Abstract
Background
The etiology of adolescent idiopathic scoliosis is multifactorial, and the influence of lifestyle factors such as sleep is not clearly understood. Differences in scoliosis incidence between urban and rural areas have been reported, but the contributing factors remain unclear. Therefore, this study investigated the association between sleep patterns and the incidence of idiopathic scoliosis and explored whether these patterns contribute to the observed urban-rural disparity.
Methods
This retrospective study utilized data from the Korea Children and Youth Panel Survey (2010–2016) and the Health Insurance Review and Assessment Service for 4,693 students (age, 7–18 years). Various lifestyle factors including sleep patterns, learning time, and activity times, were compared between urban and rural areas, and a correlation analysis was performed between these factors and the age-specific incidence of idiopathic scoliosis.
Results
Urban students, who exhibited higher idiopathic scoliosis incidence rates, tended to have later bedtimes and shorter total sleep durations than rural students. Longer learning hours were also observed in urban areas. Significant correlations were found between idiopathic scoliosis incidence and bedtime (p = 0.031), total sleep time (p = 0.026), and changes in total sleep time (p = 0.011).
Conclusions
Our findings indicate that later bedtimes and shorter sleep durations may contribute to idiopathic scoliosis development in children and adolescents. The higher idiopathic scoliosis incidence in urban students than in rural students could be partially explained by these sleep pattern differences, highlighting the need for further research into the role of sleep in scoliosis onset and prevention.
Keywords: Environmental exposure, Scoliosis, Sleep, Residence characteristics
Idiopathic scoliosis is defined as a spinal deformity with a Cobb angle of ≥ 10° in the absence of a specific cause, constituting the most prevalent form of scoliosis.1) It can manifest across all age groups during the growth phase, with a higher incidence observed in adolescent females than in adolescent males.2) Mild idiopathic scoliosis typically remains asymptomatic and does not require targeted intervention, whereas moderate to severe cases may necessitate bracing or surgical measures. Untreated children with idiopathic scoliosis may experience psychological distress due to bodily deformity, mental health exacerbation,3) and potential cardiopulmonary function impairment attributable to thoracic deformity in severe cases.4)
The etiology of idiopathic scoliosis remains elusive. It is understood that genetic and environmental factors interact complexly during idiopathic scoliosis onset. In a study using the Danish Twin Registry to investigate scoliosis, a higher concordance rate was observed among monozygotic twins than in dizygotic twins.5) These findings suggest a partial genetic influence in idiopathic scoliosis development, driving ongoing research efforts to identify relevant genes.6,7) In addition to genetic factors, numerous environmental factors play an important role in idiopathic scoliosis onset, leading to extensive research in this area. Environmental factors, such as estrogen, diet, and exercise, have been proposed;8) moreover, melatonin, a sleep-related hormone, shows particularly interesting results. Melatonin deficiency induced scoliosis in animal studies,9,10) and various clinical studies have reported a correlation between melatonin and idiopathic scoliosis development.11) In addition to melatonin, recent studies have highlighted the potential influence of sleep posture and related behavioral habits.12,13) However, there is still a lack of research focusing on specific sleep-related environmental factors, such as total sleep duration and bedtime.
Therefore, the aim of this study is to investigate the correlation between sleep patterns and idiopathic scoliosis development. We explored which environmental factors might influence idiopathic scoliosis incidence differences between urban and rural areas, as revealed in a previous study,14) and investigated the correlation between the age-specific idiopathic scoliosis incidence and environmental factors, including sleep. To achieve this, we conducted research by merging the Korean National Health Insurance and Korean Child and Youth Panel Survey databases.
METHODS
Ethics Statements
This study was approved by the Institutional Review Board and Ethics Committee of Yonsei University (IRB No. 3-2018-0041). The need for informed consent was waived since the study used anonymized data that were accessed retrospectively.
Sources of Data
We retrospectively analyzed the nationwide cohort data from 2010 to 2016. The Korean universal health coverage system, known as the National Health Insurance, covers the entire population. The Health Insurance Review and Assessment (HIRA) Service evaluates the validity of insurance claims, and its collected claims data contain comprehensive information, including diagnoses and demographic details of insured patients. We determined the age-specific incidence of idiopathic scoliosis from 2011 to 2015 using HIRA data and Korean population statistics, as reported in our previous study.14)
Additionally, the Korea Children and Youth Panel Survey (KCYPS) was used to analyze factors related to the development of idiopathic scoliosis. In Korea, the National Youth Policy Institute has been conducting the KCYPS since 2010, and this survey aims to comprehensively understand the overall conditions and trends of children, adolescents, and young adults through a series of questionnaires. The sample schools were selected on the basis of the probability proportional to size sampling method, considering urban and rural areas. One class from each school was randomly chosen to participate in the survey. For participants aged 12 years or younger, telephone interviews were conducted with their parents by trained surveyors from a professional research agency. The survey included extensive questions regarding personal development and the developmental environment. In this study, the KCYPS 2010 cohort data were used. The dataset included responses from 4,693 students, comprising 2,342 first-grade elementary school students (age, 7 years) and 2,351 first-grade middle school students (age, 13 years), who were followed up and observed from 2010 to 2016.
Differences in Sleep and Environmental Factors between Urban and Rural Areas
In a previous large-scale study using Korean HIRA data, we analyzed the incidence of idiopathic scoliosis and found that the incidences in urban areas were 1.34 times higher in males and 1.22 times higher in females compared with those in rural areas.14) As a subsequent analysis, we investigated environmental factors contributing to the incidence differences of idiopathic scoliosis between urban and rural areas using the KCYPS data. Urban areas were defined as the capital city (Seoul), the metropolitan area surrounding the capital (Gyeonggi-do), and 6 metropolitan cities (Incheon, Daejeon, Gwangju, Daegu, Ulsan, and Busan), each with a population density exceeding 2,200 persons/km2. Rural areas included the remaining provinces, with population densities < 200 persons/km2. We used the survey results, including mean sleeping patterns and other environmental factors considered relevant to idiopathic scoliosis (e.g., physical development and lifestyle patterns) and examined the differences between urban and rural areas. Specifically, learning time refers to time spent on academic activities outside of school hours (e.g., homework and private tutoring). Indoor activity time refers to time spent on non-academic indoor activities (e.g., reading for leisure, playing video games, using smartphones or tablets, and watching television). Outdoor activity time refers to the time spent on non-academic outdoor activities (e.g., playing with friends and participating in sports).
Correlation between the Incidence of Idiopathic Scoliosis and Sleep Patterns
The incidence of idiopathic scoliosis was calculated by determining the number of patients diagnosed with idiopathic scoliosis (excluding congenital and neuromuscular scoliosis) per age-matched normal population, using data obtained from HIRA and the government statistics office.14) The age-specific incidence data of idiopathic scoliosis were merged with sleep and lifestyle pattern data from KCYPS 2010, including mean bedtime, total sleep time, learning time, indoor activity time, and outdoor activity time, according to age. Additionally, the change over time compared to the previous year was obtained for each variable. Correlation analysis was also performed to examine the relationship between the incidence of idiopathic scoliosis and sleep and lifestyle patterns.
Statistical Analysis
The independent 2-sample t-test was used to compare the differences between urban and rural areas across different age groups for each factor. The correlation between the age-specific incidence of idiopathic scoliosis and sleep and activity time was examined using Spearman’s correlation analysis. Pairwise deletion was used for missing responses. All statistical analyses were performed using SAS software version 9.4 (SAS Institute), and p-values < 0.05 were considered significant.
RESULTS
Differences in Sleep Patterns and Environmental Factors between Urban and Rural Areas
The proportion of multicultural families was slightly higher in rural than in urban areas; however, the difference was not significant (Table 1). Regarding the onset of nocturnal emissions and menarche, which may be related to the children’s physical developmental status, there was no significant difference between urban and rural areas (Table 2).
Table 1. Proportion of Multicultural Families in Urban and Rural Areas.
| Overall | Urban | Rural | p-value* | ||
|---|---|---|---|---|---|
| Multicultural family (age, 7–12 yr) | 0.375 | ||||
| Yes | 30 (1.49) | 15 (1.28) | 15 (1.77) | ||
| No | 1,986 (98.51) | 1,153 (98.72) | 833 (98.23) | ||
| Multicultural family (age, 13–19 yr) | 0.433 | ||||
| Yes | 18 (0.95) | 9 (0.80) | 9 (1.16) | ||
| No | 1,882 (99.05) | 1,113 (99.20) | 769 (98.84) | ||
Values are presented as number (%).
*Comparison between urban and rural areas.
Table 2. Age Distribution of Menarche and First Nocturnal Emission in Urban and Rural Areas.
| Age (yr) | Menarche | Nocturnal emission | ||||||
|---|---|---|---|---|---|---|---|---|
| Overall | Urban | Rural | p-value | Overall | Urban | Rural | p-value | |
| 7–9 | 3 (0.31) | 2 (0.35) | 1 (0.24) | 0.7176 | 5 (0.48) | 5 (0.94) | 0 | 0.1179 |
| 10 | 57 (5.85) | 36 (6.36) | 21 (5.13) | 12 (1.16) | 4 (0.75) | 8 (1.61) | ||
| 11 | 165 (16.92) | 102 (18.02) | 63 (15.40) | 33 (3.20) | 21 (3.93) | 12 (2.41) | ||
| 12 | 290 (29.74) | 167 (29.51) | 123 (30.07) | 85 (8.24) | 37 (6.93) | 48 (9.66) | ||
| 13 | 269 (27.59) | 147 (25.97) | 122 (29.83) | 222 (21.53) | 125 (23.41) | 97 (19.52) | ||
| 14 | 139 (14.26) | 84 (14.84) | 55 (13.45) | 260 (25.22) | 136 (25.47) | 124 (24.95) | ||
| 15 | 44 (4.51) | 25 (4.42) | 19 (4.65) | 153 (14.84) | 71 (13.30) | 82 (16.50) | ||
| 16 | 7 (0.72) | 3 (0.53) | 4 (0.98) | 53 (5.14) | 28 (5.24) | 25 (5.03) | ||
| 17 | 1 (0.10) | 0 | 1 (0.24) | 8 (0.78) | 4 (0.75) | 4 (0.80) | ||
| Not started yet | 0 | 0 | 0 | 200 (19.40) | 103 (19.29) | 97 (19.52) | ||
Values are presented as number (%).
Bedtime was earlier at almost all ages and showed a tendency to become later with increasing age in rural areas compared with urban areas. Total sleep time showed a decreasing pattern with increasing age and was longer at most ages in rural than in urban areas, with a particularly noticeable difference for those aged ≥ 13 years. Learning time was significantly longer at almost all ages in urban than in rural areas. Indoor activity time was longer and showed an increasing pattern for those aged 12 years in rural areas compared with urban areas; however, for those aged ≥ 13 years, indoor activity time showed a decreasing pattern with no clear difference between urban and rural areas. Outdoor activity time showed no consistent difference between urban and rural areas across all age groups and presented mixed results (Fig. 1).
Fig. 1. Differences in sleep and lifestyle patterns by age between urban and rural areas. (A) Bedtime. (B) Total sleep time. (C) Indoor activity time. (D) Outdoor activity time. (E) Learning time. *p < 0.05, significant difference between 2 groups.
Correlation between the Incidence of Idiopathic Scoliosis and Sleep Patterns
Spearman’s correlation analysis revealed a significant correlation of the age-specific incidence of idiopathic scoliosis with bedtime (p = 0.031) and total sleep time (p = 0.026) (Fig. 2). In the additional analysis of changes in bedtime and total sleep time, a significant correlation was observed between the incidence of idiopathic scoliosis and changes in total sleep time (p = 0.011) (Fig. 3).
Fig. 2. Correlation between the age-specific incidence of idiopathic scoliosis and factors related to sleep and lifestyle patterns. (A) Incidence of idiopathic scoliosis. (B) Bedtime. (C) Total sleep time. (D) Indoor activity time. (E) Outdoor activity time. (F) Learning time. (G) Spearman’s correlation analysis of the age-specific incidence of idiopathic scoliosis.
Fig. 3. Correlation between the age-specific incidence of idiopathic scoliosis and changes in bedtime and total sleep time. (A) Incidence of idiopathic scoliosis. (B) Changes in bedtime. (C) Changes in total sleep time. (D) Spearman’s correlation analysis of the age-specific incidence of idiopathic scoliosis. Asterisks indicate a similar double peak in the incidence of idiopathic scoliosis and changes in total sleep time.
DISCUSSION
To the best of our knowledge, this study is the first to explore the possibility that sleep patterns may be a factor in the development of idiopathic scoliosis. Our findings suggest that the differences in the incidence of idiopathic scoliosis between urban and rural areas may be attributed to variations in sleep and activity times. Moreover, we observed that bedtime and total sleep time were related to the development of idiopathic scoliosis.
We examined the developmental status and environmental differences between urban and rural students to explore potential underlying factors. The most common age of first nocturnal emission for male students was 14 years in urban and rural areas, with no significant difference between the groups. Similarly, the most common age of menarche for female students was 12 or 13 years in urban and rural areas, also showing no significant difference between the groups. These developmental milestones are important indicators of adolescent growth and development, with menarche particularly known to be closely related to the progression of idiopathic scoliosis.15,16) These findings suggest that the physical developmental status is comparable between urban and rural students; thus, the difference in scoliosis incidence is likely not influenced by these developmental factors.
South Korea is a homogeneous nation, but the number of immigrants has been steadily increasing over the past few decades. Considering the potential impact of ethnic and racial variety on the differences in idiopathic scoliosis incidence between urban and rural areas, we examined the distribution of multicultural families in these regions. Although the proportion of multicultural families was slightly higher in rural than in urban areas, the difference was not significant, with both regions having a rate of < 2%. This indicates that the ethnic and racial distribution in urban and rural areas is almost uniform. Numerous studies have shown differences in the incidence and prevalence of idiopathic scoliosis according to race and ethnicity.17,18) However, since this study was conducted in South Korea, a country with a homogeneous population compared with populations of other nations, we were able to exclude racial and ethnic influences. Furthermore, as there was no significant difference in the ethnic and racial distribution between urban and rural areas, we hypothesized that the differences in the incidence of idiopathic scoliosis between these areas are related to environmental factors other than racial or ethnic differences.
In the analysis of factors reflecting sleep and lifestyle patterns, we observed differences between urban and rural areas. Bedtime became later with age in both regions, but it was consistently later in urban than in rural areas across all age groups. Total sleep time was longer in rural than in urban areas for those aged ≥ 13 years, while learning time was longer in urban than in rural areas across all ages. Indoor activity time was longer in rural than in urban areas for those aged ≤ 13 years, whereas no consistent age-related differences were observed between urban and rural areas for outdoor activity time. Overall, this suggests that students in urban areas tend to go to bed later, have less total sleep time, and spend more time studying compared with their rural counterparts. It seems possible that the high enthusiasm for education in urban areas has influenced these results.
In the correlation analysis, among sleep and lifestyle pattern factors, bedtime and total sleep time showed a significant correlation with the incidence of idiopathic scoliosis. In the graph depicting changes in bedtime and total sleep time, the changes in total sleep time showed a significant correlation with the incidence of idiopathic scoliosis, displaying a similar double-peak pattern. Considering these results, we concluded that among the various lifestyle pattern factors, sleep time and patterns are closely related to the incidence of idiopathic scoliosis.
Among the various hypotheses regarding the causes of idiopathic scoliosis, the melatonin deficiency theory is noteworthy in relation to the results of this study. Melatonin is primarily synthesized and secreted by the pineal gland, with its synthesis and secretion being promoted in darkness and inhibited by light, playing various regulatory functions, e.g., circadian rhythm and skeletal growth.19,20) Since Dubousset et al.9) first reported scoliosis occurrence in pinealectomized chickens, numerous studies have reported that scoliosis could be induced in chickens through pinealectomy, suggesting melatonin deficiency as an etiologic factor of idiopathic scoliosis.10,21) In the bipedal rat model, melatonin deficiency also induced scoliosis;22,23) however, scoliosis did not appear in primates despite pinealectomy.24) This discrepancy is speculated to be due to various differences, such as melatonin receptor distribution, between lower animals and primates.19,24) Indeed, studies in human patients with idiopathic scoliosis have shown mixed results, with most studies finding no significant difference in serum or urine melatonin levels between the scoliosis and control groups.25,26) However, recent studies have reported differences in the expression of melatonin or melatonin receptors MT1 or MT2 in bone marrow-derived mesenchymal stem cells, chondrocytes, osteoblasts, and paraspinal muscles between patients with idiopathic scoliosis and the control group, suggesting that melatonin dysfunction may play an important role in the manifestation of idiopathic scoliosis.27,28,29)
In addition to melatonin, recent studies have suggested that sleeping position itself may also influence the development of adolescent idiopathic scoliosis (AIS). Grunstein and Grivas13) proposed a novel hypothesis that habitual asymmetric sleeping positions could result in uneven mechanical loading on the spine during sleep, potentially affecting spinal alignment over time. Similarly, Khadour et al.12) found significant associations between postural habits, including sleep posture, and scoliosis prevalence in adolescents. During sleep, the spine experiences relatively less mechanical loading compared to upright activities, providing an opportunity for "unloading." For adolescents who spend a significant amount of time in asymmetrical postures during the day, insufficient sleep duration may serve as a contributing factor in the development of AIS.
Differences in the prevalence of idiopathic scoliosis based on latitude have also been reported.30) Higher latitudes correlate with higher prevalence rates of idiopathic scoliosis, which is presumed to be influenced by differences in sunlight exposure and melatonin secretion according to latitude. Similarly, in our study, among various environmental factors, sleep-related factors, such as bedtime, total sleep time, and changes in total sleep time, were correlated with the prevalence of idiopathic scoliosis. The difference in prevalence between urban and rural areas is also believed to be influenced by differences in sleep-related factors. Given that melatonin is secreted during sleep, our study results are consistent with those of the aforementioned studies.
Our study has several limitations. First, the primary variables we assessed were based on surveys. Owing to the nature of survey-based assessments, the accuracy of the evaluations may be somewhat compromised. However, as our study involved surveying a large number of participants using a consistent method, we believe the impact of potential errors is minimal. Second, the study participants were Koreans. While this can be considered a general limitation due to the lack of multi-country and multi-ethnic research, we believe our study findings are more meaningful as we controlled for national and ethnic variables. Future research involving multi-ethnic participants will be necessary. Third, we did not directly measure melatonin levels or muscle mass, which have been reported as potentially important factors in the pathogenesis of idiopathic scoliosis. This limits our ability to validate the hypothesized physiological mechanisms underlying the observed associations. Future studies incorporating biochemical and morphometric assessments will be needed to further clarify these relationships.
In conclusion, later bedtimes and shorter sleep durations may potentially influence the development of idiopathic scoliosis. The higher incidence of idiopathic scoliosis in urban than in rural students could be attributed to these differences in sleep patterns. Therefore, attention should be given to excessive study hours and insufficient sleep time among adolescent students. Further research will be needed to elucidate the detailed mechanisms by which sleep affects the development of idiopathic scoliosis.
Footnotes
CONFLICT OF INTEREST: Kyung-Soo Suk is an editorial board member of the journal but was not involved in the peer reviewer selection, evaluation, or decision process of this article. No other potential conflicts of interest relevant to this article were reported.
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