The Relationship between Sleep Duration, Body Mass Index, Physical Education Classes, Age and High Blood Pressure in Children and Adolescents: A Cross-sectional Study

Background In order to study the high blood pressure of children and adolescents and its influencing factors, it provides evidence-based evidence for the high secondary school students. Methods This study is a cross-sectional survey, which uses stratified cluster sampling method to select students from 7 schools to participate in questionnaire survey and physical examination. The correlation between single factor and high blood pressure was analyzed by chi-square test, and multivariate logistic regression analysis was used for multivariate analysis. Results A total of 1,814 subjects, including 512 primary school students (28.22%), 499 junior high school students (27.51%), 563 senior high school (31.04%), and 240 vocational high school (13.23%). There were 1000 males (55.13%) and 814 females (44.70%). The average age was 13.64±2.65 years. The detection rate of high blood pressure was 9.76%. Multivariate logistic regression analysis showed that the age groups were 10~ and 14~ (OR=20.870, 95%CI=2.372-183.666; OR=10.049, 95%CI=1.306-77.353) , sleep duration < 7h (OR=4.136, 95%CI=4.136), and physical education class times ≤2 times per week (OR=5.073, 95%CI=2.497-10.306), which were risk factors for high blood pressure in children and adolescents. Normal weight and overweight group were protective factors compared to the obesity group (OR=0.094, 95%CI=0.061-0.146; OR=0.225, 95%CI=0.134-0.378). Conclusion The age groups of 14~ and 16~ years old, physical education classes ≤ 2 times per week, and sleep duration <7h were risk factors for primary and secondary school students, and BMI of Normal weight and overweight group were protective factors compared to the obesity group students.

In 2014, the physique investigation of Chinese students found that the prevalence of high blood pressure in children and adolescents was as high as 6.4% [5]. Therefore, monitoring the high blood pressure of children and adolescents is very important for adult hypertension. The purpose of this study is to investigate the current status and influencing factors of high blood pressure in primary and middle school students, so as to provide evidence-based basis for the prevention and control of hypertension in primary and secondary school students.

Study population
The data for this study came from the results of 2017 monitoring of common diseases and health risk factors for students in a district health department of Chongqing, China. In this survey, a total of 7 schools were selected by stratified cluster sampling method, including grades 4-6 in primary school, and junior middle school, senior high school and vocational high school. These three types of schools are all grades 1-3. Each school randomly selected 1 to 3 classes, each class surveyed at least 80 students, and each school surveyed at least 240 students. The survey collected a total of 1816 primary and secondary school students.

Investigative process
This research used two methods, questionnaire survey and physical examination. And the research process consists of three parts. The first part is to investigate the basic characteristics of the study population, including the student's identity code, school, gender, nationality, date of birth, medical history and so on. The second part is the investigation of students' health risk factors. The questionnaire of students' health status and influencing factors compiled by the Chinese government, which is adopted to monitor some health risk factors that affect students' common diseases. The first part and the second part of the work were completed by the staff of the Centers for Disease Control and Prevention (CDC), who had been trained before the questionnaire. The third part is the monitoring of students' common diseases, which is jointly completed by the Health Care Center of Primary and Secondary Schools in the district and the university hospital, including height, weight, blood pressure (BP) and other demographic indicators.

Quality control
The investigation has taken strict quality control measures. Firstly, the researchers conducted unified training and assessment of the questionnaire survey workers and only the staff who successfully passed the assessment can participate in the questionnaire survey. And to avoid any inducement and suggestion of the survey results of the respondents. Secondly, the investigators are composed of medical professional, including doctors and public health workers, and the monitoring equipment must comply with national standards. Thirdly, the questionnaire contents and data entry results were reviewed by professional researchers. Finally, The supervision department carried out on-site supervision and sampling review in real time, supervised the questionnaire survey and the measurement of demographic indicators.

Determination criterion
Body mass index (BMI) = weight (kg) / height (m 2 ). Overweight and obesity were screened according to Screening for Overweight and Obesity among School-age Children and Adolescents (WS/T 586-2018) [6]. Overweight is defined as BMI greater than or equal to the overweight threshold for the corresponding gender and age group, and smaller than the obesity threshold. Those who have a BMI greater than or equal to the corresponding gender and age group "obesity" boundary points are obesity. Normal-high blood pressure and high blood pressure were screened according to Reference of Screening for Elevated Blood Pressure among Children and Adolescents Aged 7 18 years (WS/T 610-2018) [7]. Among children aged 7 to 17, according to the classification of blood pressure by age, weight and height, the normal blood pressure was defined as the systolic blood pressure (SBP) and diastolic blood pressure (DBP) < 90th percentile. The high-normal blood pressure was defined as SBP and/or DBP between the ≥ 90th percentile and the <95th percentile. The high blood pressure was defined as SBP and/or DBP ≥ 95th percentile. The blood pressure threshold of students aged 18 years and above should refer to the adult standard. Normal blood pressure was defined as SBP < 120 mm Hg and DBP < 80 mm Hg. The high-normal blood pressure was defined as SBP ≥120 mm Hg and < 139 mm Hg and/or DPB ≥80 mm Hg and < 89 mm Hg. The high blood pressure was defined as SBP ≥140 mm Hg and/or DPB ≥90 mm Hg. The definition of adequate sleep: ≥10h/d for students aged 7-12, ≥9h/d for students aged 13-15, ≥8h/d for students aged 16-18 [8], and ≥7h/d for adults over 18 years old [9].

Statistical Process
Data entry and analysis were performed with EpiData3.1 and SPSS22.0 respectively. The descriptive data are expressed as frequency and percentage (n,%), and the measurement data are expressed as mean and standard deviation (Mean± SD). Kolmogorov-smirnov test and F test were used respectively for normality test and homogeneity of variance test of samples. The rank-sum test (Kruskal-wallis H test) was used for the difference analysis of measurement data of continuous nonnormal distribution. Single-factor correlation analysis was performed by chi-square test. Spearson correlation test was used for continuous non-normal distribution data. Multivariate analysis was performed using disordered multivariate Logistic regression.

Results
The Table 1 shows the basic characteristics of the study population. A total of 1,816 students were surveyed in this study, of which the effective sample was 1,814 and the effective rate was 99.89%. Of all the 1,814 subjects, 512 were primary school students (28.22%), 499 were junior high school students (27.51%), 563 were senior high school (31.04%), and 240 were vocational high school (13.23%). And there were 1000 males and 814 females, accounting for 55.13% and 44.70% respectively. All subjects were 7 to 20 years old and the average age was 13.64±2.65 years.
According to blood pressure screening criteria, 177 primary and middle school students with high blood pressure were found in this study, accounting for 9.76%, and the normal blood pressure was 1,500, accounting for 82.69%, the high-normal blood pressure was 137, accounting for 7.55%. From primary school to vocational high school, the detection rate of high blood pressure gradually increased, among which the detection rate of high blood pressure of vocational high school students was the highest (17.08%), and the primary school students was the lowest (7.23%). All of the differences in distribution characteristics were statistically significant.  Table 2 presents the correlation between SBP, DBP and height, weight, age, BMI, and sleep duration, respectively. The results showed that the diastolic blood pressure and systolic blood pressure of primary and secondary school students were positively correlated with age, height, weight and BMI, but there is a negative correlation relationship between sleep duration and diastolic or systolic blood pressure. And all these differences were statistically significant (P<0.05).  The table 3 shows the correlation analysis results of blood pressure with different gender, different age groups and different BMI categories. In this study, 286 primary and secondary school students had a BMI of overweight, and the detection rate of overweight was 15.77%. The BMI of obese students was only 196, and the detection rate of obesity was 10.80%. However, obesity group had the highest detection rate of high blood pressure (28.57% for obesity group), followed by overweight (12.59% for overweight group), and the normal weight group had the lowest detection rate of high blood pressure (6.38%), and the difference in BMI distribution was statistically significant by Chisquare test. The detection rate of male students was higher than female students (55.13% for boys and 44.87% for girls), but there was no significant difference between the two groups. Among all age groups, the detection rate of high blood pressure in the 16~ age group and the 14~ age group was significantly higher than that in other age groups, and the difference was also statistically significant.  Table 4 shows the relationship between students' blood pressure and their sleep duration and physical exercise. In terms of exercise, most of students spent 1-3 days a week doing moderate or high intensity exercises, accounting for 53.97% and 49.95% of the total students respectively, and the two groups also had highest detection rates of high blood pressure, the detection rate of high blood pressure in moderate intensity exercise was 12.16%, and that in high intensity exercise was 11.71%. The detection rate of high blood pressure in students who did not participate in moderate or high intensity exercise was next, 9.51% and 9.91%, respectively. However, students who exercise 4-6 days of moderate or high-intensity exercise every week have the lowest rate of hypertension, the high blood pressure detection rates for moderate or high-intensity exercise for 4-6 days per week were 5.14% and 3.40%. Among all the students who joined in the study, students who attended physical education classes ≤2 times a week were the most, accounting for 45.76% of all students, and those who attended physical education classes ≥4 times a week was lowest, accounted for 20.07%. And the detection rate of high blood pressure of students who attended physical education classes ≤2 times a week was higher than that of students who attended physical education classes 3 times and ≥4 times a week15.30% > 5.59% > 4.84%). The lower the sleep time, the higher the detection rate of high blood pressure. Students who slept less than 7 hours per day had the highest detection rate of blood pressure, accounting for 15.95%, while students who slept 10 hours or more per day had the lowest detection rate, accounting for only 4.05%. Moreover, from the results of the study, most students do not have enough sleep duration, accounting for 73.10% of all subjects, and only 26.90% of students with adequate sleep. Moreover, the high blood pressure detection rate of students with insufficient sleep was 11.39%, which was higher than that of students with adequate sleep, and the proportion of students with adequate sleep was 5.33%. High intensity exercise: High-intensity exercise refers to sports that require a lot of effort and can be completed with a lot of sweat, including running, fast climbing, basketball, football, swimming, heavy loads, and so on. Compared with students who attended physical education class 3 times or more than 4 times a week, students who attended physical education class less than 2 times a week had a higher risk of high

Discussion
Hypertension is an important risk factor for cardiovascular diseases, and childhood hypertension is closely related to adult hypertension. Study have shown that the prevalence of hypertension in children and adolescents is within 3.4% ~23.7% [10]. The prevalence of hypertension in Chinese school-age children in 2010 was 14.5% [2], and the prevalence of hypertension among primary and secondary school students in Chongqing in 2013 was as high as 16.1% [11]. High blood pressure is a prerequisite for hypertension. When a blood pressure is detected ≥P95th, it needs to be measured 3 times or more on different dates. If the blood pressure is ≥P95th every time, it can be determined as hypertension [7]. From 1993 to 2011, the prevalence of high blood pressure among children and adolescents in China increased from 10% to 12.9% [3]. Therefore, the high blood pressure of children and adolescents such as primary and secondary school students deserves social attention. In this study, the detection rate of high blood pressure was 9.76%, lower than study in Suzhou during the same period (23.45%) [12], and higher than the national detection rate of high blood pressure in 2014 (6.4%) [5].
In this study, 1814 primary and secondary school students were investigated, and it was found that the detection rate of high blood pressure in males was higher than that in females. From primary school to high school, the detection rate of high blood pressure gradually increased. The detection rate of high blood pressure in vocational high school students was the highest, while that in primary school students was the lowest. Univariate correlation analysis found that factors related to high blood pressure in primary and secondary school students included BMI categories, age groups, days of moderate or high intensity exercise per week, number of physical education class per week, sleep duration and whether the sleep duration was sufficient. Students in the obesity group had higher rates of high blood pressure than the overweight and normal weight groups. The blood pressure of students in 14~ and 16~ age groups was significantly higher than that in other age groups. Students who did moderate or high intensity physical exercise 1-3 days a week and did not participate in physical activity had higher detection rates than students who did moderate or high intensity physical exercise 4-6 or 7 days a week. Compared with students who received physical education classes 3 or more times a week, students who received physical education classes ≤ 2 times a week had higher detection rates. And the less sleep duration, the higher the detection rate of high blood pressure. The high blood pressure detection rate of the students with sleep duration <7h and insufficient sleep was significantly higher than that of the students with other sleep duration groups and those with sufficient sleep.
After univariate and multivariate logistic regression analysis by adjusted gender, it was found that both the10~ and 14~ age groups were risk factors for high blood pressure of primary and secondary school students. And the risk of high blood pressure was higher in the 10~ age group than in the 14 ã ge group (20.870>10.049). The 10~ age group is the initiation period of puberty development, and children begin to enter puberty. Studies have shown that due to the regulation of the neuroendocrine system during this period, the contractile function and vascular resistance of the heart can not adapt to the sudden changes in the body, so that blood pressure rises [13]. And students in the 14~ age group began to enter the second to third year junior high school. In addition to the changes in blood pressure caused by growth and development, it is also possible that the mental stress caused by increased academic burden will increase blood pressure [14]. Therefore, this paper suggests that we should focus on the blood pressure status of students age 10~and 14~ years old groups.
This study found that in the number of physical education classes per week, most students attended physical education classes ≤2 times per week, accounting for 45.76%, and the detection rate of these students was the highest. After multivariate regression analysis, it was found that there was a higher risk of high blood pressure in physical education classes with the number of times per week ≤2 than in physical education classes with 3 or more times per week (OR=5.073). Therefore, the insufficient number of physical education classes is a risk factor for blood pressure in primary and middle school students. The Chinese government stipulates that the number of physical education classes for primary and secondary school students is 5 times a week, and not less than 20-30 minutes each time [15]. so that most of the students in this survey did not reach the standard. Several studies have shown that physical activity is a contributing factor to high blood pressure in children and adolescents [14]. With a 15 minutes increase in moderate or high intensity exercise, the risk of blood pressure will decrease by 0.65 times [16] and the SBP will decrease by 0.5 mmHg [17]. Therefore, it is recommended that all primary and secondary schools should guarantee the weekly physical education time and encourage students to participate in outdoor activity and physical education classes.
Studies have shown that children's BMI is a predictive factor in adult BMI [18] and an important basis for the diagnosis of overweight and obesity. Burke V. found that BMI also is a predictor of SBP. In this study found that 26.57% of the subjects were overweight and obesity, and 19.09% of overweight and obesity students have high blood pressure. High blood pressure detection rate: obesity group > overweight group> normal weight group. The univariate correlation analysis found that BMI is related to both SBP and DBP. Regression analysis found that compared with the obesity group, the overweight and normal weight group were the protective factors for the high blood pressure of primary and secondary school students, and the OR value was lower when the BMI was normal. This result is consistent with several studies. Researchers found that being overweight and obesity remained a risk factor for hypertension after adjusting for gender and age [19][20]. The WGOC also recommends BMI as a sensitive factor in predicting blood pressure. Juonala M.'s [18] studies have shown that decreasing BMI, overweight and obesity can reduce the risk of cardiovascular disease.
Therefore, it is recommended that students in primary and secondary schools should actively control their BMI values, control their weight, and reduce obesity, thereby reducing the risk of high blood pressure.
Several studies have linked sleep duration to high blood pressure [21][22]. There are many studies on adult sleep duration and hypertension, and there are few studies on children and adolescents. In recent years, researchers have begun to pay attention to the relationship between sleep and hypertension in children and adolescents. Studies have shown a significant correlation between sleep and cardiovascular disease in children and adolescents [23][24], and increased sleep duration can decrease blood pressure [25]. Guo et al. found in their sleep study on children aged 5-18 that for every 1h increase in sleep time, SBP and DBP decreased by 1.04mmHg and 0.55mmHg, respectively [26]. In this study, Most of students sleep 7 and 8 hours a day, accounting for 48.84%. However, the detection rate of high blood pressure in the students who slept for < 7h per day was higher than that in the students who slept for other duration groups. Multivariate analysis showed that sleep duration <7h was a risk factor for high blood pressure in primary and middle school students, which was consistent with the results of Kuciene R.'s study [23] . The study also found that students with insufficient sleep time accounted for the highest proportion of all students (73.10%), and the high blood pressure detection rate was higher than those with adequate sleep (11.39%). The sleep adequacy ratio is only 26.9%, which is lower than the 33.78% sleep duration pass rate of Chinese students in 2010 [8]. The results of the study indicate that the sleep duration of primary and secondary school students in the area needs to be improved. They should ensure adequate sleep by reasonable arrangement of work and rest, and the sleep time should not be lower than 7h.
The limitation of this study is that the sample size is not large enough. The study population only selected one district of Chongqing, but did not expand to the entire city. Moreover, the number of students in each grade from primary school and high school is not enough. In order to popularize the results of this study, more sample sizes are needed to better prove the results of this study.

Conclusion
In this study, it was found that age, body mass index, weekly physical education classes and sleep were associated with high blood pressure in children and adolescents. The age groups of 14~ and 16~ years old, physical education classes ≤ 2 times per week, and sleep duration <7h were risk factors for primary and secondary school students, and BMI of Normal weight and overweight group were protective factors compared to the obesity group students.
the statistical analysis. Both authors collaborated on interpreting the results and writing the paper.
And all authors have read and approved the manuscript.