Ten-Year Secular Trends in Sleep/Wake Patterns in Shanghai and Hong Kong School-Aged Children: A Tale of Two Cities
To compare the secular trends of sleep/wake patterns in school-aged children in Hong Kong and Shanghai, two major metropolitan cities in China with two different policies that school start time was delayed in Shanghai, but advanced in Hong Kong in 10 years’ time.
Participants were from two waves of cross-sectional school-based surveys of children aged 6 to 11 years. In Shanghai, 4,339 and 13,795 children participated in the 2005 and 2014 surveys, respectively. In Hong Kong, 6,231 and 4,585 children participated in the 2003 and 2012 surveys, respectively. Parents reported their children’s bedtime and wakeup time, and thus sleep duration, short sleep (≤ 9 hours) and weekend oversleep (difference in sleep duration between weekday and weekend > 2 hours) were determined.
Hong Kong children had later bedtime and wakeup time and slept consistently less than their Shanghai counterparts at both survey time points. The shorter sleep duration was particularly marked during weekdays. Over the interval period, weekday sleep duration significantly decreased from 9.2 to 8.9 hours as wakeup time became earlier for Hong Kong children, but increased from 9.4 to 9.6 hours as wakeup time became later for children in Shanghai. Children from both cities slept longer on the weekends. Prevalence of weekend oversleep significantly increased in Hong Kong children, but no interval change was found in Shanghai children.
The findings indicate subcultural differences in sleep/wake patterns in Shanghai and Hong Kong school-aged children. In particular, sleep duration had increased for Shanghai children, but decreased for Hong Kong children over 10 years. The benefits and barriers of delaying school start time for optimizing sleep health in school-aged children should be further explored.
Wang G, Zhang J, Lam SP, Li SX, Jiang Y, Sun W, Chan NY, Kong APS, Zhang Y, Li S, Li AM, Jiang F, Shen X, Wing YK. Ten-year secular trends in sleep/wake patterns in Shanghai and Hong Kong school-aged children: a tale of two cities. J Clin Sleep Med. 2019;15(10):1495–1502.
Current Knowledge/Study Rationale: Evidence about secular trends of sleep/wake patterns in children is scarce and conflicting. No study has compared the secular trends of sleep/wake patterns in Chinese school-aged children in Hong Kong and Shanghai, which implemented two different policies that affected schools’ start time in 10 years’ time.
Study Impact: We found that Hong Kong children had later bedtime and wakeup time, and slept consistently shorter than Shanghai children. Weekday sleep duration significantly decreased for Hong Kong children, but increased for Shanghai children over the past decade. This study provides insights into the secular trends of sleep/wake patterns, subcultural difference, and population-level policy regarding healthy and adequate sleep in school-aged children.
Substantial evidence supports the beneficial effects of adequate sleep on child’s health, growth, well-being, and neuroplasticity,1–4 whereas insufficient sleep is linked to various adverse outcomes such as cardiometabolic, neurocognitive, and mental problems in children and adolescents.5–11 However, a significant proportion of school-aged children report habitual sleep duration less than the recommended 9 hours per day.12–15 In parallel to the cross-sectional evidence of insufficient sleep, a secular trend of decline in sleep duration in children is evident.16 A decrease of more than 1 hour of sleep per night in children and adolescents over the past century from 1905 to 2008 has been noted in a recent review of worldwide literature, albeit there were substantial differences in age, sex, region, and country.17 In addition, there has been mixed evidence suggesting either no such secular decrease or even an increase in sleep duration in children.18 Thus, further research about the secular changes of sleep duration in children is required to inform the public, educational sectors, health care providers, and policymakers.19
Childhood sleep/wake patterns are shaped by multiple biopsychosocial and environmental factors. While Asian children have consistently reported a shorter sleep duration than their Caucasian counterparts,12,20,21 subcultural or regional variations in the amount of sleep within the same ethnicity or country remain overlooked.22 In recent years, westernization together with the increasing dependence on electronic devices and rapid-paced lifestyle are believed to result in reduced sleep in children and adolescents, albeit there are limited supporting data.19 The interactive relationships among school schedule, parental sleep/wake patterns, socioeconomic status, and daytime activities in determining the sleep/wake patterns in children further implicate a complex scenario with regard to the secular trend of children’s sleep.23 School start time has been recognized as a factor that significantly affects wakeup time and sleep duration in Chinese school-aged children.13,23 Thus, policy changes regarding school start time may contribute to the secular changes of sleep/wake patterns in Chinese school-aged children. With the emerging evidence on the benefits of delaying school start time on improving sleep duration in school-aged children,24,25 Shanghai government launched a new policy in 2007 that the start time of primary schools should not be earlier than 8:15 am. This was an important change for the local students because many of them used to have school start time no later than 7:30 am. It was estimated that 89% of primary schools had school start time earlier than 8:15 am in 2005, whereas 36% of primary schools had later school start time in 2014 than 2005 in Shanghai. However, the Hong Kong Education Bureau has gradually phased out half-day schooling (ie, having classes either in the morning or in the afternoon) to a full-day schedule since 2005, which led to significant changes in school start time for many local students (ie, those who used to attend afternoon school had to get up much earlier to attend the full-day classes). In 2003, 18.6% of Hong Kong children were attending afternoon school; this percentage decreased to 1.3% in 2011. The different direction of policy change with respect to school start time could potentially affect the secular trends of sleep/wake patterns in school-aged children in both cities. In view of the significant effect of school start time on children’s sleep duration,13,23–25 we hypothesized that there were significant changes of sleep/wake patterns in children between Hong Kong and Shanghai along with the policy change on school start time. By using two waves of large-scale surveys, we aimed to investigate and compare the secular trends of sleep/wake patterns over 10 years among Chinese school-aged children in these two metropolitan cities, Shanghai (2005 to 2014) and Hong Kong (2003 to 2012).
Shanghai 2005 Survey
This was part of a national investigation of sleep and health in Chinese school-aged children in 2005.7,26–28 There were 4,997 children randomly selected from 10 schools in Shanghai, representing the socioeconomic diversity of the city in eastern China, and 4,399 questionnaires were returned (response rate: 88%).29 The analysis of this study was based on 4,339 children aged 6 to 11 years.
Shanghai 2014 Survey
Data came from a survey on Shanghai Children’s Health, Education and Lifestyle Evaluation (SCHEDULE) conducted in July 2014. Briefly, 17,624 children were recruited from 26 schools across Shanghai, and 17,318 questionnaires were returned (response rate: 98.3%). There were 16,495 children aged 6 to 11 years. A total of 2,700 children from a public school in Chongming County (rural area) and two private schools were excluded to make the sample comparable to that of the 2005 survey. The analysis of this study was based on 13,795 children aged 6 to 11 years.
Hong Kong 2003 Survey
The sample from the Hong Kong 2003 survey was part of an epidemiologic study of sleep problems and sleep/wake patterns in Hong Kong Chinese children. Students from 13 randomly selected primary schools were invited to take part in this study. A total of 9,172 questionnaires were delivered and 6,447 children returned their questionnaires (response rate: 70.3%).15 The analysis of this study was based on 6,231 children aged 6 to 11 years.
Hong Kong 2012 Survey
The sample from the Hong Kong 2012 survey was part of a large-scale sleep education program that recruited students from both primary and secondary schools (ChiCTR-TRC-12002798).30 The current study was based on the baseline data that included 4,585 children aged 6 to 11 years old from 16 primary schools in Hong Kong (response rate: 61.9%).31
The institutional review board of the Shanghai Children’s Medical Center and the Chinese University of Hong Kong approved the respective studies. Parents of the children gave written informed consent in all the studies.
Parent-reported children’s bedtime and wakeup time were collected by the following questions, adapted from the Children’s Sleep Habits Questionnaire (CSHQ): “What time does your child usually go to bed on weekdays and weekends?” and “What time does your child usually get up on weekdays and weekends?”23,32
Sleep duration was operationalized as the proxy-reported time in bed, taken to mean the elapsed time between turning off the light to go to sleep and waking in the morning.17 Short sleep duration was defined as < 9 h/night.33 Irregular sleep was defined as weekend oversleep (difference in sleep duration between weekend and weekday) > 2 hours.34,35 Habitual napping was defined as taking naps at least 3 times/week.23 Only the samples from Hong Kong provided information on napping.
Demographic factors measured in all four surveys included child’s sex, age, school start time, and parental education level. Parental education level was considered as a proxy of socioeconomic status. School start time in all surveys was provided by individual schools.
Descriptive statistics were used to characterize the participants. A series of 2 (city: Shanghai and Hong Kong) × 2 (survey year: baseline and 10-year follow-up) analyses of covariance were conducted to compare sleep patterns in girls and boys separately, after adjusting for age, school start time, and parental education level. In consideration of multiple comparisons, all tests were considered statistically significant at an α level of .004 according to Bonferroni correction for the analyses of covariance (6 parameters for sleep patterns × 2 sexes). Prevalence of short sleep and irregular sleep were compared with χ2 tests. Statistical analyses were performed with SPSS 22.0 (IBM, Armonk, New York).
Table 1 presents demographic characteristics of the participants. No significant differences in age and sex were noted between the Hong Kong 2003 and 2012 surveys, whereas average age was slightly older in the Shanghai 2014 survey than the 2005 survey (9.1 ± 1.5 versus 8.8 ± 1.5 years, P < .001), and fewer girls were included in the Shanghai 2014 survey (46.4% versus 50.1%, P < .001). School start time was slightly later in 2014 than 2005 in Shanghai (8:02 am ± 5 minutes versus 8:00 am ± 13 minutes, P = .01). In Hong Kong, school start time was 76 minutes earlier in 2012 than in 2003. However, if excluding children who used to attend afternoon schools in 2003, school start time was 30 minutes later in 2012 for those who used to attend morning schools in 2003. Parents participating in 2012 and 2014 surveys in Hong Kong and Shanghai, respectively, had a higher education level than their counterparts from 2003 and 2005 surveys.
Table 2 presents weekday and weekend sleep in girls and boys, respectively, adjusting for age, school start time and parental education. For weekday sleep, Hong Kong girls went to bed earlier by 12 minutes, woke up earlier by 46 minutes, and slept shorter by 19.8 minutes in 2012 than in 2003. Shanghai girls had similar bedtime, but woke up later by 19 minutes and had longer sleep duration by 10.2 minutes in 2014 than in 2005. Girls in Hong Kong had considerably later bedtime, later wakeup time, and shorter sleep duration on weekdays than Shanghai girls in both surveys. The patterns of the changes in weekday sleep were generally similar for boys and girls.
For weekend sleep, there was a similar trend in Hong Kong and Shanghai children, with slightly earlier bedtime, later wakeup time, and longer sleep duration, and the pattern was more marked in the second survey. Hong Kong children reported consistently later bedtime, later wakeup time but comparable sleep duration on weekends when compared to Shanghai children in both surveys. These results were similar for both girls and boys.
Figure 1 and Figure 2 depict the secular trends of sleep patterns by sex, age, and day of the week. A notable feature was the variations of secular changes across age. For example, secular changes of weekday sleep duration were more prominent in older children (Figure 1E and Figure 1F). In addition, the prevalence of habitual napping decreased significantly from 2003 to 2012 in girls (from 6.5% to 3.6%) and boys (from 4.6% to 2.8%) in Hong Kong.
Short Sleep Duration and Irregular Sleep
Figure 3A presents the prevalence of weekday short sleep duration by sex. The prevalence of short sleep duration decreased from 15.3% in 2005 to 11% in 2014 for boys and from 16.3% in 2005 to 10.7% in 2014 for girls in Shanghai, but increased from 39.3% in 2003 to 45.1% in 2012 for boys and from 38.6% in 2003 to 49.7% in 2012 for girls in Hong Kong. Figure 3B presents the prevalence of weekend short sleep by sex. Fewer children had short sleep duration on the weekends in the later survey year in both Shanghai (from 6.8% to 4.4% for boys, and from 3.9% to 2.7% for girls) and Hong Kong (from 8.2% to 5.8% for boys and from 3.9% to 2.7% for girls).
As shown in Figure 4, the prevalence of irregular sleep in Hong Kong increased from 10.2% to 19.6% for boys, and from 15.4% to 26.5% from 2003 to 2012 for girls, whereas it was comparable from 2005 to 2014 in Shanghai children (from 4.7% to 4.6% for boys, and from 6.0% to 6.2% for girls).
Subgroup Analysis After Excluding Afternoon Schools in Hong Kong
As shown in Table 1, if we excluded afternoon schools in Hong Kong, the school start time was delayed for approximately 30 minutes in those morning-start schools. There may be concern about the extreme situation of the phase-out effect of afternoon schools in Hong Kong. We further ran analyses on sleep patterns after excluding afternoon schools (Table S1 and Table S2 in the supplemental material). We found that Hong Kong boys and girls in 2012 had minimal changes in bedtime but delay in wakeup time and slightly longer sleep duration than in 2003, along with later school start time. Nonetheless, the sleep duration of the children in the morning start schools in Hong Kong in both 2003 and 2012 were sleeping much shorter than their Shanghai counterparts.
This is the first study to investigate the secular trends of sleep/wake patterns over the past decade in Chinese school-aged children using large-scale and representative samples from two metropolitan cities in China, namely Shanghai and Hong Kong. Several important findings were worthy of note. Hong Kong children had significant later bedtime and wakeup time and shorter sleep duration than Shanghai children, especially during weekdays at both baseline (early 2000s) and 10 years later. Along with the policy change on school start time (eg, delayed school start time in Shanghai and cancellation of afternoon schools in Hong Kong), Hong Kong children, on average, got less sleep whereas Shanghai children got more sleep during weekdays. Nonetheless, if we excluded afternoon schools in 2003, Hong Kong children attending the morning start schools actually got more sleep alongside with their later school start time. Nonetheless, the irregularity of sleep in Hong Kong children got worse but remained unchanged in Shanghai children. These findings suggested there were significant subcultural differences in sleep patterns in Chinese children. Policy change on school start time and other subcultural influences, such as consistent late bedtime in Hong Kong, seemed to have a significant effect on sleep patterns in children.
Using similar study design in each city to examine secular changes, our current study has clearly demonstrated opposite changes in sleep duration between Hong Kong and Shanghai children. The direction of change mirrored the stance taken by each city with regard to school start time. The change in school start time may be one of the significant contributors to the secular trend of sleep duration in our children.13,23 Delaying school start time has been found to increase sleep duration in children and adolescents.36 In Hong Kong, the abolition of half-day afternoon school has advanced the school start time for many local students over the past decade. Nonetheless, the effect of school start time on sleep duration should be considered in the following context. Despite the fact that Hong Kong had later school start time than Shanghai at both survey time points, Hong Kong children slept considerably less than their counterparts in Shanghai and the gap became more pronounced over the past decade (from about 0.2 to 0.7 hour). The main reason was that Hong Kong children went to bed much later compared with children in Shanghai. In other words, the abolition of half-day afternoon school with subsequent advancement in the school start time, coupled with persistent late bedtime, has resulted in a decrease in sleep duration of children in Hong Kong. Because children’s bedtime was significantly affected by parental bedtime,13,23 the effort of extending sleep duration for children in Hong Kong should also be directed to advancing bedtime, with parental involvement being an essential element. For example, it has been shown that parent-set bedtime is a significant protective factor against of short sleep duration in the pediatric population.37 Further research is warranted to inform cost-effective public health approaches to promote healthy sleep practice and adequate sleep duration among school-aged children across different regions and cities.
Another interesting finding worthy of note was that there was only a 2-minute difference in the delay of school start time in Shanghai from 2005 to 2014, despite a public policy requesting schools to start no earlier than 8:15 am. In actual practice, the long-term compliance to the delay school start policy in the Shanghai schools remained unclear. Indeed, previous research found that it was rather challenging to motivate the schools to delay their school start time due to various practical concerns and logistic issues (for example, concerns about timing for after-school activities and transportation arrangements).25 Therefore, there is a need to address the barriers to a successful implementation of policies for promoting sleep health in school-aged children, such as delaying school start time.
This study also captured a consistent upward trend of weekend sleep duration with slightly earlier bedtime and delayed wakeup time in the two cities. This upward trend was a mere reflection of increasing catch-up sleep on the weekends due to sleep restriction during weekdays, which was especially true for Hong Kong children.25 This was further confirmed by our findings that the prevalence of irregular sleep was increased over the survey years. Considering potential negative effects of irregular sleep on health and daytime functioning,34,35 it is suggested that school-aged children should keep sufficient sleep on a regular basis rather than having weekend compensation for weekday restricted sleep.
In this regard, the noted differences in the secular trends of sleep/wake patterns between Shanghai and Hong Kong linked to policy change on school start time implicate subcultural differences, and population-level policy regarding healthy sleep in school-aged children. For example, the later school start time has benefited children in Shanghai (morning lark city) with increasing sleep duration, albeit the intended effect and sustainability of the policy was not as good as one would have expected. However, the alarmingly high and marked increase in the prevalence of short sleep duration (over 45% in 2012) in Hong Kong children would argue for an urgent need to look for the underlying reasons for a much delayed bedtime and effective interventions to improve the sleep health of children in Hong Kong, a late owl city.
This study is the first to investigate and compare the secular trends of sleep duration in school-aged children across two metropolitan cities in China. The parallel survey time points, large-scale samples, and similar measurements across surveys enabled an accurate examination of the secular trends and comparisons.16 However, several limitations should be noted when interpreting our findings. First, sleep patterns were reported by parents without objective assessments, such as actigraphy or polysomnography. However, it has been shown that parent-reported measures have satisfactory agreement with objective tools in measuring sleep duration in children.38 In addition, the application of parent-reported measures would not have affected the observed secular trends of sleep patterns. Second, data were collected during the school year that might not reflect the sleep patterns during a long holiday and might be subject to the potential influence of seasonality.39 Third, this was a naturalistic observational study without any intervention. Although we speculated that policy changes regarding school start time might be the key factor contributing to the secular changes in sleep/wake patterns in these two cities, no definite conclusion can be drawn due to the study design. In addition, other factors that could significantly influence sleep patterns, such as media use and seasonality, were not able to be put into the statistical model. In addition, habitual napping was only measured in Hong Kong children, and we were not able to compare the changes of this nap behavior across two cities. Finally, despite random selection, the schools that participated in the first survey were not the same as those schools that participated in the later survey. As such, it was possible that the secular trends in sleep/wake patterns might also be influenced by individual school characteristics.
In summary, we have captured 10-year secular trends in sleep/wake patterns in school-aged children in Shanghai and Hong Kong, and noted between-city differences. Overall, weekday sleep duration was significantly decreased from 9.2 to 8.9 hours as wakeup time became earlier for Hong Kong children, but increased from 9.4 to 9.6 hours as wakeup time became later for Shanghai children. Both Shanghai and Hong Kong children slept longer during weekends. The prevalence of irregular sleep as indicated by weekend oversleep significantly increased in Hong Kong children but no change was found in Shanghai children. The findings provided insights into the secular trends of sleep/wake patterns, subcultural differences, and population-level policy regarding healthy and adequate sleep in school-aged children. In particular, the benefits and barriers of delaying school start time for optimizing sleep health in school-aged children should be further explored.
All authors have read and approved the final version of the manuscript. Work for this study was performed at Shanghai Children’s Medical Center and The Chinese University of Hong Kong. The study was supported by the Chinese National Natural Science Foundation (81602868, 81601162, 81728017, 81773443); Fourth Round of Three-Year Public Health Action Plan (2015-2017) (GWIV-36); Ministry of Science and Technology of China (2016YFC1305203); Shanghai Science and Technology Commission (17411965300, 17XD1402800, 14441904004, 19QA1405800, 19411968800); Shanghai Municipal Commission of Health and Family Planning (20164Y0095, 20164Y0001, 2016ZB0104); Shanghai Pudong District Technology Development Funds (PKJ2018-Y45); Shanghai Jiao Tong University (YG2016ZD04); Shanghai Jiao Tong University School of Medicine (Innovation Team on Pediatric Research Funds, Child Developing Brian Research Center Construction Funds) Project of Shanghai Children’s Health Service Capacity Construction(GDEK201708); Fundamental Research Funds for the Central Universities from the Key Laboratory of Child Development and Learning Science (Southeast University) and the Ministry of Education (CDLS-2018-03), Public Policy Research of University Grants Committee (CUHK4012-PPR-11), and Research Grants Council (CUHK4161/02M), Hong Kong SAR, China. The funding body had no role in the conception, design, conduct, interpretation, or analysis of the study nor in the approval of the publication. Dr. Wing has received sponsorship from Lundbeck Export A/S, Servier Hong Kong Ltd, Pfizer Company Ltd, and Celki Medical Company. Dr. Kong has received honorarium for consultancy or giving lectures from Astra Zeneca, Pfizer, Sanofi, Novo Nordisk, Eli Lilly, Merck Serono, and Nestle. The other authors report no conflicts of interest.
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