大脑如何从清醒状态过渡到睡眠（并再次醒来）
内容总结：
【本报综合报道】人类大脑如何实现睡眠与觉醒间的自如切换？这一过程长期以来被视为生命科学的重要谜题。最新研究表明，睡眠并非简单的非此即彼状态，而是一个充满创造性潜能的动态谱系。
睡眠启动：意识转化的精妙过程
当人类进入睡眠状态时，大脑会启动一系列复杂变化：脑部血流量减缓，脑脊液循环加速，神经元活动逐渐同步化。麻省理工学院睡眠科学家亚当·霍洛维茨指出：“进入睡眠需要全身系统协同转变。”在此过程中，人们会经历被称为“入睡期”的过渡阶段，此时可能出现既非完全清醒又非完全沉睡的独特意识状态。
科学溯源：从脑波解密到创造力迸发
自1930年代金融大亨阿尔弗雷德·李·卢米斯开创脑电图睡眠研究以来，科学家已识别出包括快速眼动睡眠在内的多个睡眠阶段。值得注意的是，超现实主义艺术家达利早在上世纪就通过“钥匙坠盘”实验探索入睡初期的创造性潜能。2021年巴黎脑研究所研究证实，在入睡初期（N1阶段）被唤醒的实验对象，解决数学难题的创造力提升近三倍。
觉醒机制：大脑的“开机”信号
瑞士洛桑大学研究员奥雷莉·斯蒂芬近期通过对上千次觉醒过程的分析，发现优质睡眠者从非快速眼动睡眠醒来时会出现独特的慢脑波信号。该信号从大脑前额叶向后部传导，如同按下“开机键”般启动觉醒程序。值得注意的是，这种信号强度与醒来后的清醒程度直接相关。
睡眠障碍：状态转换的失调
研究表明，失眠、睡眠瘫痪等常见睡眠问题本质上是大脑不同区域状态转换失调所致。巴黎脑研究院认知神经科学家托马斯·安德里永指出，即使在清醒状态下，部分大脑区域仍可能处于局部睡眠模式，这种“碎片化睡眠”现象挑战了传统睡眠定义。
目前，全球科学家正通过脑电图、功能磁共振等先进技术，持续解码睡眠意识的神秘疆界。这些研究不仅深化了人类对意识本质的理解，更为治疗睡眠障碍开辟了新途径。
中文翻译：
大脑如何从清醒步入睡眠（再重返清醒）
引言
枕面触颊生凉。楼上邻居踩着吱呀作响的地板走过天花板。你合上双眼，光与影在视野中翩跹起舞。一只猫在嗅闻奶酪碎屑，彩色圆点坠入湖中。这一切都显得如此理所当然——尽管你从未养猫，也远离湖泊。
你已开启通往睡眠的旅程。这种神秘状态是你和大多数动物赖以生存的生理需求。睡眠以我们尚未完全理解的方式焕新大脑与身体：修复组织、清除毒素、巩固记忆。但失眠者都能证明，进入这种状态无论在生理还是心理层面都绝非易事。
“要实现睡眠，万物皆需蜕变。”麻省理工学院睡眠科学研究员亚当·霍罗威茨如是说。流向大脑的血液减速，脑脊液循环加速。神经元释放的神经递质改变着大脑化学环境，它们开始以同步节律齐鸣共舞。心象浮光掠影，思维渐生涟漪。
麻省理工学院睡眠研究员劳拉·刘易斯指出：“我们的大脑能瞬间将我们从感知环境切换至无意识状态，甚至体验虚无幻象。这为人类体验提出了深邃迷人的命题。”
大脑如何安全高效地实现状态转换仍是未解之谜。但针对入睡与醒觉过渡阶段的研究，正逐渐揭示这些中间状态的神经生物学基础。这种认知进步有望解释当转换机制失常时，失眠或睡眠瘫痪等睡眠障碍如何形成。
刘易斯表示，传统睡眠观认为睡眠是非此即彼的状态，非醒即眠。但新研究发现“这更像一道光谱，而非泾渭分明的分类”。
脑波探秘
1930年代，华尔街百万富翁、律师兼业余科学家阿尔弗雷德·李·卢米斯喜欢在纽约州北部宅邸中扫描小憩宾客的大脑。他开创性地运用脑电图仪研究睡眠，为每位休憩者佩戴电极帽无创监测脑部活动。仪器以每秒1厘米速度在纸卷上绘制出峰谷起伏的波形图，形成脑电图。
这些波动表征着神经元的整体活动。入睡时神经元开始同步放电，齐鸣共寂（成因至今未明）。随着睡眠深入，这种同步性增强，产生频率更低、振幅更高的脑波。整夜睡眠中，脑波会以循环节律加速放缓。卢米斯将不同脑波类型归类为睡眠状态，建立了描述无意识阶段的命名体系。
脑电图技术催生了睡眠研究革命。通过解读脑波成为神经科学家无创推断人脑状态的主流方法。借助该技术，我们既能解析睡眠中的神经元活动，也能探索不同睡眠意识形态衍生的主观体验（如梦境）。
1950年代初，芝加哥大学生理学家纳撒尼尔·克莱特曼与学生尤金·阿瑟林斯基首次描述了快速眼动睡眠阶段——大脑每夜重复多次的周期，梦境多诞生于此。REM睡眠期的脑波较非REM期更接近清醒状态。数年后，克莱特曼与同校睡眠研究员威廉·德门特整合出改良版睡眠分期模型：基于卢米斯研究的四阶段非REM睡眠，加上REM睡眠阶段。经修订的版本（最后两个非REM阶段合并）沿用至今。
巴黎脑研究所认知神经科学家托马斯·安德里永指出，这种划分虽构建了清晰边界，却模糊了过渡期的精微变化，导致领域内形成“非醒即非REM，或REM”的三选一范式。
尽管早有证据表明大脑可能存在清醒与睡眠的混合态，但因其复杂多变背离主流研究的精确界定而长期被忽视。安德里永表示，新一代神经科学家逐渐开始质疑这种范式，并意识到“或许正是过渡地带蕴藏着真知”。
入眠时分
萨尔瓦多·达利想必深有同感。
当卢米斯在宅邸进行脑电实验时，这位超现实主义艺术家正探索自己的入眠过渡。据其1948年著作《魔法技艺的50个秘诀》所述，他端坐于“骨感扶手椅（优选西班牙款式）”，掌心虚握重钥悬于地面倒置的盘子上空。恍惚间手指松驰，钥匙坠盘铿然，将他惊醒。
达利深信此刻被唤醒能重燃灵性、激发创意，随即伏案作画。托马斯·爱迪生、埃德加·爱伦·坡等巨匠同样痴迷于这种催眠幻觉状态——清醒末期开始出现心象的睡眠窗口期。
2021年，安德里永等巴黎脑研究所团队验证了这些自省者的直觉：从N1期（睡眠最初阶段）苏醒似乎让人进入“创意甜蜜点”。经历约15秒催眠状态后醒来者，发现数学问题隐藏规律的概率提升近三倍。几年后，麻省理工学院霍罗威茨团队的后续研究表明，通过引导梦境可进一步增强该状态下的创造力。
催眠期提升创造力的机制尚不明确。曾在西北大学研究清醒梦、现任职霍罗威茨联合创立的Dust Systems公司的卡伦·康科利解释：“入睡过程要求释放对思维的掌控。当执行控制放松时，我们或许能触及更广阔语义网络，从而激发创意。”安德里永赞同睡眠过渡会产生“自由流转的意识”，使大脑摆脱常规思维定式。
如同入夜后渐次熄灯的城镇，大脑逐步切换至夜间模式。睡眠从大脑中枢启动：下丘脑等深部神经元发出信号抑制唤醒回路。邻近的丘脑（负责向大脑传递感官信息）率先关闭。数分钟后，掌管高阶思维的皮层随之沉寂，从负责计划决策的前额叶向处理视觉等感官的后部依次关闭。
在此过渡期，当部分脑区休眠而其他区域仍清醒时，我们可能体验梦幻思绪。霍罗威茨描述，许多人在此状态“一脚踏入梦境，一脚留在现实”。有人听见声响，有人看见幻象。这些体验似梦却更轻盈，是在尚能感知的现实框架上投射的浮光掠影。
巴西北大河联邦大学神经科学家西达塔·里贝罗认为：“这些精神体验或许存在功能，也可能只是大脑活动的副产品。”
闭目凝神，感官渐弱，外界输入急剧减少，但脑内信号仍在涌动，或许是白日经历的残影。里贝罗团队最新研究发现，白天经历会显现在入睡早期的催眠意象中，印证了此前多项研究的结论。
部分研究者正利用这种半梦半醒的状态探索意识本质。巴黎脑研究所睡眠与意识研究方向研究生尼古拉·德卡指出：“追踪这两个对立世界切换时的大脑活动，能极大揭示意识波动机制。”在尚未经同行评审的初步研究中，他记录百余位受试者入睡时的脑波，借鉴达利与爱迪生的方法，让参与者持瓶入睡，坠瓶声响将其唤醒。
比对脑波与主观报告后，德卡发现有些梦样意象发生于技术性清醒期，而某些自主思维出现在技术性睡眠期。例如有受试者报告蚂蚁爬背幻觉时，脑电图却显示清醒期特有的高频脑波；另一受试者在低频慢波定义的睡眠期，仍保持对自身入睡过程的清醒认知。
这项未公开数据质疑了用睡眠状态划分睡眠意识的传统。德卡强调：“清醒与否不能完全决定意识内容。数据挑战了流行观念——清醒时产生理性思维，睡眠时出现梦样意象，现实远非如此。”
入睡过渡可持续数十分钟，这使研究者易于观测。而觉醒过程不仅更为迅疾难以控制，其发生时机也更难预测。
晨光苏醒
瑞士洛桑大学博士后研究员奥雷莉·斯蒂芬在研究矛盾性失眠时对觉醒过程产生兴趣。与彻夜未眠的失眠者不同，矛盾性失眠者虽脑波显示已入睡，却坚信自己整夜清醒。“他们的实际睡眠时长与正常睡眠者相当……这成了难解之谜。”斯蒂芬说道。
为破解此谜，她首先需要研究典型觉醒过程：正常睡眠者苏醒时，其大脑正在经历什么？
近期研究中，她以秒为单位审视千余次觉醒案例，发现正常睡眠者从非REM睡眠苏醒时会出现奇特的慢波。基于既往动物实验，斯蒂芬团队推测该慢波源自大脑深部。此信号出现后，她观察到皮层自管理执行功能的前额叶向处理视觉的后部依次激活（脑波加速）。而从REM睡眠苏醒时，皮层虽以相同方式激活，却无前置慢波。
斯蒂芬发现这种独特慢波与觉醒体验相关：出现该信号的受试者醒后困倦感更轻。她表示这暗示（虽未证实）可能是辅助觉醒的激活信号。
斯坦福大学研究动物睡眠转换的分子生物学家路易斯·德莱塞（未参与该研究）评价：“他们对睡眠-觉醒转换特征的研究非常出色。”刘易斯认为该研究绘制了“精细图谱”，揭示“我们觉醒方式存在差异”的成因。
尽管脑电图读数粗糙，无法探测深部脑区细节，但先前运用功能磁共振与深部电极的研究已揭示潜在激活信号的深层机制：觉醒神经信号始于下丘脑与脑干等大脑深部区域，这些区域唤醒丘脑，再由丘脑向皮层传递指令。
斯蒂芬指出，觉醒虽通常快于入睡，但仍需时间。其发现的睡眠特征信号需数秒从前额叶传至后脑。但意识恢复、认知能力重建及睡眠惯性完全消退可能需要数分钟至一小时。该研究与其他实验均表明，通常与睡眠相关的慢波有时却预示觉醒——界限实难划定。
即便自认完全清醒地感知世界，我们大脑的某些区域可能仍在沉睡。这种“局部睡眠”现象使过度工作的神经元得以休整，与海豚半脑睡眠、鸟类飞行中休憩的机制异曲同工。安德里永解释道：“这些人确实醒着，睁着眼甚至能执行任务。”但他们部分脑区却呈现典型的睡眠慢波。正因如此，局部睡眠对“睡眠”本质提出了挑战。
转换困境
在醒睡转换乃至睡眠状态交替时，不同脑区神经元同步与去同步化会形成多种波型并存的混乱节律。这种镶嵌模式可能引发催眠幻觉、清醒梦及睡眠障碍。
刘易斯指出：“睡眠障碍极为普遍，其本质常是状态转换故障。”
这些障碍可表现为入睡困难的失眠，或觉醒异常的夜惊、睡眠瘫痪与梦游。多数情况下，问题源于本应休眠的脑区保持清醒，或反之。
失眠本质是启动或维持睡眠转换的困难；睡眠瘫痪是皮层先于控制身体的深部脑区苏醒，导致意识清醒却无法动弹；矛盾性失眠中，斯蒂芬新发现的潜在激活信号微弱，“未能完全唤醒他们，却制造出清醒感”。她的团队在梦游者脑中发现了相同信号，但出现在深睡期“不恰当的时间窗口”。梦游者脑活动与做梦状态相似，暗示两种状态源于相似的睡眠意识机制。
德卡正继续探索睡眠意识的表现形式。他通过问卷调查收集人们入睡时的心理体验。这些思绪与心象难以捕捉，因为回忆它们需要我们先行醒来。
有时我们在刚入睡或深睡期不合时宜地惊醒——或许是同床者的翻身扰动，或许是钥匙坠地的脆响，又或许是大脑自身对区域唤醒时机的误判。
你的睡眠意识被骤然打断。从睡眠边缘抽身而出，眼帘缓缓睁开。
英文来源：
How the Brain Moves From Waking Life to Sleep (and Back Again)
Introduction
The pillow is cold against your cheek. Your upstairs neighbor creaks across the ceiling. You close your eyes; shadows and light dance across your vision. A cat sniffs at a piece of cheese. Dots fall into a lake. All this feels very normal and fine, even though you don’t own a cat and you’re nowhere near a lake.
You’ve started your journey into sleep, the cryptic state that you and most other animals need in some form to survive. Sleep refreshes the brain and body in ways we don’t fully understand: repairing tissues, clearing out toxins and solidifying memories. But as anyone who has experienced insomnia can attest, entering that state isn’t physiologically or psychologically simple.
To fall asleep, “everything has to change,” said Adam Horowitz, a research affiliate in sleep science at the Massachusetts Institute of Technology. The flow of blood to the brain slows down, and the circulation of cerebrospinal fluid speeds up. Neurons release neurotransmitters that shift the brain’s chemistry, and they start to behave differently, firing more in sync with one another. Mental images float in and out. Thoughts begin to warp.
“Our brains can really rapidly transform us from being aware of our environments to being unconscious, or even experiencing things that aren’t there,” said Laura Lewis, a sleep researcher at MIT. “This raises deeply fascinating questions about our human experience.”
It’s still largely mysterious how the brain manages to move between these states safely and efficiently. But studies targeting transitions both into and out of sleep are starting to unravel the neurobiological underpinnings of these in-between states, yielding an understanding that could explain how sleep disorders, such as insomnia or sleep paralysis, can result when things go awry.
Sleep has been traditionally thought of as an all-or-nothing phenomenon, Lewis said. You’re either awake or asleep. But the new findings are showing that it’s “much more of a spectrum than it is a category.”
Riding the Brain Wave
In the 1930s, the millionaire Wall Street tycoon, lawyer and amateur scientist Alfred Lee Loomis liked to scan the brains of his guests as they napped in his upstate New York mansion. He was pioneering the use of a machine known as an electroencephalograph to study sleep. Every napper wore a cap with electrodes, which could noninvasively measure their brain activity. The machine would use a pen to physically scribble waves with peaks and troughs onto paper scrolling at a rate of 1 centimeter per second to create an electroencephalogram (EEG).
The waves represented the gross activity of neurons. As we fall asleep, neurons start to synchronize, which means they fire together and go silent together. (No one knows exactly why this happens.) As a person sleeps, this synchrony grows, producing brain waves that are lower in frequency and higher in amplitude. Over the course of a night’s sleep, the waves will speed up and slow down in a cyclical fashion — all night, every night. Loomis categorized the different types of brain waves into what became known as sleep states, and created a nomenclature to describe the phases of unconsciousness.
Electroencephalography catalyzed sleep research. Measuring the waves recorded on an EEG became a common way for neuroscientists to infer a person’s brain or sleep state without invasive surgery. It became the go-to method for understanding both the activity of neurons as we sleep and the subjective experiences, such as dreams, that they create as we move through different forms of sleep consciousness.
In the early 1950s, the physiologist Nathanial Kleitman at the University of Chicago and his student Eugene Aserinsky first described the sleep stage categorized by rapid eye movement, or REM sleep — a cycle the brain repeats multiple times throughout the night, and during which we tend to dream. In REM sleep, brain waves are faster than in non-REM sleep and look more like those produced when we’re awake. A few years later, Kleitman and the sleep researcher William Dement, also at the University of Chicago, put together an improved sleep-stage schema: four non-REM sleep stages, based on Loomis’ original work, and one REM stage. A modified version (with the last two non-REM stages combined into a single stage) is still in use today.
However, by creating sharp boundaries, the schema obscured the subtleties of what happened between them. It became a norm in the field that “you have three options: You are either awake, in non-REM [sleep] or in REM sleep,” said Thomas Andrillon, a cognitive neuroscientist at the Paris Brain Institute.
Though there was some evidence that the brain could exist in a state that mixed sleep and wakefulness, it was largely ignored. It was considered too complicated and variable, counter to most researchers’ tightly defined view of sleep.
But little by little, a new wave of neuroscientists started questioning this status quo, Andrillon said. And they realized, “well, maybe that’s where things are interesting, actually.”
Drifting Off
Salvador Dalí might agree.
Around the time that Loomis was conducting EEG experiments in his mansion, the surrealist artist was experimenting with his own transitions into sleep. As he described it in his 1948 book 50 Secrets of Magic Craftsmanship, he would sit in a “bony armchair, preferably of Spanish style,” while loosely holding a heavy key in one palm above an upside-down plate on the floor. As he drifted off, his hands would slacken — and eventually the key would fall through his fingers. The sudden clack of the key hitting the plate would wake him.
Convinced that being aroused during this period revived his psychic being and boosted creativity, Dalí would then sit down and start painting. Other great minds, including Thomas Edison and Edgar Allan Poe, shared his interest in and experimentation with what is known as the hypnagogic state — the early window of sleep when we start to experience mental imagery while we’re still awake.
In 2021, a group of researchers at the Paris Brain Institute, including Andrillon, discovered that these self-experimenters had gotten it right. Waking up from this earliest sleep stage, known as N1, seemed to put people in a “creative sweet spot.” People who woke up after spending around 15 seconds in the hypnagogic state were nearly three times more likely to discover a hidden rule in a mathematical problem. A couple years later, another study led by Horowitz at MIT found that it’s possible to further boost creativity in people emerging from this state by guiding what they dream about.
It’s not exactly clear why hypnagogia appears to boost creativity. One possibility is that the process of falling asleep “requires us to release control over our thoughts,” said Karen Konkoly, who studied lucid dreaming as a postdoctoral fellow at Northwestern University and now consults for the sleep start-up Dust Systems (co-founded by Horowitz). “As our executive control over our mind relaxes, we can perhaps access a broader semantic network of information, which could help creativity.” Andrillon agrees that the sleep transition produces a state of “free-wheeling consciousness” that unshackles the brain from its regular ways of thinking.
Like houses slowly shutting off their lights as a town falls into slumber, the brain gradually turns to night mode. Sleep starts at the center of town: Neurons deep in the brain, such as those in the ancient control center known as the hypothalamus, fire signals to suppress arousal circuits. Nearby brain regions such as the thalamus, which relays information from your senses to the rest of your brain, shut off first. Minutes later, the cortex, which is involved in more conscious, high-order thinking, follows suit. It shuts down from the front of the brain, where planning and decision-making occurs, to the back, where senses such as vision are analyzed.
During this transition, as some parts of the brain shut down while other parts remain awake, we can sometimes experience dreamlike thoughts. In this hypnagogic state, many people are “one foot in dreams and one foot in the world,” Horowitz said. Some people hear things; others have visions. These are like dreams but lighter: projections against the scaffold of the real world, which is still in our grasp.
“We could think that there’s a function” to these mental experiences, said Sidarta Ribeiro, a neuroscientist at the Federal University of Rio Grande do Norte in Brazil. “But maybe there isn’t. Maybe it’s a by-product of what’s going on in the brain.”
With your eyes shut and your senses powering down, you’re no longer getting much input from the outside world. But you’re still getting signals from inside the brain, maybe remnants of the day’s experiences. Ribeiro and his team recently reported that a person’s daytime experience can show up in hypnagogic imagery early in the process of drifting to sleep, adding to other studies that made similar findings.
Some researchers are using this state between sleep and wakefulness to study the nature of consciousness itself. “If you can track what’s going on in the brain when you go from those two opposite worlds, that would give you a lot of insights as to how consciousness fluctuates,” said Nicolas Decat, a graduate student studying sleep and consciousness at the Paris Brain Institute. In preliminary research that’s not yet peer-reviewed, Decat used an EEG to record the brain waves of more than 100 people as they were falling asleep. Following the techniques of Dalí and Edison, he had participants hold bottles so that as they drifted off, the bottles would fall and make a sound to wake them up.
By comparing the participants’ brain waves with their self-reports about what crossed their minds, Decat realized that some dreamlike imagery had occurred while they were technically awake, and some voluntary thinking had occurred while they were technically sleeping. For example, one participant reported ants crawling on her back even as the EEG documented the fast and frequent brain waves of wakefulness. Another reported having conscious thoughts about how they were falling asleep while they were technically asleep, based on slow and infrequent brain waves.
The unpublished data suggests that sleep states may not be the best way to categorize sleep consciousness. “Being awake or asleep does not fully determine what crosses your mind,” Decat said. The data “challenges the popular view that when you’re awake, you have certain thoughts. When you’re asleep, you have dreamlike imagery. It’s not necessarily like that.”
The transition to sleep can last for tens of minutes. That means it’s fairly easy for researchers to study — far easier than the process of waking up, which happens much more quickly and in a less controlled way. It’s much harder to predict when someone’s going to wake up.
Good Morning, Sunshine
Aurélie Stephan, a postdoctoral researcher at the University of Lausanne in Switzerland, grew interested in the wake-up process when she was studying a phenomenon known as paradoxical insomnia. Unlike people with insomnia, who are up all night without sleeping, people with paradoxical insomnia believe that they’re up all night even though their brain waves show that they’re asleep. “They sleep as much as good sleepers … so it’s a mystery,” Stephan said.
To understand this problem, she needed to first study a more typical wake-up process. When a good sleeper wakes up, what is their brain doing?
In a recent study, she examined more than 1,000 different awakenings or arousals — transitions from being asleep to being awake — on a timescale of seconds. She observed a curious slow brain wave in the data from good sleepers as they woke from non-REM sleep. Based on past animal studies, Stephan and her team hypothesized that this slow wave emanated from a spot deep in the brain.
After this signal, she saw the cortex wake up (as brain waves grew faster) from the front, which manages executive function, to the back, where vision and other senses are processed. When people woke from REM sleep, their cortex woke up in the same way, but without the preceding slow wave.
The presence of this unique slow wave was correlated with how people felt when they awoke, Stephan found. Participants who showed the signal woke up less drowsy than those without it. This suggested, but didn’t prove, that this might be an arousal signal that assists the wake-up process, Stephan said.
“They have done a very good job of finding this signature of the sleep-to-wake transitions,” said Luis de Lecea, a molecular biologist who studies sleep transitions in animals at Stanford University and was not involved with the study. They created a “detailed portrait,” Lewis said, and showed why “we don’t always wake up the same way.”
Still, EEG readings are coarse and can’t probe the deep brain or provide great detail. However, previous studies that used fMRI and electrodes unearthed some of the deeper mechanisms from which such arousal signals might arise. They found that neural signals for waking begin in deep, inner regions of the brain, such as the hypothalamus and the brainstem. These areas wake up the thalamus, which projects the instructions to the cortex.
Though typically faster than falling asleep, waking up can also take some time. Stephan’s sleep signature took a few seconds to travel from the front of the cortex to the back. But recovering consciousness and cognitive abilities, and shedding all sleep inertia, can take minutes to an hour, she said. This study and others also showed that slow waves, usually associated with sleep, can sometimes indicate arousal. The lines are blurry.
Even when we think we’re fully awake and wandering about the world, parts of our brain could be sleeping. This phenomenon, known as local sleep, is thought to occur so that overworked neurons in the brain can rest and be refreshed. It is not unlike how dolphins can sleep with only one brain hemisphere at a time or how some birds sleep on the wing. Sometimes when we’re really tired, some neurons need to refresh and recharge, even if we’re still up and going about our day.
“These people are awake. They have their eyes open. They can be even doing things,” Andrillon said. And yet parts of their brain are undergoing the classic slow waves of sleep. Given that, local sleep challenges what “sleep” actually is.
Troubled Transitions
As we wake up and fall asleep, or even move between sleep states, different types of waves happen at the same time, as neurons synchronize and desynchronize in different regions, in a cacophony of rhythms. This mosaic can lead to experiences such as hypnagogia, lucid dreaming and sleep disorders.
“Sleep disorders are incredibly common,” Lewis said. “They really are often defined by problems with the state switching.”
These disorders might manifest as insomnia, where people don’t fall asleep properly, or as night terrors, sleep paralysis or sleepwalking, where they don’t awaken as expected. In many cases, parts of the brain are awake when they should be sleeping, or vice versa.
Insomnia is fundamentally a difficulty with initiating the transition into sleep or maintaining it. In sleep paralysis, the cortex wakes up before deeper brain regions that control the body, resulting in full consciousness without the ability to move. In paradoxical insomnia, the potential arousal signal Stephan observed in her new study is weak, “so instead of waking them up completely it makes them feel awake,” she said. Her team found the same signal in sleepwalkers, but in those cases it happened “in an inappropriate time window” during deep sleep, she said. They also found that the brain activity of sleepwalkers is similar to that seen during dreaming, suggesting that both states result from similar mechanisms of sleep consciousness.
Decat is continuing to probe what that sleep consciousness looks like. He is running a survey to learn more about the mental experiences people have while falling asleep. Those thoughts and mental images can be hard to remember because to do so, we have to wake up.
Sometimes we wake up right as we’re falling asleep or from the depths of our sleep cycle — times we’re not really supposed to. Maybe it’s a bedmate turning in their sleep that disturbs us. Maybe it’s the clink of keys on a hard floor. Maybe it’s the brain itself, miscalculating when it’s supposed to arouse certain regions.
Your sleep consciousness is disrupted. You pull back from the edge of sleep, and your eyes blink open.