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Chronobiol Med > Volume 6(3); 2024 > Article
Sunwoo, Lee, and Kwon: Sixteen-Year Secular Trends in Sleep Duration Among Korean Children and Adolescents

Abstract

Objective

This study aimed to analyze trends in adolescent sleep duration over a 16-year period according to sex, educational level, residential area, and obesity.

Methods

In this cross-sectional study, data from the 2007–2022 Korean National Health and Nutrition Examination Survey were analyzed. Total sleep duration was measured using a self-administered questionnaire, and a survey-weighted linear regression analysis, considering the survey year as a continuous variable, was performed to examine the trend. Total sleep duration trends were also analyzed across subgroups categorized according to sex, educational level, residential area, and obesity level.

Results

Total sleep duration showed an overall increasing trend in boys (coefficient=0.027, ptrend<0.001), but no substantial trend was observed in girls (coefficient=0.002, ptrend=0.816). In the residential area-specific analyses, total sleep duration increased only in boys whose residential areas were urban (coefficient=0.017, ptrend<0.001). When classified by educational level, statistically significant increases in total sleep duration were observed among elementary school graduates or less (coefficient=0.018, ptrend=0.001) and middle school graduates (coefficient=0.015, ptrend=0.015). Depending on the obesity level, total sleep duration increased in normal-weight and overweight boys (normal weight: coefficient=0.014, ptrend=0.003; overweight: coefficient=0.038, ptrend=0.025).

Conclusion

This study found increasing or stable trends in sleep duration among adolescents aged 13 to 18 in Korea, with variations observed across subgroups based on sex, residential area, educational level, and obesity status.

INTRODUCTION

Adequate sleep duration is crucial for the growth and development of children and adolescents [1,2]. Growth hormone, which facilitates the growth and development of children and adolescents by increasing both the number and size of soft tissue cells and stimulating slowly dividing pre-chondrocytes in the epiphyseal growth plates of long bones, increases significantly during slow-wave sleep [3].
According to the Consensus Statement of the American Academy of Sleep Medicine on the recommended amount of sleep for healthy children, teenagers aged 13–18 years are advised to obtain 8–10 hours of sleep per night for optimal health [2]. Obtaining the recommended amount of sleep is associated with lower adiposity indicators, better emotional regulation, higher academic achievement, and improved quality of life and well-being [4]. However, short sleep duration is correlated with obesity, impaired glucose metabolism, measurable impairments in short-term and working memory, attention deficits, greater levels of attention deficit hyperactivity disorder-like symptoms, poorer self-reported academic performance, and negative behavior [5]. It is also associated with internalizing and externalizing behavioral problems [5].
Despite the importance of sleep, the prevalence of short sleep durations among adolescents is increasing worldwide [6,7]. According to a 2020 survey on the health and lifestyle habits of adolescents published by the Korea Youth Policy Institute, the average sleep time of Korean adolescents is 7 hours and 18 minutes, which is approximately 1 hour less than the Organisation for Economic Co-operation and Development (OECD) average of 8 hours and 22 minutes [8]. Against the backdrop of the fact that sleep deprivation among adolescents appears to be more serious in Korea than in other countries, this study aimed to analyze trends in adolescent sleep duration based on data from a nationwide health survey conducted by a national organization to determine whether sleep deprivation problems are being alleviated or worsened.

METHODS

Study population

In this cross-sectional study, we analyzed data from the 2007– 2022 Korean National Health and Nutrition Examination Survey (KNHANES), which includes data from a total of 7,523 adolescents aged 13 to 18 years. The KNHANES is a nationwide, representative, and population-based survey conducted by the Korea Disease Control and Prevention Agency in accordance with the National Health Promotion Act to produce representative and reliable statistics at the national level on the health status, health behavior, and nutritional intake of the Korean population [9]. From 1998 to 2005, the survey was conducted every 3 years but has been implemented annually since 2007 to improve the timeliness of national statistics. The KNHANES used the most recent population and housing census data available at the time of sample design as the basic sampling frame and applied a two-stage stratified cluster sampling method with survey districts and households as the primary and secondary sampling units to extract a representative sample of people over the age of one living in the Republic of Korea. The survey comprised health interviews, self-administered questionnaires, medical examinations, and nutritional surveys of household members. Detailed information about the survey is provided on the KNHANES website (https://knhanes.kdca.go.kr/knhanes/main.do, accessed May 24, 2024).
This study complied with the ethical guidelines of the 1964 Declaration of Helsinki and its amendments and was approved by the Institutional Review Board of Severance Hospital (IRB No. 4-2023-1115). In accordance with the National Health Promotion Act, study participants voluntarily participated in the survey and were able to discontinue participation at any time. Written informed consent was obtained from all participants or their legal representatives registered in this study.

Total sleep duration measurement

Total sleep duration per day was measured using a self-administered questionnaire with the following survey questions: First, in the 2007–2015 KNHANES, survey participants were asked to answer the subjective question, “How many hours do you usually sleep per day?” However, since 2016, total sleep duration has been investigated separately on weekends and weekdays. In the 2019, 2020, and 2022 surveys, the question “How many hours do you usually sleep per day?” was used as before, but the response boxes were divided into 1) weekdays (or working days) and 2) weekends (or nonworking days). In the 2016–2018 and 2021 surveys, participants were asked to answer questions regarding typical weekdays (or workdays): “What time do you go to bed, and what time do you wake up? On typical weekends (or days when you do not work, or the day before you do not work), what time do you go to bed and what time do you wake up?” Total sleep duration was then calculated using the time taken to go to bed and wake up.

Covariates

Anthropometric values were obtained by professionally trained medical staff according to a standardized protocol using a mobile examination vehicle. Body height (cm) and body weight (kg) were measured to the first decimal place, 0.1 cm and 0.1 kg, respectively. Body mass index (BMI) was calculated as body weight divided by height squared (kg/m2) and rounded to the nearest 0.1 kg/m2. Based on BMI percentiles, the study participants were categorized into the following obesity groups according to the 2017 Korean National Growth Charts: normal, BMI less than the 85th percentile; overweight, 85th percentile to less than the 95th percentile; and obese, BMI equal to or greater than the 95th percentile [10].
Waist circumference (cm) was measured directly at the midline between the inferior margin of the rib and the superior margin of the iliac crest by professionally trained medical staff. Abdominal obesity was defined as a waist circumference >90th percentile based on age- and sex-specific criteria. After resting for at least 5 minutes, systolic blood pressure (BP) and diastolic BP (mm Hg) were measured three times in the sitting position, and the mean values of the second and third measurements were defined as systolic BP and diastolic BP, respectively.
Blood samples were collected by professionally trained medical staff after participants fasted for at least 8 hours. Levels of fasting plasma glucose (mg/dL), serum total cholesterol (mg/dL), triglyceride (mg/dL), and high-density lipoprotein cholesterol (mg/dL) were measured using a Hitachi automatic analyzer 7600/7600–210 (Hitachi, Tokyo, Japan).
The educational level of the survey participants was measured through a self-administered survey and categorized into elementary school graduates or less (currently middle school students) and middle school graduates (currently high school students). The residential area was divided into urban (“dong” in the street address) and rural area (“eup, myeon” in the street address). Household income and parents’ educational levels were investigated through health interviews and self-administered questionnaires. Monthly household income was classified into quartiles, and parents’ educational levels were classified into three categories according to their graduation status: middle school graduate or less, high school graduate, and college graduate or higher.

Statistical analysis

We analyzed the trends in total sleep duration per day for the entire study population and for sex, obesity, educational level, and residential area subgroups. All data are presented as means±standard error for continuous variables and as weighted percentages for categorical variables. One-way analysis of variance was performed to compare continuous variables, and the Rao–Scott chi-square test was used to compare categorical variables (sex, obesity, educational level, and residential area).
To determine the p-value for the total sleep duration trend, a survey-weighted linear regression analysis was performed, considering the survey year as a continuous variable. For each subgroup, survey-weighted linear regression analysis was used to assess the statistical significance of differences in total sleep duration trends, and the analysis was performed after adjusting for age. We used sampling weights in all analyses to report estimates that were representative of the Korean population.
All statistical analyses were performed using SAS version 9.4 (SAS Inc., Cary, NC, USA), and statistical significance was set at p<0.05.

RESULTS

Characteristics of the study population and trends in total sleep duration

Table 1 shows the clinical and socioeconomic characteristics of the study population. A total of 3,988 boys and 3,535 girls participated in the study, with a mean age of 15.6±1.7 years. Over the 16-year study period (2007–2022), the proportion of the normal-weight group barely changed, from 89.4% to 88.8%; however, the proportion of the overweight group increased from 5.2% to 7.6%, whereas the proportion of the obese group decreased from 5.4% to 3.5%. The distribution ratio of the educational level, which reflects the age group of the participants, was approximately 40% for elementary school graduates or less and approximately 60% for middle school graduates. Additionally, the proportion of residential areas was within the range of 80%–90% in urban areas and 10%–20% in rural areas.
We confirmed the extent to which sleep duration increases linearly over time through coefficient values. The trend in total sleep duration from 2007 to 2022 is shown in Figure 1. The total sleep duration had an overall trend of increasing from 7.18±0.11 hours in 2007 to 7.22±0.06 hours in 2022 (coefficient=0.015, ptrend<0.001). When analyzing trends by sex, boys had an overall trend of increasing total sleep duration from 7.03±0.12 hours in 2007 to 7.26±0.08 hours in 2022 (coefficient=0.027, ptrend<0.001); by contrast, there was no substantial trend in girls (coefficient=0.002, ptrend=0.816). These trends were maintained even after adjusting for age, with an overall increasing trend observed in boys but not in girls.

Trends in total sleep duration according to sex and residential area subgroups

Table 2 presents the trends in total sleep duration in sex- and residential area-specific analyses from 2007 to 2022. When both sexes were included, the total sleep duration showed a statistically significant increase from 7.09±0.12 hours in 2007 to 7.23±0.07 hours in 2022 in the urban group (coefficient=0.017, ptrend<0.001), but there was no statistically significant trend in the rural group (coefficient=0.008, ptrend=0.506). These trends were maintained even after adjusting for age.
In sex-specific analyses, total sleep duration significantly increased in boys whose residential areas were urban (coefficient=0.028, ptrend<0.001), whereas there was no statistically significant increase in boys whose residential areas were rural (coefficient=0.025, ptrend=0.119). Unlike boys, no statistically significant increasing trend was observed among girls in either urban or rural areas of residence (urban: coefficient=0.005, ptrend=0.561; rural: coefficient=-0.011, ptrend=0.643). The findings remained consistent even after adjusting for age.

Trends in total sleep duration according to sex and educational level subgroups

Table 3 illustrates the trends in total sleep duration in sex- and education level-specific analyses from 2007 to 2022. When classified solely by educational level, regardless of sex, statistically significant increases in total sleep duration were observed in elementary school graduates or less (coefficient=0.018, ptrend=0.001) and middle school graduates (coefficient=0.015, ptrend=0.015).
Among boys, there were statistically significant increases in total sleep duration among both elementary school graduates or less (coefficient=0.032, ptrend<0.001) and middle school graduates (coefficient=0.024, ptrend=0.002). Conversely, among girls, no statistically significant changes were observed in either elementary school graduates or less (coefficient=0.001, ptrend=0.886) or middle school graduates (coefficient=0.006, ptrend=0.586). These trends were also observed in age-adjusted analyses.

Trends in total sleep duration according to sex and obesity subgroups

Table 4 shows the trends in total sleep duration in the sex- and obesity-specific analyses from 2007 to 2022. When considering boys and girls together, total sleep duration increased significantly in the normal weight and overweight groups (normal weight: coefficient=0.014, ptrend=0.003; overweight: coefficient=0.038, ptrend=0.025), whereas there was no statistically significant increase in the obese group (coefficient=0.014, ptrend=0.496).
The sex-specific results were as follows: there were statistically significant increases in total sleep duration in boys of normal weight and overweight (normal weight: coefficient=0.027, ptrend<0.001; overweight: coefficient=0.045, ptrend=0.041). However, no significant increase was observed in the obese boys (coefficient=0.012, ptrend=0.599). By contrast, in girls, there were no statistically significant changes in total sleep duration in any of the three groups (normal weight: coefficient=0.001, ptrend=0.912; overweight: coefficient=0.017, ptrend=0.526; obese: coefficient=0.023, ptrend=0.614). These results remained consistent after adjusting for age.

DISCUSSION

According to our sleep duration trend analysis, the total sleep duration for adolescents aged 13–18 years increased over 16 years from 2007 to 2022. However, given that adolescent sleep problems have long been a significant social concern, we initially anticipated a decrease in adolescent sleep duration over time. This unexpected result was presumed to be due to a shift towards later bedtimes over time. Data from 2016 to 2018 and 2021 revealed a statistically significant decrease in sleep duration with delayed bedtime. Because the KNHANES only assessed bedtime during these years, our analysis of the relation between bedtime and total sleep duration was limited to data from these 4 years.
Among girls, there were no significant trends in sleep duration across residential areas, educational levels, and obesity levels. Conversely, among boys, statistically significant increases in sleep duration were noted in certain subgroups, mirroring the trends observed in the overall group. These findings suggest that sex differences influence sleep duration trends. There are sex-based differences in circadian rhythm, and one possible reason is that boys are more likely to be “night owls” with a preference for staying up late, which can lead to increased total sleep time as their wake-up time gets pushed ahead [11,12].
Analyzing the change trends in sleep duration among different subgroups in boys, it was evident that sleep duration increased across all ages from 13 to 18 years, as observed in both elementary school graduates or less and middle school graduates. Regarding the analysis of subgroup sleep duration trends based on residential areas, a statistically significant increase in sleep duration was observed only among adolescents living in urban areas. This finding contradicts previous studies suggesting that exposure to outdoor artificial light at night, which is typically greater in urban areas owing to higher streetlight density and surrounding commercial facilities, is associated with shorter sleep duration [13-15]. This discrepancy with previous studies is presumed to be due to the implementation of the 9 o’clock school start-time system in Gyeonggi-do, an urban area, in 2014. This policy change increased sleep duration as school start times were delayed [16].
In the analysis of sleep duration trends based on obesity level, sleep duration increased in both the normal-weight and overweight groups, whereas no distinct trend was observed in the obese group. Given the overall increasing trend in sleep duration across the entire group, it can be inferred that the obese subgroup exhibited a different pattern from the overall trend. This outcome likely occurred due to the established positive correlation between short sleep duration and obesity in children and adolescents [17,18]. Short sleep duration increases the time available to consume food and central neuronal responses to unhealthy foods, which potentially leads to overeating and reduces physical activities due to fatigue caused by sleep deprivation, thereby leading to obesity. Simultaneously, because obstructive sleep apnea, a highly prevalent consequence of obesity, worsens sleep quality, obesity may affect sleep duration, contrary to a previous causal relation [19].
Most previous studies on adolescent sleep duration were studies that confirmed the correlation between sleep duration and specific health conditions such or clinical indicators [20-24]. There was a previous study that analyzed trends in sleep duration using KNHANES data, similar to our study, but it was conducted on adults aged 18 years or older and used data from 2007 to 2015, making it difficult to identify recent trends [25]. In addition, there was also a study that analyzed trends in sleep sufficiency and sleep duration among Korean adolescents, but it focused on how the trends changed during the COVID-19 period [26]. Among the numerous studies on sleep, there has been no study that has confirmed how adolescent sleep time has changed, even though sleep problems among adolescents have continued to be a social issue in Korea. Therefore, our study differs from previous studies in that we limited the study population to adolescents and focused on sleep duration itself. While most studies on sleep are cross-sectional studies, our study can be differentiated in that it is a longitudinal study that identified trends in sleep duration among adolescents aged 13 to 18 over 16 years.
Our study had several limitations. First, sleep duration was self-reported and was not objectively measured. Total sleep time was determined by respondents answering open-ended questions, such as “How many hours do you usually sleep per day?” or “When do you usually go to bed and wake up?” There is a potential risk of recall bias in the process of thinking about the amount of sleep they usually receive at the time of the survey. Second, a significant increase in sleep duration has been observed since 2016, which may be attributed to changes in the survey questions regarding sleep duration. Starting in 2016, the survey separately investigated the average sleep duration on weekdays and weekends and then calculated the total sleep duration as (average sleep duration on weekdays×5 + average sleep duration on weekends×2)/7. While weekend sleep duration tends to be longer than weekday sleep duration, respondents might have answered with their weekday sleep duration when asked about the average sleep time in the survey, without distinguishing between weekdays and weekends. This could potentially bias the reported sleep duration, particularly if weekdays represent a larger portion of the week for the respondents. Third, we could not evaluate sleep quality, which substantially influences health outcomes.
Despite of these limitations, our study has strength. While previous research has established correlations between sleep duration and specific health conditions or clinical indicators, there has been a lack of studies confirming how adolescent sleep duration has changed over time, despite ongoing concerns about sleep problems among adolescents in Korea. Our study fills this gap by identifying trends in sleep duration among adolescents aged 13 to 18 over a 16-year period. Additionally, we conducted subgroup analyses based on sex, residential area, educational level, and obesity level to further explore these trends.
In conclusion, our study revealed an overall increasing trend in sleep duration among adolescents aged 13–18 years in the Republic of Korea. Specifically, this trend was observed predominantly in boys, urban-dwelling adolescents, and those with normal or overweight status. Notably, the increasing trend persisted across all educational levels, which was indicative of the participants’ age. Our findings underscore the influence of sex, residential area, and obesity level on sleep duration among Korean adolescents, providing valuable insights into sleep patterns in this population. Although an increasing trend of sleep duration was observed only in boys, it was confirmed that sleep duration did not decrease for both boys and girls over the past 16 years. Therefore, we anticipate that this result can be used as evidence that the problem of sleep deprivation in adolescents has not worsened, and thus unnecessary social concerns about the sleep deprivation problem in adolescents can be allayed. Furthermore, our study suggests that public health policies related to time going to bed, sleep quality, and sleep environment rather than sleep duration itself, are needed to promote healthy sleep habits in adolescents.

NOTES

Conflicts of Interest

The authors have no potential conflicts of interest to disclose.

Availability of Data and Material

The data analyzed in this study were obtained from the Korea National Health and Nutritional Examination Survey and are available at the following website: https://www.kdca.go.kr/contents.es?mid=a4050401000.

Author Contributions

Conceptualization: Hyun Sunwoo, Yu-Jin Kwon. Data curation: Yaeji Lee. Formal analysis: Yaeji Lee. Funding acquisition: Yu-Jin Kwon. Investigation: Hyun Sunwoo. Methodology: Yaeji Lee. Project administration: Yu-Jin Kwon. Software: Yaeji Lee. Supervision: Yu-Jin Kwon. Validation: all authors. Visualization: Yaeji Lee. Writing—original draft: Hyun Sunwoo. Writing—review & editing: all authors.

Funding Statement

This research was supported by the Technology Innovation Program [grant no. 20018384; development of an intelligent health monitoring and management platform for children and adolescents based on smart device nutritional body analysis] funded by the Ministry of Trade, Industry, & Energy (MOTIE, Republic of Korea).

Acknowledgments

We thank all the citizens who participated in the 2010–2011 KNHANES survey.

Figure 1.
Trends of total sleep duration.
cim-2024-0014f1.jpg
Table 1.
Characteristics of study population
Characteristic Overall 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022
Unweighted number 7,523 297 678 830 599 540 501 538 423 443 476 440 401 407 353 321 276
Age (yr) 15.6±1.7 15.4±1.6 15.6±1.6 15.4±1.7 15.5±1.7 15.6±1.7 15.5±1.6 15.6±1.7 15.5±1.6 15.6±1.7 15.7±1.7 15.7±1.7 15.7±1.7 15.7±1.7 15.6±1.7 15.5±1.7 15.5±1.7
Sex
 Boys 53.5 54.4 53.2 53.1 53.6 52.7 56 51.5 53.2 54 52.8 52.4 51.3 53.7 55.3 54.8 55.1
 Girls 46.5 45.6 46.8 46.9 46.4 47.3 44 48.5 46.8 46 47.2 47.6 48.7 46.3 44.7 45.2 44.9
Height (cm) 166.4±8.4 165.7±8.6 166.2±8.6 166.4±8.6 165.9±8.6 166.4±8.7 166.4±8.1 166.1±8.2 165.8±8.1 166.0±8.2 166.4±8.0 166.5±8.4 167.1±8.3 167.3±8.6 167.1±8.1 167.0±8.1 167.8±8.3
Weight (km) 60.0±13.4 59.1±13.0 59.0±13.0 58.9±12.2 58.1±12.0 59.7±13.2 58.9±12.0 59.6±13.3 60.0±13.2 61.1±13.5 59.7±13.2 60.0±12.2 61.1±13.7 62.0±14.6 62.5±15.2 61.8±15.6 62.0±15.2
SBP (mm Hg) 108.6±10.4 106.1±10.0 105.4±10.7 109.0±10.8 109.1±11.5 108.2±10.0 108.6±9.9 108.7±10.3 109.0±10.2 109.9±9.9 109.8±10.2 108.9±9.6 109.2±10.2 110.1±10.6 110.2±11.0 108.0±10.2 107.6±9.4
DBP (mm Hg) 68.1±8.6 67.4±9.5 67.0±8.9 70.3±8.4 68.9±8.7 68.0±8.1 68.2±9.0 67.6±8.3 67.0±8.5 67.7±8.3 68.0±8.1 67.9±8.3 68.0±8.7 68.9±8.7 70.0±8.7 67.4±8.1 66.8±7.8
BMI (kg/m2) 21.5±3.9 21.4±3.8 21.2±3.6 21.2±3.4 21.0±3.5 21.4±3.8 21.2±3.4 21.5±3.7 21.7±3.9 22.1±4.0 21.4±3.9 21.5±3.7 21.8±3.8 22.0±4.3 22.3±4.5 22.0±4.4 21.9±4.3
Obesity group
 Normal 90.0 89.4 92.5 92.4 92.1 89.1 91.1 91.2 89.4 86.2 90.7 92.6 90.2 88.6 85.0 87.9 88.8
 Overweight 5.6 5.2 4.7 4.6 3.9 5.8 6.8 4.5 4.9 7.7 5.4 5.2 4.5 4.8 7.6 7.0 7.6
 Obese 4.4 5.4 2.9 2.9 4.0 5.1 2.1 4.3 5.7 6.1 3.8 2.1 5.3 6.5 7.4 5.0 3.5
WC (cm) 72.4±10.2 73.1±10.5 71.8±9.7 71.0±8.8 70.8±9.3 71.9±9.7 70.7±8.9 71.0±10.0 72.5±10.0 74.1±10.0 72.5±10.4 71.8±9.7 72.4±10.5 74.3±11.0 75.1±11.7 73.9±11.8 73.7±11.3
Glucose (mg/dL) 89.8±8.6 87.6±11.4 89.2±6.7 88.2±6.3 88.1±6.4 88.5±9.7 88.0±6.0 89.7±6.6 91.8±11.2 91.3±7.7 91.2±8.2 90.7±10.2 90.6±6.9 91.7±8.4 91.7±13.2 91.3±6.8 89.7±6.7
TC (mg/dL) 158.3±27.5 156.6±26.8 153.8±26.4 155.5±27.9 155.4±25.2 155.5±28.4 156.7±27.7 154.1±25.0 155.2±26.4 159.2±26.5 163.3±28.6 164.5±27.7 162.9±28.3 162.1±28.1 162.9±26.3 159.0±27.5 161.6±29.4
TG (mg/dL) 86.2±50.6 92.8±48.3 89.1±67.2 86.6±49.0 83.0±46.6 81.9±45.4 81.5±41.3 83.5±47.8 85.3±46.1 88.3±53.3 82.6±40.9 86.1±53.9 85.6±45.2 90.1±50.0 92.9±54.2 77.9±41.5 93.6±70.2
HDL-C (mg/dL) 50.5±10.0 47.4±8.9 49.2±9.1 48.4±9.2 49.0±8.7 50.1±10.6 51.0±10.8 51.5±9.9 50.8±9.4 50.4±9.6 51.8±9.9 51.0±9.8 50.3±9.3 51.6±10.0 51.1±9.2 50.5±10.2 55.8±12.7
Household income
 Low 12.7 12.4 10.8 15.9 19.4 17 12.2 14.2 11.1 15.6 13.3 10.8 10.6 11.9 5.5 8.3 7.7
 Mid-low 26.1 29.3 23.2 20.5 30.2 29.4 26.3 30.2 25.8 25.2 23.5 24 21.6 28.5 29.1 25.8 25.3
 Mid-high 30.9 28.4 32.1 30.7 24.3 29.1 28.2 28.1 35.2 33.4 33 29.7 34.4 29.2 37.5 33.3 32.4
 High 30.3 30 33.9 32.9 26 24.5 33.3 27.5 28 25.8 30.2 35.6 33.5 30.4 27.8 32.5 34.6
Parent’s education level
 Middle school 12.7 27.7 20.3 18.5 20.4 16.3 13.3 10.5 12.2 12.8 8.5 6 7 5.7 4 7 1.7
 High school 43.1 42.1 47.4 46.3 45.7 45.5 47 53.7 45.8 45 40.3 43.9 32.1 35.9 43.2 35.1 31.1
 College 44.2 30.2 32.3 35.2 33.8 38.2 39.8 35.8 42 42.2 51.1 50.1 60.9 58.3 52.8 57.9 67.2
Education level
 Elementary school graduates or less 40.1 41.9 42.1 42.2 42.3 38.4 39.3 41.3 39.6 40.8 36.2 35.4 37.2 38.4 40.9 43.3 43.1
 Middle school graduates 59.9 58.1 57.9 57.8 57.7 61.6 60.7 58.7 60.4 59.2 63.8 64.6 62.8 61.6 59.1 56.7 56.9
Residential area
 Urban 84.5 80.2 83.4 85.5 79.7 84.1 83.1 81.8 82.7 87.6 87.9 89.6 88.6 82.9 88.2 84.8 85.1
 Rural 15.5 19.8 16.6 14.5 20.3 15.9 16.9 18.2 17.3 12.4 12.1 10.4 11.4 17.1 11.8 15.2 14.9

Continuous values are expressed as mean±standard deviation, and categorical values are expressed as the percentage. SBP, systolic blood pressure; DBP, diastolic blood pressure; BMI, body mass index; WC, waist circumference; TC, total cholesterol; TG, triglyceride; HDL-C, high density lipoprotein cholesterol

Table 2.
Trends in total sleep duration by sex and residential area
Group Total sleep duration (weighted mean±standard error)
Age-unajusted
Age-adjusted
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 Coefficient ptrend Coefficient ptrend
Overall 7.18±0.11 7.00±0.08 7.15±0.07 7.18±0.08 7.00±0.08 6.96±0.09 6.91±0.09 6.99±0.07 6.95±0.08 7.43±0.06 7.27±0.08 7.30±0.07 7.27±0.07 7.19±0.06 7.29±0.08 7.22±0.06 0.015 0.001 0.017 <0.001
 Urban 7.09±0.12 6.98±0.09 7.11±0.08 7.19±0.09 6.96±0.09 6.92±0.09 6.92±0.11 7.04±0.08 6.93±0.09 7.41±0.07 7.29±0.08 7.22±0.07 7.23±0.08 7.17±0.07 7.26±0.08 7.23±0.07 0.017 <0.001 0.020 <0.001
 Rural 7.54±0.21 7.08±0.19 7.40±0.21 7.17±0.14 7.23±0.19 7.19±0.22 6.90±0.17 6.76±0.14 7.08±0.22 7.57±0.18 7.14±0.14 7.97±0.23 7.49±0.16 7.39±0.13 7.45±0.29 7.11±0.20 0.008 0.506 0.008 0.491
Boys 7.03±0.12 6.89±0.10 7.30±0.09 7.17±0.09 7.12±0.10 7.07±0.11 7.12±0.10 7.14±0.10 7.10±0.11 7.42±0.10 7.31±0.09 7.50±0.08 7.48±0.09 7.20±0.08 7.53±0.12 7.26±0.08 0.027 <0.001 0.028 <0.001
 Urban 6.99±0.13 6.86±0.11 7.23±0.10 7.12±0.10 7.07±0.11 7.07±0.12 7.14±0.10 7.22±0.11 7.04±0.11 7.40±0.10 7.37±0.10 7.40±0.08 7.43±0.09 7.21±0.09 7.51±0.12 7.23±0.09 0.028 <0.001 0.030 <0.001
 Rural 7.14±0.27 7.03±0.22 7.68±0.24 7.42±0.22 7.49±0.32 7.08±0.36 7.03±0.25 6.71±0.23 7.48±0.27 7.57±0.33 6.88±0.19 8.56±0.22 7.77±0.32 7.13±0.14 7.69±0.40 7.53±0.22 0.025 0.119 0.022 0.178
Girls 7.35±0.22 7.11±0.11 6.99±0.10 7.19±0.13 6.86±0.11 6.83±0.12 6.69±0.14 6.82±0.11 6.77±0.11 7.43±0.08 7.23±0.12 7.10±0.10 7.03±0.09 7.19±0.12 6.98±0.11 7.16±0.12 0.002 0.816 0.005 0.551
 Urban 7.19±0.21 7.11±0.13 6.98±0.10 7.27±0.15 6.82±0.13 6.71±0.12 6.68±0.17 6.82±0.13 6.81±0.11 7.41±0.09 7.21±0.13 7.01±0.12 6.98±0.10 7.12±0.12 6.96±0.10 7.24±0.13 0.005 0.561 0.007 0.362
 Rural 8.21±0.77 7.15±0.24 7.06±0.28 6.91±0.20 7.04±0.22 7.29±0.26 6.77±0.22 6.82±0.16 6.49±0.36 7.57±0.25 7.46±0.18 7.61±0.22 7.23±0.15 7.82±0.50 7.13±0.44 6.77±0.27 -0.011 0.643 -0.008 0.738
Table 3.
Trends in total sleep duration by sex and education level
Group Total sleep duration (weighted mean±standard error)
Age-unajusted
Age-adjusted
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 Coefficient ptrend Coefficient ptrend
Overall 7.18±0.11 7.00±0.08 7.15±0.07 7.18±0.08 7.00±0.08 6.96±0.09 6.91±0.09 6.99±0.07 6.95±0.08 7.43±0.06 7.27±0.08 7.30±0.07 7.27±0.07 7.19±0.06 7.29±0.08 7.22±0.06 0.015 0.001 0.017 <0.001
 Elementary school graduates or less 7.54±0.12 7.48±0.10 7.73±0.09 7.54±0.08 7.50±0.11 7.48±0.10 7.48±0.11 7.54±0.10 7.54±0.10 8.04±0.07 7.88±0.07 7.85±0.09 7.78±0.10 7.72±0.09 7.81±0.12 7.53±0.11 0.018 0.001 0.017 0.002
 Middle school graduates 6.91±0.18 6.64±0.10 6.73±0.09 6.92±0.12 6.69±0.10 6.63±0.12 6.51±0.12 6.62±0.10 6.54±0.11 7.08±0.08 6.94±0.10 6.98±0.09 6.96±0.08 6.83±0.10 6.89±0.11 6.97±0.08 0.015 0.015 0.015 0.020
Boys 7.03±0.12 6.89±0.10 7.30±0.09 7.17±0.09 7.12±0.10 7.07±0.11 7.12±0.10 7.14±0.10 7.10±0.11 7.42±0.10 7.31±0.09 7.50±0.08 7.48±0.09 7.20±0.08 7.53±0.12 7.26±0.08 0.027 <0.001 0.028 <0.001
 Elementary school graduates or less 7.59±0.14 7.43±0.13 7.79±0.10 7.52±0.10 7.43±0.16 7.49±0.14 7.63±0.12 7.71±0.14 7.67±0.11 8.04±0.10 7.95±0.08 8.00±0.11 8.03±0.12 7.91±0.11 8.00±0.17 7.67±0.12 0.032 <0.001 0.030 <0.001
 Middle school graduates 6.60±0.18 6.54±0.13 6.91±0.13 6.96±0.15 6.92±0.13 6.77±0.18 6.76±0.12 6.77±0.14 6.70±0.15 7.08±0.13 6.96±0.12 7.18±0.11 7.13±0.12 6.74±0.10 7.14±0.16 6.94±0.11 0.024 0.002 0.023 0.002
Girls 7.35±0.22 7.11±0.11 6.99±0.10 7.19±0.13 6.86±0.11 6.83±0.12 6.69±0.14 6.82±0.11 6.77±0.11 7.43±0.08 7.23±0.12 7.10±0.10 7.03±0.09 7.19±0.12 6.98±0.11 7.16±0.12 0.002 0.816 0.005 0.551
 Elementary school graduates or less 7.49±0.19 7.54±0.14 7.65±0.12 7.56±0.14 7.57±0.14 7.48±0.16 7.33±0.16 7.36±0.14 7.40±0.16 8.04±0.10 7.81±0.11 7.68±0.15 7.48±0.13 7.50±0.16 7.55±0.16 7.35±0.18 0.001 0.886 0.001 0.904
 Middle school graduates 7.26±0.33 6.77±0.15 6.54±0.11 6.88±0.19 6.42±0.14 6.47±0.16 6.26±0.19 6.45±0.14 6.34±0.14 7.07±0.10 6.92±0.17 6.78±0.15 6.76±0.11 6.95±0.18 6.61±0.14 7.02±0.15 0.006 0.586 0.005 0.639
Table 4.
Trends in total sleep duration by sex and obesity subgroups
Group Total sleep duration (weighted mean±standard error)
Age-unajusted
Age-adjusted
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 Coefficient ptrend Coefficient ptrend
Overall
 Normal 7.18±0.12 7.04±0.08 7.19±0.08 7.20±0.08 7.02±0.08 6.99±0.09 6.91±0.10 7.02±0.07 6.97±0.09 7.45±0.07 7.30±0.08 7.32±0.07 7.31±0.08 7.18±0.08 7.32±0.09 7.21±0.07 0.014 0.003 0.016 0.001
 Overweight 7.07±0.45 6.43±0.22 6.46±0.27 7.08±0.39 7.27±0.22 6.47±0.32 6.57±0.35 6.50±0.34 6.73±0.27 7.45±0.31 6.89±0.20 6.97±0.37 7.34±0.23 7.37±0.21 7.11±0.26 7.20±0.29 0.038 0.025 0.042 0.007
 Obese 7.18±0.39 6.36±0.38 7.16±0.41 6.84±0.36 6.23±0.35 7.58±0.72 7.39±0.47 6.93±0.22 6.93±0.21 6.89±0.20 6.86±0.31 7.16±0.27 6.74±0.23 7.09±0.22 6.93±0.30 7.44±0.46 0.014 0.496 0.015 0.466
Boys
 Normal 7.00±0.13 6.95±0.10 7.34±0.10 7.20±0.10 7.20±0.11 7.07±0.12 7.12±0.10 7.16±0.11 7.09±0.12 7.46±0.11 7.34±0.10 7.50±0.09 7.51±0.10 7.20±0.10 7.68±0.14 7.24±0.08 0.027 <0.001 0.028 <0.001
 Overweight 7.06±0.78 6.54±0.29 6.65±0.24 6.52±0.44 7.04±0.19 6.85±0.35 6.68±0.35 6.85±0.56 7.12±0.26 7.34±0.40 6.90±0.26 7.38±0.37 7.43±0.26 7.51±0.29 7.00±0.32 7.22±0.33 0.045 0.041 0.045 0.022
 Obese 7.27±0.40 6.45±0.44 7.21±0.17 7.31±0.23 6.14±0.41 7.64±0.80 7.47±0.52 7.13±0.21 7.15±0.20 6.60±0.30 7.19±0.47 7.40±0.29 7.22±0.12 6.88±0.24 6.93±0.30 7.76±0.47 0.012 0.599 0.013 0.557
Girls
 Normal 7.39±0.24 7.14±0.12 7.02±0.10 7.20±0.13 6.84±0.11 6.90±0.12 6.70±0.15 6.88±0.11 6.82±0.11 7.43±0.09 7.26±0.12 7.14±0.10 7.09±0.10 7.16±0.14 6.96±0.11 7.17±0.13 0.001 0.912 0.003 0.675
 Overweight 7.09±0.36 6.17±0.29 5.97±0.68 8.16±0.82 7.53±0.39 5.86±0.49 6.36±0.82 6.10±0.33 6.32±0.47 7.68±0.45 6.88±0.29 6.23±0.77 6.89±0.23 7.06±0.24 7.44±0.43 7.15±0.51 0.017 0.526 0.034 0.204
 Obese 5.00±0.00 6.03±0.73 7.11±0.81 6.26±0.59 6.40±0.67 7.34±0.63 7.00±0.98 5.80±0.56 6.57±0.33 7.28±0.13 6.43±0.34 6.44±0.47 6.16±0.37 7.87±0.26 NA 6.36±0.09 0.023 0.614 0.017 0.700

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