第一章 睡眠的演化逻辑：为什么人类必须睡觉？ Chapter 1: Evolutionary Logic of Sleep: Why Must Humans Sleep?

> 你有没有过这样的经历：考前通宵刷了3套数学卷，第二天上考场看到熟悉的题型却怎么都想不起公式？或者周末一口气睡到中午12点，起来反而头昏脑涨，连最喜欢玩的游戏都提不起劲？很多人会把这些问题归咎于“没睡够”或者“睡多了”，但如果把时间线拉到几百万年的演化长河里看，你会发现：睡眠从来不是人类可以随意压缩、随意补回的“可选任务”，而是刻在我们基因里的生存刚需——它是我们的大脑在数十亿年的演化中，为自己量身定做的“维修车间”和“升级工厂”。 > > 【English】Have you ever had this experience: you stayed up all night before an exam and did 3 sets of math papers, but when you took the exam the next day, you couldn't recall the formula even when you saw the familiar question? Or you slept in until noon on the weekend, but when you got up you felt dizzy and even had no interest in playing your favorite game? Many people attribute these problems to "not getting enough sleep" or "oversleeping", but if we pull the timeline back to the millions of years of evolution, you will find: sleep is never an "optional task" that humans can compress or make up at will, but a survival necessity engraved in our genes—it is a "maintenance workshop" and "upgrade factory" custom-made for our brain during hundreds of millions of years of evolution.

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一、睡眠的起源：比大脑更古老的生存机制

（中文）从单细胞生物到高等动物：睡眠是生命演化的“默认配置”

很多人认为只有拥有大脑的动物才需要睡觉，但2016年法国国家科学研究中心（CNRS）的一项研究彻底颠覆了这个认知：研究人员发现一种名为多头绒泡菌（*Physarum polycephalum*，俗称黏菌）的单细胞生物，在营养匮乏时会进入一种类似睡眠的状态：代谢率下降30%，对外界的触碰刺激反应速度降低40%，细胞活动周期会出现明显的“活跃-静止”交替，这种交替周期和我们人类的睡眠-觉醒周期高度相似。更神奇的是，如果给黏菌施加轻微的周期性刺激（相当于人类的“闹钟”），它们会在刺激结束后快速恢复到静止的睡眠状态，和人类被吵醒后很快能重新入睡的特征几乎一致。

这一发现说明：睡眠的起源远比神经系统、甚至远比动物的出现更早。在生命演化的早期，单细胞生物就已经进化出了“休息-活动”的交替机制，这种机制随着生物的复杂化不断升级，最终形成了我们现在看到的多样化睡眠模式。哪怕是地球上最简单的多细胞生物水螅，也会表现出周期性的活动抑制状态，而这种状态就是睡眠的雏形。

> 【纸书页码：P12】【电子版侧边栏提示】💡 小知识：2013年之前，科学界一直认为大脑没有淋巴系统，因为血脑屏障会阻止免疫细胞和废物进入大脑。直到罗切斯特大学的Maiken Nedergaard团队发现类淋巴系统，才彻底改写了我们对大脑代谢的认知。相关论文发表于《Science》期刊，DOI: 10.1126/science.1232251，感兴趣的读者可以搜索查阅。

（English）The Origin of Sleep: A Survival Mechanism Older Than the Brain

Many people think only animals with brains need to sleep, but a 2016 study by the French National Centre for Scientific Research (CNRS) completely subverted this认知: researchers found a single-celled organism called *Physarum polycephalum* (commonly known as slime mold) that enters a sleep-like state when nutrients are scarce: its metabolic rate drops by 30%, its response speed to external touch stimuli decreases by 40%, and its cell activity cycle shows a clear "active-rest" alternation, which is highly similar to the sleep-wake cycle of humans. What's more amazing is that if slime molds are subjected to mild periodic stimuli (equivalent to a human "alarm clock"), they will quickly return to a resting sleep state after the stimulus ends, which is almost the same as the characteristic that humans can fall asleep again quickly after being woken up.

This finding shows that the origin of sleep is much earlier than the nervous system, even much earlier than the emergence of animals. In the early stage of life evolution, single-celled organisms have evolved the alternating mechanism of "rest-activity", which has been continuously upgraded with the complexity of organisms, and finally formed the diversified sleep patterns we see now. Even the simplest multicellular organism on Earth, hydra, will show a periodic activity inhibition state, and this state is the prototype of sleep.

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二、动物的睡眠密码：生存策略决定睡眠模式

（中文）不同动物的睡眠时长差异：不是“想睡多久就睡多久”

如果把地球上所有动物的睡眠时长排个序，你会发现结果完全超出你的想象：每天睡22小时的考拉排第一，睡20小时的蝙蝠排第二，每天只睡2小时的长颈鹿排在末尾。这种巨大的差异从来不是随机的，而是和它们的生存策略、代谢水平、捕食压力直接相关。

我们可以把动物的睡眠模式分成三类： 1. 高警惕型睡眠：代表动物是长颈鹿、马、大象等食草动物。它们每天只需要2-4小时的睡眠，而且大多是浅睡眠，站着就能睡，哪怕睡着了也会保持对外界刺激的敏感度，一旦有危险可以立刻逃跑。长颈鹿的睡眠时间最短，每天只有2小时，而且每次深睡眠只有几分钟——因为它们的脖子太长，低头喝水的时候很容易被狮子、鬣狗等捕食者攻击，所以它们进化出了“少睡、浅睡、随时能醒”的睡眠模式。 2. 低耗型睡眠：代表动物是考拉、狮子、棕熊等。考拉每天睡22小时，因为它们的主食是桉树叶，这种食物的热量极低，消化需要消耗大量能量，所以它们只能通过长时间睡眠来降低代谢，节省能量。狮子作为顶级捕食者，没有天敌，食物充足，所以每天可以睡16小时，把能量用在狩猎和繁殖上。 3. 特殊适应型睡眠：代表动物是海豚、鸟类、部分鱼类。它们进化出了“单半球睡眠”（unihemispheric slow-wave sleep, USWS）：一半大脑进入深度睡眠，另一半大脑保持清醒。海豚用这种方式一边睡觉一边浮出水面呼吸，不会被淹死；候鸟在迁徙的时候可以连续飞行几十个小时不落地，就是靠单半球睡眠来让一半大脑休息，另一半大脑控制飞行方向。

> 【纸书页码：P13】下面这张图展示了不同动物的日均睡眠时长对比，你可以直观地看到不同生存策略对应的睡眠模式差异： > > ```mermaid > bar > title 不同动物日均睡眠时长对比 > x-axis 动物种类 > y-axis 睡眠时长（小时） > bar 考拉 22 > bar 蝙蝠 20 > bar 狮子 16 > bar 家猫 14 > bar 人类（青少年） 9 > bar 黑猩猩 10 > bar 长颈鹿 2 > bar 马 3 > ```

（English）Animal Sleep Codes: Survival Strategies Determine Sleep Patterns

If we rank the sleep duration of all animals on Earth, you will find the result is completely beyond your imagination: koalas that sleep 22 hours a day rank first, bats that sleep 20 hours a day rank second, and giraffes that sleep only 2 hours a day rank at the end. This huge difference is never random, but directly related to their survival strategies, metabolic levels, and predation pressure.

We can divide animal sleep patterns into three categories: 1. High-alertness sleep: Representative animals are giraffes, horses, and elephants. They only need 2-4 hours of sleep a day, and most of them are light sleep, they can sleep standing up, and even when they are asleep, they remain sensitive to external stimuli, so they can escape immediately when there is danger. Giraffes have the shortest sleep time, only 2 hours a day, and each deep sleep only lasts a few minutes—because their necks are too long, they are easily attacked by predators such as lions and hyenas when they lower their heads to drink water, so they have evolved a sleep pattern of "less sleep, light sleep, and can wake up at any time". 2. Low-consumption sleep: Representative animals are koalas, lions, and brown bears. Koalas sleep 22 hours a day because their staple food is eucalyptus leaves, which have extremely low calories and take a lot of energy to digest, so they can only reduce metabolism and save energy by sleeping for a long time. As top predators, lions have no natural enemies and abundant food, so they can sleep 16 hours a day and put their energy into hunting and reproduction. 3. Special-adaptive sleep: Representative animals are dolphins, birds, and some fish. They have evolved "unihemispheric slow-wave sleep (USWS)": one half of the brain enters deep sleep, while the other half remains awake. Dolphins use this method to sleep while floating to the surface to breathe, so they will not drown; migratory birds can fly continuously for dozens of hours during migration without landing, relying on unihemispheric sleep to let one half of the brain rest, while the other half controls the flight direction.

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三、人类的单相睡眠：演化出来的“最优解”

（中文）为什么人类不用像长颈鹿那样“站着睡碎片觉”？

和其他灵长类动物相比，人类的睡眠模式有一个非常明显的特征：单相睡眠（monophasic sleep），也就是把7-9小时的睡眠集中在夜间一次性完成，中间很少醒来。而我们的近亲黑猩猩、大猩猩每天睡10小时左右，但会分成3-4段，白天睡2-3次，晚上再睡一次，属于多相睡眠（polyphasic sleep）。

这种单相睡眠模式是直立人阶段（距今约200万年）才逐渐演化出来的，它的出现和人类的生存策略变化直接相关： 1. 降低夜间被捕食的风险：远古人类在演化出直立行走之后，白天外出狩猎、采集，晚上回到集体居住的洞穴里。集中睡眠可以让群体中一部分人负责放哨，其他人可以进入深度睡眠，而且单相睡眠的深度睡眠占比更高，对外界刺激的反应更弱，不容易被野兽的动静惊醒，比多相睡眠的生存安全性更高。 2. 提升复杂认知的效率：人类的脑容量是同等体重哺乳动物的3倍，大脑每天消耗全身20%的能量，产生的代谢废物也远多于其他动物。单相睡眠可以给大脑提供完整的、不被中断的修复时间，让大脑可以完成记忆巩固、突触修剪、代谢废物清除等复杂过程，这些过程如果被频繁打断，效率会下降60%以上。 3. 适应高复杂度的社会活动：人类是唯一会大规模协作、制造复杂工具、发展语言和文化的物种，这些活动都需要连续的时间投入。如果像多相睡眠那样频繁醒来，会打断工作、社交的连续性，降低群体的生产效率。

> 【纸书页码：P15】【电子版互动思考】🤔 思考：如果让你选择，你愿意像考拉一样每天睡22小时，吃低热量的食物，还是愿意像人类一样每天睡8小时，有更多的时间学习、娱乐、和朋友相处？为什么？

（English）Human Monophasic Sleep: The "Optimal Solution" Evolved

Compared with other primates, human sleep patterns have a very obvious characteristic: monophasic sleep, that is, concentrating 7-9 hours of sleep at night to complete at one time, with little waking in the middle. Our close relatives, chimpanzees and gorillas, sleep about 10 hours a day, but they are divided into 3-4 segments, sleeping 2-3 times during the day and once at night, which belongs to polyphasic sleep.

This monophasic sleep pattern was gradually evolved during the stage of Homo erectus (about 2 million years ago), and its emergence is directly related to the change of human survival strategies: 1. Reduce the risk of being preyed on at night: After ancient humans evolved upright walking, they went out to hunt and collect during the day, and returned to the communal cave at ni