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In modern society, a "good night's sleep" is universally understood to mean a single, uninterrupted eight-hour block of rest, known as ‘monophasic sleep.’ We unconsciously assume this sleep pattern is a universal and biologically fixed norm for humanity. However, this belief is far from historical reality. Modern monophasic sleep is a relatively recent cultural construct in the long span of human history, a product of specific historical pressures.
This report begins with the premise that the modern concept of monophasic sleep is not a historical or biological standard. For millennia, humanity maintained a pattern of ‘segmented sleep’ or ‘biphasic sleep,’ dividing the night's rest into two distinct periods. The core argument of this report is that the transition from a natural, segmented sleep pattern to a rigid, monophasic one is one of the most significant yet overlooked changes in human daily life, profoundly impacting our physical health, mental well-being, and ultimately, our lifespan.
To substantiate this thesis, this report will first delve into historical records to explore the nature of the pre-industrial night and the culture of segmented sleep that thrived within it. It will then trace how two powerful forces—the invention of artificial lighting and the Industrial Revolution—conquered the human night and reshaped the rhythm of sleep. Furthermore, from the perspective of modern sleep science, it will precisely analyze the neurobiological and endocrinological advantages and vulnerabilities of monophasic sleep. Finally, by synthesizing the results of large-scale epidemiological studies, it will seek to determine the tangible impact of sleep patterns on human health and longevity. Through this comprehensive analysis, readers will be encouraged to move beyond fixed notions of sleep, understand its historical and biological complexity, and redefine the meaning of healthy sleep in contemporary society.
To understand that modern sleep patterns are not the universal standard of human history, we must first travel back to a time when the concept of monophasic sleep did not exist. In the pre-industrial world, the night was not merely a time when activity ceased, nor was sleep a task to be completed in one go. Sleep during this era was a unique experience, intricately woven with social, cultural, and environmental factors.
The pre-industrial night was a world of profound darkness, difficult for modern individuals to imagine. Except for moonlight or the faint glow of a candle, the night was enveloped in pitch-black darkness, which deeply influenced people's lifestyles and psychology.1 According to the research of historian A. Roger Ekirch, the night of this era was not just dark but an ‘alternate reality’ where fear and opportunity coexisted.1 In the darkness lurked not only supernatural threats like wolves and demons but also real dangers such as thieves and arsonists.3 In urban areas, curfews were imposed and night watchmen patrolled, but these measures did not completely eliminate the perils of the night.3
This environment induced high levels of stress, a major factor disrupting deep and peaceful sleep. According to the records of the 17th-century healer Richard Napier, about 20% of his patients suffered from insomnia.3 People slept with weapons within reach for fear of burglars and installed devices like bells on shutters to guard against intruders.3 Thus, the pre-industrial night was far from a romantic tranquility; it was a time filled with noise, stress, and anxiety. However, the darkness also offered freedom from the social constraints of the day and provided a private space. The low light narrowed the physical and emotional distance between people, and in an environment where sight was limited, hearing, touch, and smell became more important, fostering an intimate atmosphere.1
While a private, individual bedroom is taken for granted in modern society, in the pre-industrial era, it was common to share a bed not only with family members but even with guests.3 Beds were very expensive pieces of furniture, sometimes accounting for more than a third of the value of an ordinary family's entire household assets.3 Consequently, it was a universal experience for several siblings to sleep together in one bed, and sometimes an entire family of five or six would use a single bed.3
From a modern perspective, this communal sleeping culture might seem unhygienic and uncomfortable. Indeed, sleep disruption due to pests like fleas and bedbugs was a very common occurrence, and a pre-sleep "hunt" to remove insects from bedding was a daily ritual.1 Additionally, various noises such as barking dogs, meowing cats, and the sounds of livestock broke the silence of the night.3 Such cramped, uncomfortable, and noisy environments inherently created conditions where sleep was frequently interrupted. Paradoxically, this made waking up in the middle of the night seem neither abnormal nor problematic. In other words, fragmented sleep was a natural part of daily life, not considered a medical condition.
Historian Roger Ekirch of Virginia Tech, through more than 16 years of research, revealed that the dominant sleep pattern in pre-industrial Western society was not the monophasic sleep of today, but a biphasic or segmented sleep, divided into two parts.2 He analyzed a vast array of historical sources, including diaries, court records, medical literature, and literary works, to prove that terms like ‘first sleep’ and ‘second sleep’ were used in everyday language.5
According to this pattern, people typically went to bed between 9 and 10 p.m. after sunset and had their ‘first sleep’ for about 3-4 hours.5 Around midnight, they would naturally wake up and spend about one to three hours in a state of wakefulness before returning to their ‘second sleep,’ which lasted until dawn.5 Ekirch found 83 references to ‘first sleep’ in English sources from 1300 to the 1800s alone, and this evidence suggests that segmented sleep was not a phenomenon limited to a specific class or region but was a universally practiced norm throughout Western society.10
The period of wakefulness between the two sleeps, known as ‘the watch,’ was by no means a time of suffering from sleeplessness.11 On the contrary, it was a culturally significant time filled with highly productive and meaningful activities. During this quiet time, people engaged in introspection through prayer or meditation, reflected on the meaning of their dreams, had marital relations, or chatted with neighbors.4 It was also a time for light household chores like tending the fire or sewing.5
This ‘watch’ period served important social and psychological functions. Pre-industrial life was characterized by relentless labor and extreme stress from external threats.3 In such circumstances, ‘the watch’ might have been the only time when external stimuli were minimized, allowing individuals to focus entirely on themselves. Considering that modern psychology recommends quiet, reflective activities like meditation or journaling to manage anxiety and enhance creativity, the ‘watch’ of the past could have been an institutionalized mechanism that naturally integrated such activities into daily life. In other words, the segmented sleep pattern was not inefficient but likely contributed to maintaining the mental health and enhancing the psychological resilience of people at the time. This lost ‘intermediate awakening’ might hold a key to explaining some of the chronic anxiety and stress experienced by modern individuals.
One of the most important functions of ‘the watch’ was to interpret dreams and reflect on their meaning.13 Because people had just awakened from their first sleep, they could remember the content of their dreams vividly. In this era, dreams were considered messages from God or the devil, or prophecies of the future, so reflecting on and understanding their meaning was a crucial spiritual activity.15 Ekirch argues that this time "afforded fresh visions to absorb before returning to unconsciousness," suggesting it was a source of self-revelation and spiritual solace.10
This liminal state, on the border between sleep and wakefulness, was recognized as an ideal time for creative inspiration.19 In this state, where the constraints of rational thought are loosened and unconscious associations become active, people could gain insights into problems they couldn't solve during the day or come up with new ideas.19 Therefore, ‘the watch’ was not merely a rest period before resuming sleep but an active mental space for exploring the inner world and expressing creativity.
While Ekirch's research clearly established the pattern of segmented sleep in Western society, caution is needed when generalizing this to be the universal past of all humanity. Traditional sleep culture in East Asia, particularly Korea, developed in a different context. Records from the Joseon Dynasty show that the ruling class, including the king, went to bed after the 10 p.m. curfew bell (Injeong) and woke up around 3 a.m., indicating a relatively short and consolidated sleep.22 While direct records of the common people's sleep patterns are scarce, it is presumed that the unique floor heating system called
ondol and the communal sleeping culture where families slept together in one room greatly influenced the sleep environment.24 The
ondol could provide warmth throughout the night, possibly preventing awakenings due to cold and enabling more continuous sleep than in the West.27 It is important to recognize that the technological, cultural, and environmental factors of each society shaped sleep patterns differently.
Anthropological studies of existing hunter-gatherer societies provide another crucial perspective on the history of sleep. Contrary to the common belief that they would sleep more than modern urban dwellers, studies on groups like the Hadza of Tanzania and the San of Namibia show their average sleep duration is between 5.7 and 7.1 hours, similar to the lower end of modern industrial societies.28 More interestingly, their sleep is a monophasic, consolidated block, not segmented.29
The most powerful factor regulating their sleep was not light, but ‘temperature.’ They remained active for an average of 3.3 hours after sunset, going to bed as the temperature began to drop and waking up at the coldest point of the day.28 This suggests that the human sleep rhythm is deeply attuned not only to the sun's light cycle but also to the daily temperature cycle (thermo-cycle).
Another theory explaining the diversity of human sleep patterns is the ‘sentinel hypothesis’.30 This hypothesis notes that sleep is the most vulnerable time for group-living animals to predators. Therefore, the sleep-wake cycles of group members are slightly staggered, so that while some are asleep, others are awake to stand guard, ensuring the safety of the entire group.32 The difference between ‘morning people’ and ‘evening people’ (chronotypes) in modern society may be an evolutionary legacy of this.33 In other words, diverse sleep patterns may not be an individual problem but an evolutionary strategy to increase the group's survival probability.
Synthesizing these diverse pieces of evidence, it becomes clear that human sleep patterns are not determined by a single biological program. Rather, sleep is a flexible adaptation to the environmental (sunlight, temperature), technological (fire, heating), and social (safety, community culture) conditions of each society. The segmented sleep of the West may have been an adaptation to long, cold winter nights, while the monophasic sleep of hunter-gatherer societies near the equator was likely an adaptation to a distinct temperature cycle. Therefore, the question "What sleep pattern is natural?" should be replaced with "What is the most optimized adaptive strategy for a given environment?"
The natural and flexible segmented sleep pattern of the pre-industrial era was rapidly dismantled amidst the great social transformations that began in the late 18th century. The invention of artificial lighting and the standardization of working hours brought by the Industrial Revolution completely changed the human night, redefining the very meaning and purpose of sleep.
The most fundamental technological change that altered human sleep patterns was the invention and spread of artificial lighting. From the gas lamps that began to illuminate city streets in the early 19th century to the incandescent light bulb commercialized by Edison in 1879, artificial light drove out the darkness of night and dramatically extended human activity hours.34 The time of rest, which previously began with the setting of the sun according to the rhythm of nature, was now transformed into a time for labor and leisure.35
Artificial lighting directly affects the human biological clock. Our sleep-wake cycle is regulated by a biological clock located in the hypothalamus of the brain, which is highly sensitive to light.37 Exposure to bright light at night, especially LED lighting with a high proportion of blue light, can suppress the secretion of the sleep-inducing hormone melatonin, making it difficult to fall asleep and causing disruptions to the sleep-wake cycle.38 The spread of artificial lighting naturally pushed people's bedtimes later, eroding the temporal leeway that had allowed the old segmented sleep pattern to exist.4
The Industrial Revolution fundamentally changed the concept of time. The factory system demanded a time discipline entirely different from that of the preceding agricultural society. Workers had to report to work at a fixed time and labor for long hours at the pace of machines.41 During the Industrial Revolution in the 1800s, the average workday was 16-17 hours, with only one day off per week.42 In such extreme working conditions, the leisurely practice of sleeping in two segments became an impossible luxury.
Sleep was now reduced to a necessary ‘recharge’ time to recover from a day of hard labor and reproduce the workforce for the next day. The long, segmented sleep had to be transformed into a short, compressed, single block.35 The slogan "Eight hours' labour, Eight hours' recreation, Eight hours' rest" emerged as a protest against extreme long working hours, but it paradoxically contributed to defining and solidifying sleep as a single eight-hour block.42 The logic of efficiency and productivity demanded by industrial capitalism permeated even the realm of sleep, and seemingly unproductive time like ‘the watch’ was dismissed as waste.35
This transformation went beyond a mere change in sleep schedules; it resulted in redefining the purpose of sleep itself. Whereas in pre-industrial society, sleep was deeply connected with social and spiritual dimensions such as prayer, reflection, and community activities, after the Industrial Revolution, the sole value of sleep was tailored to the economic utility of reproducing labor power. Sleep was stripped of its social and cultural meaning, leaving only its pure biological function. This utilitarian perspective is the root of modern sleep anxiety—the tendency to evaluate sleep by the yardstick of efficiency and to view the inability to sleep as a failure.
Within the new time discipline brought by the Industrial Revolution and artificial lighting, the once perfectly normal middle-of-the-night awakening was gradually redefined as an abnormal and problematic state, a disease called ‘insomnia’.5 References to insomnia in literature began to surge in the late 19th century, a period that intriguingly coincides with the disappearance of segmented sleep.5
The 19th-century medical community questioned the necessity of segmented sleep and spread the perception that a single, continuous sleep was more efficient.44 Especially among the upper classes, who valued productivity and efficiency, reducing sleep was sometimes seen as evidence of being a noble and important person.44 Being awake was no longer a time for reflection and rest but a pathological symptom that needed treatment.
A significant portion of the sleep problems experienced by modern individuals is a legacy of this historical transition. We have internalized the socially constructed ideal of an ‘eight-hour monophasic sleep’ and feel anxious when we fail to achieve it. When we wake up in the middle of the night, instead of accepting it as a natural phenomenon like our ancestors did, we are consumed by worries such as ‘Do I have insomnia?’ or ‘What if this affects my work tomorrow?’.46
This anxiety itself is a powerful awakening factor. The pressure to sleep promotes the secretion of the stress hormone cortisol and activates the sympathetic nervous system, putting the body in a state of tension.47 Ultimately, the effort to sleep paradoxically drives sleep away, a situation known as ‘psychophysiological insomnia’.35 Thus, modern insomnia is not a purely biological problem but a complex result of historically formed cultural pressures and the psychological anxiety they cause.
Monophasic sleep, which became the human standard through the industrial era, possesses both powerful advantages and clear vulnerabilities from the perspective of modern sleep science. Understanding the structure and function of monophasic sleep is essential for grasping the nature of sleep problems faced by modern individuals and evaluating their health implications.
Modern sleep science, through electroencephalography (EEG) measurements, has revealed that sleep is not merely a state of rest but a highly dynamic and structured process.49 Monophasic sleep is like an architectural structure where two main states, Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep, repeat in approximately 90-minute cycles.50
These sleep stages cycle 4-5 times throughout the night. In the early part of the night, deep NREM sleep (N3) is dominant, while the proportion of REM sleep increases towards the latter part. Each stage performs unique physiological functions, so the seamless continuation of these cycles is a key determinant of sleep quality.
One of the most powerful biological advantages of monophasic sleep is that it maximizes the efficiency of the glymphatic system, the brain's waste removal system. The glymphatic system circulates cerebrospinal fluid (CSF) through brain tissue, washing away toxic proteins (such as beta-amyloid and tau) that accumulate as byproducts of neural activity during the day.58 These toxic proteins are known to be major causal agents of neurodegenerative diseases like Alzheimer's disease.53
Crucially, this cleaning process is most active during deep slow-wave sleep (N3). During this stage, the brain's astrocytes shrink, expanding the space between cells by about 60%, which facilitates the flow of CSF and dramatically increases waste removal efficiency.61 Therefore, monophasic sleep, by allowing for uninterrupted entry into and maintenance of deep sleep, provides a very important biological advantage in protecting brain health and lowering the risk of neurodegenerative diseases.
Monophasic sleep optimizes the secretion cycles of hormones essential for maintaining the body's homeostasis.
While monophasic sleep offers powerful benefits due to its consolidated structure, this very structure makes it highly vulnerable to the various disruptive factors of the modern environment. Key physiological processes like the activation of the glymphatic system or the secretion of growth hormone depend on specific sleep stages, particularly deep slow-wave sleep. However, modern society is filled with factors that disrupt sleep and make it shallow, such as light pollution, noise, the blue light from smartphones, caffeine, alcohol, and work-related stress.38
If sleep is frequently interrupted or fails to reach deep stages due to these factors, one may not reap the core benefits of monophasic sleep, even if they have been lying down for 7-8 hours. This means that monophasic sleep is a kind of ‘high-risk, high-reward’ strategy, with a large gap in health outcomes between ‘success’ and ‘failure.’ Successful monophasic sleep ensures optimal brain cleaning and hormonal regulation, but the numerous disruptive factors of modern life make failure highly probable. And that failure leads to serious consequences beyond mere fatigue, such as the accumulation of neurotoxic substances in the brain over decades and an increased risk of metabolic diseases. While segmented sleep incorporated intermediate awakenings as a natural process, monophasic sleep defines any interruption as a failure, making its vulnerability even more pronounced.
As discussed in Part 2, the cultural pressure to achieve a perfect, uninterrupted sleep itself becomes a major cause of insomnia. Even though waking up briefly in the middle of the night can be a natural human physiological phenomenon, modern individuals label it as a ‘failure’ and become anxious.5 This anxiety stimulates the secretion of cortisol, which awakens the brain and makes it even harder to fall back asleep.47 Ultimately, the pursuit of the monophasic ideal results in the fragmentation of sleep. This is a psychological burden unique to modern society, born after industrialization.
Through historical and biological analysis, we have confirmed that sleep is not a fixed entity but a product of adaptation that has changed with time and environment. Based on this understanding, we will now examine the specific effects of modern sleep patterns on health and lifespan and propose a direction for future sleep health.
Comparing the health advantages and disadvantages of different sleep patterns from a modern medical perspective is of great significance.
To clearly understand these complex relationships, the table below compares the main characteristics and health impacts of each sleep pattern.
| Feature | Monophasic Sleep (Ideal) | Monophasic Sleep (Fragmented) | Historical Biphasic Sleep | Modern Biphasic Sleep (Siesta) |
|---|---|---|---|---|
| Typical Schedule | 7-9 hours continuous sleep at night | 7-9 hours sleep at night with multiple awakenings | First sleep (3-4h) + Wake (1-2h) + Second sleep (3-4h) | Night sleep (5-7h) + Nap (20-90 min) |
| Total Sleep Time | 7-9 hours | 7-9 hours (actual sleep time may be shorter) | 6-8 hours | 6-8.5 hours |
| Continuity | High | Low | Medium (two long blocks) | Medium (long night sleep and short nap) |
| Key Supported Biological Processes | Glymphatic system activation, maximized growth hormone secretion | Reduced efficiency of key biological processes | Unclear (potential for dream reflection, psychological stability) | Cognitive function enhancement (short nap), memory consolidation (long nap) |
| Main Hormone Profile | Early night Melatonin/GH surge, dawn Cortisol rise | Cortisol rhythm disruption, GH secretion inhibition | Potential for two GH/Prolactin surges, altered Cortisol rhythm | Night sleep pattern similar to monophasic, hormonal changes during nap |
| Cognitive Impact | Optimized memory consolidation and learning | Decreased concentration, memory decline, delayed reaction time | Potential for enhanced creativity and self-reflection 18 | Improved alertness and cognitive performance (especially short naps) 68 |
| Confirmed Health Benefits | Optimal brain health and metabolic function maintenance | None | Unclear (potential for anxiety relief from mid-night waking) 5 | Cardiovascular protective effects (short naps) 71 |
| Confirmed Health Risks | None | Increased risk of cardiovascular disease, diabetes, obesity, cognitive decline 4 | Unclear | Increased risk of metabolic syndrome, hypertension (long naps) 70 |
Large-scale epidemiological studies analyzing the relationship between sleep and lifespan consistently show a ‘U-shaped curve’ relationship.75 This means that the mortality rate is lowest in the group that sleeps an average of 7-8 hours per day, and the risk of death from all causes increases as sleep duration becomes shorter or longer than this.
In the U.S. Nurses' Health Study, which followed over 80,000 women for 14 years, the group sleeping 5 hours or less had a 15% higher risk of death compared to the group sleeping 7 hours, while the group sleeping 9 hours or more had a staggering 42% higher risk.78 This trend was maintained even after adjusting for other risk factors such as age, smoking, and obesity. Too little sleep puts a strain on the cardiovascular, endocrine, and immune systems, increasing disease risk 75, while too much sleep is likely not a problem in itself but an indicator of underlying conditions that degrade sleep quality, such as sleep apnea or depression.52
Recent studies show that the ‘quality’ and ‘regularity’ of sleep may have a more significant impact on lifespan than the ‘duration.’
Sleep problems not only affect physical health but also have a direct impact on brain health and the aging process. The Whitehall II Study followed civil servants in their middle age over a long period to analyze the impact of changes in sleep patterns on cognitive function.82 The results showed that when individuals who had been getting an adequate 6-8 hours of sleep experienced a ‘negative change,’ such as a decrease or increase in sleep duration, their scores on most cognitive function tests declined. The extent of this cognitive decline was equivalent to aging by 4-7 years.82 In particular, the group that newly developed sleep disorders during the transition to retirement showed a faster decline in reasoning ability compared to the group that did not.84 This is consistent with the biological mechanism that poor quality sleep disrupts the brain's waste removal (glymphatic system) and memory consolidation processes, thereby accelerating cognitive aging and increasing the risk of dementia in the long term.60
This report has provided a multifaceted view of the history of human sleep, from the segmented sleep culture of the pre-industrial era to the modern ideal of monophasic sleep and its biological and health implications. The key conclusion reached through this analysis is that the ‘uninterrupted 8-hour monophasic sleep,’ which modern society considers the standard, is not a biological absolute but a historical construct formed in the relatively recent past.
Our journey began in the pre-industrial night, where darkness and danger coexisted with communal intimacy. In that world, humanity accepted segmented sleep, divided into a ‘first sleep’ and a ‘second sleep,’ as a natural rhythm of life. The waking period between the two sleeps, ‘the watch,’ was not a time of anxious tossing and turning but a precious time for prayer and reflection, creativity, and social interaction. This shows that sleep was not merely an act to satisfy a biological need but a complex cultural activity with social and spiritual significance.
However, the great waves of the invention of artificial lighting and the Industrial Revolution completely changed this rhythm of the night. Under the logic of efficiency and productivity, sleep was relegated to a utilitarian tool for reproducing labor power, and middle-of-the-night awakenings were stigmatized as a disease called ‘insomnia.’ This great transformation has left modern individuals with a central tension. We have adopted a monophasic sleep pattern that, when perfectly executed, offers powerful benefits optimized for brain health (especially waste removal via the glymphatic system). Yet, at the same time, we have created a world filled with light pollution, noise, stress, and digital devices that make its perfect execution nearly impossible.
Consequently, modern individuals are forced into a ‘high-risk, high-reward’ sleep strategy. Success can maintain optimal brain function and health, but failure can lead to severe consequences such as the accumulation of neurotoxic substances, metabolic diseases, and cognitive decline. And many modern individuals are losing this gamble.
Therefore, the path to future sleep health lies not in obsessing over the difficult-to-achieve ideal of perfect monophasic sleep, but in redefining our relationship with sleep in a more flexible, informed, and less anxious way. As large-scale epidemiological studies clearly show, the most powerful predictors of lifespan and health are not the absolute hours of sleep but its ‘regularity,’ ‘quality,’ and ‘sufficient total amount.’
By understanding that occasionally waking up in the middle of the night may be a part of our human historical legacy, rather than viewing it as a pathological failure, we can reduce unnecessary anxiety. Paradoxically, this psychological stability can help us fall back asleep, thereby improving the overall quality of our sleep. The future of sleep health will not be about imposing a single pattern on the entire society, but about using tools like wearable technology to find a sustainable sleep rhythm that fits each individual's biological rhythm and lifestyle.86 We may not be able to fully recover the lost rhythm of the night, but by simply understanding its historical existence, we can begin to see sleep not as an object of suppression and control, but as a process of dialogue and harmony with our bodies.