The trouble is that we will never be able to experience any animal's dreams. That goes for other humans' dreams too. But we can try to imagine what these dreamscapes are like by meeting animals on their own terms. For example, vision is the dominant sense for many humans, and so our dreams are heavily visual too. Dogs primarily navigate the world using smell while spiders rely much more on vibrations.

问题在于，我们永远无法体验任何动物的梦境，其他人类的梦境也是如此。但我们可以通过以动物自身的视角去接触它们，来尝试想象这些梦境世界是什么样子的。例如，对许多人来说，视觉是主要的感官，因此我们的梦境也以视觉为主。狗主要依靠嗅觉来感知世界，而蜘蛛则更多依赖振动来导航。

It is likely that dreaming has served multiple purposes since the first complex animals evolved. And if this is the case, it is possible that better understanding of these purposes might shed light on the true purpose of our own dreams.

自最早出现复杂动物以来，做梦很可能具有多种目的。如果真是如此，那么更深入地理解这些目的或许能揭示我们自身梦境的真正意义。