0 point by karyan03 2 months ago | flag | hide | 0 comments

Sleep is a fundamental biological process essential for maintaining human health and well-being. Recent advances in sleep science have clearly established that sleep deprivation leads to a wide range of negative consequences, including cognitive decline, mental health problems, and an increased risk of chronic diseases. Despite this scientific consensus, there are subtle but significant differences in the "recommended sleep duration" presented by public health agencies in various countries. This suggests that the issue involves more than just a numerical difference; it reflects a complex interplay of scientific interpretation, public health philosophy, and sociocultural realities in each nation.
This report aims to provide an in-depth answer to the question, "Do recommended sleep durations differ by country?" It begins by clarifying the confusion raised by the user regarding the 2022 Nature Aging study that utilized data from the UK Biobank. We will precisely analyze the background and scientific significance of that study's conclusion, which proposed 7 hours—not 8—as the optimal sleep duration.
Building on this, the report will comparatively analyze the official sleep guidelines of major countries such as the United States, the United Kingdom, Japan, and South Korea. We will explore how each country's recommended duration was established and what scientific evidence and policy considerations underlie it. Furthermore, by comprehensively examining the impact of genetic and ethnic factors, as well as cultural norms on sleep patterns and health as revealed by the latest research, we will argue that the concept of "optimal sleep" is not a universal biological constant but a complex "biocultural" concept.
Ultimately, this report seeks to provide a more sophisticated and multidimensional framework for understanding sleep duration for both individuals and policymakers. It will present a path toward pursuing healthy sleep by comprehensively considering scientific evidence, individual characteristics, and sociocultural context, rather than chasing a single "magic number."
At the core of the user's question lies an influential study published in the international journal Nature Aging in 2022. This research explored the optimal sleep duration for middle-aged and older adults using the vast dataset of the UK Biobank, and its results posed a significant challenge to conventional wisdom. This chapter will clarify the key findings of the study, resolve the user's confusion, and conduct an in-depth analysis of its implications for sleep science.
The user recalled that the study recommended 8 hours of sleep, but this is incorrect. The key conclusion of the study, published by a joint research team from the University of Cambridge in the UK and Fudan University in China, is that the optimal sleep duration for cognitive function and mental health in middle-aged and older adults (aged 38-73) is approximately 7 hours.1
The research team analyzed data from nearly 500,000 participants in the UK Biobank. Participants reported on their sleep habits, mental health, and well-being, and completed a series of cognitive tests to assess processing speed, visual attention, memory, and problem-solving skills.2 The analysis revealed a
nonlinear relationship between sleep duration and both cognitive function and mental health, specifically a U-shaped or quadratic relationship.5
This means that both sleeping less than 7 hours (insufficient sleep) and sleeping more than 7 hours (excessive sleep) were associated with poorer cognitive performance.1 Similarly, groups with shorter or longer sleep durations reported more symptoms of anxiety and depression and lower overall well-being compared to the group that slept 7 hours.1 Furthermore, sleep consistency also emerged as an important factor. Consistently sleeping around 7 hours each night with little fluctuation had a more positive impact on cognitive function and mental health.1
In conclusion, this study statistically identified that the "sweet spot" for sleep duration that yields the best health outcomes in a specific age group and population is approximately 7 hours. The user's mention of "announced as 7 hours in Shanghai" likely stems from the fact that one of the study's lead institutions was Fudan University, located in Shanghai, China; the research finding itself clearly points to 7 hours as the optimum.1
The significance of this study extends beyond simply demonstrating a correlation between sleep duration and health outcomes; it also explored the potential biological mechanisms behind it. The research team provided deeper insights by analyzing brain imaging and genetic data from approximately 40,000 participants.1
The analysis showed that sleeping less or more than 7 hours was associated with structural changes in brain regions related to cognitive processing and memory, particularly the hippocampus, precentral cortex, and lateral orbitofrontal cortex.2 This provides strong evidence that non-optimal sleep can affect the physical structure of the brain, leading to cognitive decline.
The researchers pointed to the disruption of slow-wave sleep, or "deep sleep," as one possible reason for this association.3 Deep sleep plays a crucial role in memory consolidation and the removal of waste products from the brain. Specifically, the "glymphatic system," which clears out toxins like amyloid proteins—a hallmark of some dementias such as Alzheimer's disease—is most active during deep sleep.3 Therefore, insufficient or excessive sleep can disrupt this process, leading to the accumulation of harmful substances in the brain, which in the long term can increase the risk of cognitive decline.
Furthermore, the team proposed an integrated model that incorporates Polygenic Risk Score (PRS), sleep, brain structure, cognition, and mental health.6 This suggests the existence of a complex, interconnected pathway where genetic predispositions influence sleep patterns and brain structure, which in turn affect cognitive function and mental health.
While this study made significant discoveries through its vast dataset and sophisticated analytical methods, there are clear limitations that must be considered when interpreting and applying its results.
The most critical limitation is the ethnic homogeneity of the study population. 94% of the UK Biobank participants included in this study were white and of European ancestry.6 This means that while the "7-hour optimum" conclusion is very strong for this specific population, it cannot be definitively stated that it applies universally to people of other races, ethnicities, or cultural backgrounds. The research team themselves acknowledged this point, stating that it is unknown how these results would manifest in other populations.11
This limitation serves as a crucial starting point for exploring the core theme of this report: "differences in recommended sleep duration by country." In other words, the Nature Aging study should be understood not as having discovered a "universal optimal sleep time for humans," but as having presented strong evidence for the "optimal sleep time for Western middle-aged and older adults."
Another important point revealed by this study is the conceptual difference between a "research optimum" and a "public health guideline."
The Nature Aging study identified the nadir of the U-shaped risk curve, the point where risk is statistically minimized, at approximately 7 hours.5 This is a precise scientific finding for a specific population. In contrast, public health agencies like the U.S. Centers for Disease Control and Prevention (CDC) or the UK's National Health Service (NHS) typically recommend a
range, such as "7-9 hours."13
This difference arises because the two types of information serve different purposes. Public health guidelines are intended for the entire population, which is highly diverse in terms of genetics, lifestyle, and health status.15 Therefore, presenting a single number to everyone would be overly prescriptive and could be inaccurate for many individuals. The "7-9 hours" range provides a safe and effective target within which the vast majority of people can find their own optimal sleep duration. It can be seen as a kind of "confidence interval" around the scientific optimum, embracing natural individual variability.
Therefore, the "7 hours" finding from the Nature Aging study does not contradict the "7-9 hours" public health guideline. Rather, they provide different kinds of information and are complementary. One represents a scientific optimum under specific conditions, while the other represents a practical guideline for the general public. This conceptual distinction will provide an important framework for the comparative analysis of national sleep recommendations in the following chapters.
Despite the scientific consensus on the importance of sleep, the recommended sleep durations presented by governments and health organizations in different countries show subtle variations. These differences go beyond mere numbers, reflecting each country's public health philosophy, sociocultural realities, and its approach to interpreting scientific evidence. This chapter systematically compares and analyzes the official adult sleep guidelines and actual sleep conditions in the United States, the United Kingdom, Japan, and South Korea to explore the underlying context.
The United States and the United Kingdom show a strong and consistent consensus on healthy sleep duration for adults. This can be considered the "standard model" in the Western world and serves as a benchmark for comparing recommendations from other countries.
In the United States, major organizations such as the Centers for Disease Control and Prevention (CDC), the American Academy of Sleep Medicine (AASM), and the National Sleep Foundation (NSF) all offer similar recommendations. For adults aged 18 to 64, the recommended sleep duration is 7 to 9 hours per day.13 The CDC also more broadly recommends "7 or more hours per day."18 These guidelines are the result of a consensus reached by a panel of 13 experts in sleep medicine and research after an extensive review of published scientific evidence.15 It is emphasized that adhering to this recommended duration is associated with improvements in overall health outcomes, including attention, behavior, learning, memory, emotional regulation, quality of life, and mental and physical health.15
The United Kingdom's National Health Service (NHS) takes a very similar stance. It states that healthy adults generally need "around 7 to 9 hours" of sleep.14 While there are slight variations in wording in materials from some NHS-affiliated bodies, such as "8-9 hours" 19 or "7-9 hours," 20 the overall message is consistent with that of the US.
These Western recommendations are based on an "ideal-state" philosophy aimed at "optimal health." That is, they clearly state that sleeping within this range is a goal for preventing disease and achieving the best possible physical and mental function.
Japan's sleep recommendations show a distinctly different approach from those in the West. This is the product of a unique public health policy that reflects Japan's serious societal problem of sleep deprivation.
In late 2023, Japan's Ministry of Health, Labour and Welfare (MHLW) released its revised "Sleep Guidelines for Health Promotion." The most noteworthy point here is the recommendation for adults to secure at least 6 hours of sleep.21 This is closer to setting a "minimum standard" to avoid serious health problems rather than presenting an "ideal" sleep duration. In contrast, the recommendations for elementary school students (9-12 hours) and middle and high school students (8-10 hours) are not significantly different from Western standards.23
Behind this policy decision lies the reality of chronic sleep deprivation in Japan. According to a 2021 survey by the Organisation for Economic Co-operation and Development (OECD), the average sleep time for Japanese people was 7 hours and 22 minutes, the lowest among 33 member countries and nearly an hour shorter than the OECD average.22 It was found that about 40% of Japanese adults get less than 6 hours of sleep per day on average.21
Therefore, Japan's "6 hours or more" guideline can be interpreted as a "pragmatic-intervention" or "harm reduction" strategy. The MHLW cites research showing that less than 6 hours of sleep can increase the risk of death from stroke, heart disease, and cancer by 2 to 3 times, strongly warning against falling below this baseline.22 This approach focuses on sending a warning message to the most urgent risk groups by acknowledging the social reality, rather than presenting an unrealistically high goal for the entire population.
South Korea exhibits a complex and dualistic pattern regarding sleep recommendations. While the official medical community's advice follows Western standards, the actual sleep situation of the public and some domestic research findings tell a different story.
In terms of official recommendations, professional academic societies like the Korean Sleep Society and the Korean Sleep Research Society often cite the recommendations of the US National Sleep Foundation (NSF) and advise 7-9 hours of sleep for adults.26 Seoul National University Hospital also suggests 7-8 hours as the appropriate sleep duration for Koreans based on research.28 This shows that the domestic medical community has adopted Western scientific standards.
However, the reality for the public is a state of severe sleep deprivation, similar to Japan. The average sleep time for Koreans is among the lowest in OECD countries 29, and many citizens suffer from chronic sleep deprivation.26
What is more interesting is the discrepancy in the results of large-scale domestic epidemiological studies.
These conflicting research findings reveal the complex situation South Korea faces. Official medical recommendations follow the Western "golden rule," but actual population data suggest other possibilities. This is a powerful example of the limitations of applying universal recommendations directly to a specific population. While it cannot be concluded that 5-6 hours of sleep is "healthy," it suggests that the health effects may differ in a population chronically adapted to short sleep, or that other confounding factors affecting the relationship between sleep duration and health may be uniquely at play.
The table below summarizes the recommended adult sleep durations and their backgrounds for the United States, the United Kingdom, Japan, and South Korea.
| Country | Key Institution | Recommended Adult Sleep Duration | Key Nuances & Rationale | Average Sleep Duration (Reference) |
|---|---|---|---|---|
| United States | CDC, AASM, NSF | 7-9 hours | Presents an ideal standard aiming for optimal health 13 | No data available |
| United Kingdom | NHS | Approx. 7-9 hours | Similar to the US, based on scientific consensus for optimal health 14 | Approx. 7.6 hours 31 |
| Japan | MHLW | 6+ hours | A pragmatic "minimum standard" and "harm reduction" strategy to address a national sleep crisis 21 | Approx. 7 hours 22 mins 24 |
| South Korea | Korean Sleep Society, etc. | 7-9 hours (Official) | Medical community recommendation adopting Western scientific standards. However, some domestic studies show different results 27 | Lowest level in OECD 29 |
This comparative analysis shows that the differences in national recommended sleep times are not due to fundamental disagreements about the science, but are rather a reflection of each country's public health philosophy, policy priorities, and the specific realities of its population. The next chapter will explore the deeper causes that create these differences: the roles of culture and genetics.
As observed in the preceding chapters, there are clear differences in national sleep recommendations and actual sleep patterns. These differences are not merely the arbitrary decisions of policymakers but are the complex result of how scientific findings are interpreted, cultural norms, and potential biological factors. This chapter will comprehensively analyze how these elements interact to form different concepts of "optimal sleep."
As mentioned in Chapter 1, there is a difference in purpose and form between the results of scientific research and public health guidelines. A clear understanding of this difference is essential for interpreting the diversity of national recommendations.
Scientific investigations like the Nature Aging study aim to identify a precise point (a statistical mean or nadir) where disease risk is minimized or function is optimized in a specific population.1 This is a statistical "optimum" obtained under controlled conditions.
On the other hand, public health guidelines issued by agencies like the CDC or NHS focus on practicality. They must be applicable to the general public, which exhibits enormous diversity in genetic background, age, health status, and lifestyle.15 Therefore, a range like "7-9 hours" is a strategic choice to accommodate this individual variability. This range provides a safe and flexible guideline within which most people can find a healthy sleep duration that suits them. In essence, if research presents a "peak," guidelines present a "safe zone" that includes that peak. The two are not contradictory but are complementary information systems that explain sleep health at different levels.
The idea that sleep is a purely biological phenomenon is no longer valid. The latest research shows that cultural norms and the environment have a profound influence on determining sleep patterns and their health effects.
Sleep habits vary greatly around the world. Examples include inemuri (napping at work) in Japan, the siesta in Southern Europe, and co-sleeping, which is common in many East Asian cultures.32 These practices are not simply individual choices but are deeply rooted in social structures and values. For instance, in Japan,
inemuri can be positively perceived as a sign that an individual has worked hard.32
A groundbreaking study published in the Proceedings of the National Academy of Sciences (PNAS) in 2024 provides crucial clues about the health impacts of these cultural differences. By analyzing data from multiple countries, this study discovered the following key facts 33:
This research has implications that are close to a "paradigm shift." It helps explain why countries like Japan, with short average sleep durations, do not have correspondingly worse health indicators when other variables are controlled for.34 It suggests the possibility that their social, and perhaps even physiological, systems have adapted to a different sleep schedule. "Optimal sleep" may not be a fixed biological constant but a flexible concept that is culturally mediated and adjusted.
In addition to cultural factors, the biological basis, particularly differences according to genetics and race, is another important variable to consider.
Several studies conducted within the United States have reported significant differences in sleep duration and quality among racial and ethnic groups. For example, non-Hispanic white Americans tend to have longer and better-quality sleep compared to Black Americans.36 While these differences are compounded by various social factors such as socioeconomic status, living environment, and stress levels, the possibility of potential genetic predispositions cannot be ruled out.
The Nature Aging study explored genetic risk factors within a European-ancestry population 6, but the question of whether there are genetic predispositions that cause different sleep needs among global populations remains an active area of research. While the study on cultural influences 34 suggests that environment and norms are powerful drivers, it is also possible that specific cultural norms evolved in populations with certain genetic predispositions. In other words, biology and culture likely have a complex, reciprocal relationship.
The table below compares the methodologies and key findings of the two crucial studies discussed in this report—the Nature Aging study and the PNAS study—to visually highlight the difference between a biological optimum and a culturally mediated optimum.
| Study (Year, Journal) | Primary Focus | Study Population | Key Finding | Implication |
|---|---|---|---|---|
| Li et al. (2022, Nature Aging) | Identifying biological optimal sleep duration | Middle-aged and older adults from UK Biobank (primarily European ancestry) | Approx. 7 hours of sleep is the optimum associated with cognitive function and brain structure (U-shaped relationship) 6 | Suggests a strong biological link between sleep and brain health in a specific population. |
| Ou et al. (2024, PNAS) | Exploring cultural diversity in sleep duration | Multinational cohorts | Optimal sleep duration varies by culture, and health is better when aligned with cultural ideals rather than absolute duration 33 | Suggests that "healthy sleep" is not a universal constant and should be defined flexibly according to cultural context. |
While the dangers of sleep deprivation are widely known, a growing body of evidence indicates that consistently sleeping too long is also associated with a variety of negative health outcomes. Numerous large-scale epidemiological studies have confirmed a U-shaped or J-shaped relationship between sleep duration and health risks, showing that both short and long sleep, relative to an optimal duration (typically 7-8 hours), are associated with increased morbidity and mortality.38 The user's question about risks like thrombosis touches upon some of the most significant findings in this area: cardiovascular and metabolic complications.
Increased Risk of Cardiovascular and Cerebrovascular Disease
The link between long sleep duration (usually defined as 9 or more hours per day) and cardiovascular health is one of the most consistently reported findings.
Underlying Mechanisms: Inflammation and Metabolic Dysfunction
Researchers have proposed several biological pathways that may explain these associations.
A Complex Relationship: Cause or Symptom?
It is crucial to note that the relationship between long sleep and poor health is complex. In some cases, oversleeping may not be the cause of health problems but rather a symptom of an underlying condition. Depression, low socioeconomic status, obstructive sleep apnea, and other undiagnosed illnesses can lead to increased sleep duration.40 Furthermore, a sedentary lifestyle is often associated with long sleep times and is, in itself, an unhealthy risk factor.43
In summary, while occasional long nights of sleep are unlikely to be harmful, consistently sleeping 9 or more hours is a significant marker for an increased risk of cardiovascular diseases, particularly stroke. This association appears to be mediated by factors such as chronic inflammation and metabolic dysregulation, but it may also be a signal of other underlying health conditions.
Through an in-depth analysis of the differences in recommended sleep duration by country, this report has revealed that this issue is a multidimensional phenomenon involving a complex interplay of science, culture, and policy, rather than a simple numerical difference. By synthesizing the findings from the Nature Aging study's "7 hours," comparing the guidelines of major countries, and exploring the importance of cultural context, we present the following conclusions and recommendations.
The clear answer to the question, "Do recommended sleep durations differ by country?" is "Yes." This difference arises from the interaction of three key factors:
In conclusion, the "7 hours" result from the UK Biobank study, which caused the user's confusion, is an important biological finding for a specific European-ancestry population, but it should not be regarded as the sole golden rule applicable to everyone worldwide. Optimal sleep can be defined differently for each individual and each society at the intersection of biological needs and cultural context.
Considering this complexity, how should individuals manage their sleep health? The following are expert recommendations based on the analysis in this report.
The journey toward healthy sleep is not a process of finding a single correct answer, but a continuous process of finding the optimal balance that suits your body and lifestyle, based on scientific knowledge. We hope this report serves as a sophisticated and reliable compass on that journey.