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American Journal of Epidemiology Advance Access originally published online on August 13, 2007
American Journal of Epidemiology 2007 166(9):1053-1058; doi:10.1093/aje/kwm175
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American Journal of Epidemiology © The Author 2007. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

ORIGINAL CONTRIBUTIONS

Short Duration of Sleep and Unintentional Injuries among Adolescents in China

Lawrence T. Lam1 and L. Yang2

1 Royal Alexandra Hospital for Children, Westmead, New South Wales, Australia
2 School of Public Health, Guangxi Medical University, Nanning, Guangxi, China

Correspondence to Lawrence T. Lam, Royal Alexandra Hospital for Children, Locked Bag 4001, Westmead, New South Wales 2145, Australia (e-mail: lawrencl{at}chw.edu.au).

Received for publication March 27, 2007. Accepted for publication May 15, 2007.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Using a population-based cross-sectional health survey, the authors investigated the association between nightly duration of sleep and unintentional injuries among high school students in Nanning, China. The survey utilized a two-stage random cluster-sampling design. In March 2005, adolescents aged 13–17 years were recruited from students attending the first 3 years of high school in Nanning. Sleep duration was measured by self-reported usual times of going to bed and rising during a normal school week. Unintentional injury was assessed via a structured personal interview. Data were analyzed using multinomial logistic regression with adjustment for the effects of cluster sampling. After adjustment for potentially confounding factors, adolescents who slept less than 7 hours per night during a normal school week were approximately two times more likely to have experienced multiple episodes of unintentional injury during the 3-month presurvey period (odds ratio = 2.2, 95% confidence interval: 1.1, 4.8) than those who slept 7 hours or more (p < 0.05). There was also a nonsignificantly (p > 0.05) increased risk of single injury for adolescents with short sleep durations (odds ratio = 1.5, 95% confidence interval: 0.9, 2.3). Findings suggest that a short nightly duration of sleep can be considered a potential risk factor for multiple unintentional injuries among adolescents.

adolescent; China; health surveys; sleep; wounds and injuries

Abbreviations: CI, confidence interval; OR, odds ratio


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Injury has long been identified as a major cause of mortality in most developed countries. It has also been recognized as an emerging health problem in developing countries, including the People's Republic of China (1–4).

Nightly duration of sleep as a health issue has not received much attention until recently. There is a growing body of evidence for an association between shorter nightly duration of sleep and major health problems, including all-cause mortality, breast cancer, diabetes mellitus, Parkinson's disease, hypertension, and obesity (5–14). For example, among subjects in a longitudinal US study (the First National Health and Nutrition Examination Survey), self-reported short duration of sleep was significantly associated with an increased risk of hypertension (11).

In terms of the relation between sleep duration and injury, few studies have been conducted. In a recent prospective study, Choi et al. (15) reported that sleeping for less than 7.5 hours per night increased the risk of injury by 61 percent (relative risk = 1.61, 95 percent confidence interval (CI): 1.21, 2.15) among rural adults in Iowa when compared with persons who slept longer. In a case-crossover analysis, Valent et al. (16) employed a study period of 48 hours for each injured pediatric patient, with 24 hours before presentation used as the case period and 25–48 hours used as the control period. They found that children slept longer during the control period than during the case period. There was also a significant association between injury risk and sleeping less than 10 hours per night among boys (relative risk = 2.33, 95 percent CI: 1.07, 5.09), though not among girls (16).

While the sleep patterns of these adults and young children were rather stable, adolescents' sleep behavior may differ. To our knowledge, so far no study has evaluated the relation between nightly duration of sleep and health problems, especially unintentional injuries, among adolescents. Hence, our aim in this study was to examine that relation.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
In March 2005, a population-based health survey utilizing a two-stage random cluster-sampling design was conducted in Nanning, a city in Guangxi Province in southwestern China. The sampling frame of the study was the total adolescent population attending the first 3 years of high school (ages 13–17 years) in Nanning, accounting for more than 60,000 young people. Students attending special schools due to physical or mental disabilities were excluded from the study. Sampling of students consisted of a two-stage process with stratification done according to year of high school, using individual schools and classes as the primary and secondary sampling units. Institutional ethical approval for the study was granted by the Ethics Committee of Guangxi Medical University. All parents agreed to their children's participation in the study.

All high schools in Nanning City are government schools overseen by the Education Department of Guangxi Province. Health care is provided for all students by a school physician. Hence, health information on the students could be obtained directly from the resident school physician. Through the Guangxi Education Department, all resident physicians in the selected schools received standardized training in administering the structured health interview prior to the study. All selected students were referred to their school physician for a brief medical examination, anthropometric measurements, and a structured health interview. They also filled in a health survey questionnaire that asked for information on other variables, including nightly duration of sleep, snoring, weekly duration of physical activity, and demographic factors. Students were asked to indicate the usual time at which they went to bed and the usual time at which they arose on normal weekdays. Duration of sleep on normal weekdays was calculated from these times. The body mass index (weight (kg)/height (m)2) of each student was calculated from his/her body weight and height.

Injuries were assessed during the medical examination and the structured health interview. Students were asked to indicate whether they had experienced any injuries in the past 3 months. For any injuries reported, students were further questioned about the details of each incident, including place of injury and the kind of activity the student was involved in while injured. This information was then cross-checked with medical records for verification, or for updating if the incident(s) had not been reported to the school health-care center. Of particular interest was any episode of unintentional injury that had occurred. Unintentional injuries were defined as any injuries that were not deliberately caused by the student or another person. Multiple episodes of injury were defined as having more than one injury incident within the 3-month period prior to the survey, whereas a single episode of injury was defined as having only one incident.

Data were analyzed using the Stata statistical software program (17). Since the study had a cluster-sampling design, data were set up using the survey design function, utilizing the svy commands for handling the cluster-sampling effect. For the outcome variable, the number of injury episodes was categorized as none, a single episode, or multiple episodes. Nightly duration of sleep was categorized as short (<7 hours) versus longer (≥7 hours). We conducted bivariate analyses to examine unadjusted relations between all variables of interest and the outcome variable. Unadjusted odds ratios for the exposure variable and potentially confounding factors were calculated for each category of the outcome variable using multinomial logistic regression. In order to identify potentially confounding variables to be included in the multinomial logistic regression analyses, we applied chi-squared tests to examine the bivariate relations between the exposure variable, demographic variables, and the outcome. All statistical tests of the study hypothesis were adjusted for the cluster-sampling design. Because there were multiple categories for the outcome variable, we further analyzed the data via multinomial logistic regression modeling, using the no-injury group as the referent group to examine the relation between duration of sleep and injury risk after adjustment for potentially confounding factors identified in the bivariate analyses. For a variable to be included in the regression model, it had to have a p value of 0.20 or less for a bivariate association or be a known risk factor for unintentional injury. All p values were two-sided.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
A cluster random sample was obtained from nine different schools and 24 different classes, resulting in a total of 1,429 students. The characteristics of these students are summarized in table 1. One hundred and sixty-three students (10.7 percent, adjusted for cluster-sampling effects) reported getting less than 7 hours of sleep per night during a normal school week. The mean duration of sleep was 7.6 hours per night (standard error, 0.1), with a median of 8.0 hours. In terms of unintentional injuries, 336 students reported having experienced an injury during the 3-month period prior to the survey. Of these students, approximately 6 percent (adjusted for cluster sampling; n = 85) had had more than one episode of injury and approximately 17 percent (adjusted for cluster sampling; n = 251) had had a single episode. There were 455 episodes of injury, with 130 falls, 113 sprains and strains, and 110 collisions. The remaining injuries were mainly minor burns, cuts, and crush injuries.


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  		TABLE 1. Unadjusted odds ratios for unintentional injury according to nightly duration of sleep and other variables among adolescents (n = 1,429) in Nanning, China, March 2005

 
The bivariate relations between duration of sleep, other variables, and unintentional injuries were examined. The results are summarized in table 1. There was a significant unadjusted association between nightly duration of sleep and injury, particularly with multiple injuries (unadjusted odds ratio (OR) = 2.4, 95 percent CI: 1.2, 4.9). The results also suggested that there was a slight increase in the risk of single injury (OR = 1.4, 95 percent CI: 0.8, 2.4). However, the unadjusted association was not significant at the 5 percent level. In terms of other variables, significant associations were found between unintentional injury and sex, father's educational level, and family income (table 1). These variables and others that satisfied the above-mentioned criteria were considered as potentially confounding factors to be included in the multinomial logistic regression analyses. Because of a strong correlation between father's educational level and mother's educational level, which might have posed a collinearity problem in the regression analyses, we decided to retain the more significant variable, namely father's educational level.

To further examine the potentially confounding nature of these selected variables, we investigated bivariate associations between these variables and duration of sleep. Results are summarized in table 2. Involvement in physical activity for 6 or more hours per week and potential sleep problems, as indicated by snoring, were known risk factors for injury. These variables were included in the next phase of the analyses.


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  		TABLE 2. Distributions of potentially confounding variables according to nightly duration of sleep and related unadjusted odds ratios for unintentional injury among adolescents (n = 1,429) in Nanning, China, March 2005

 
Results obtained from the multinomial logistic regression analyses are presented in table 3. After adjustment for potential confounders, the association between nightly duration of sleep and multiple unintentional injuries remained significant (p < 0.05). After adjustment for potentially confounding factors, the results suggested that adolescents who slept less than 7 hours per night during a normal school week were approximately two times more likely to have experienced multiple episodes of unintentional injury during the 3 months prior to the survey (OR = 2.2, 95% CI: 1.1, 4.8) than those who slept 7 hours or more. The results also suggested an increased risk of single-episode injury for those who had shorter durations of sleep. After adjustment for the same variables, there was an increase of 50 percent in the risk of single-episode injury for short-duration sleepers (<7 hours/night) in comparison with longer-duration sleepers (≥7 hours/night) (OR = 1.5, 95 percent CI: 0.9, 2.3). However, the adjusted association between short duration of sleep and single-episode injury was insignificant at the 5 percent level.


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  		TABLE 3. Adjusted odds ratios for unintentional injury obtained from multinomial logistic regression analysis among adolescents (n = 1,429) in Nanning, China, March 2005

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
In this study, we aimed to examine the relation between nightly duration of sleep and all types of unintentional injuries among adolescents in China. After adjustment for potential confounders, the risk of multiple injuries was increased more than twofold among adolescents who slept less than 7 hours per night as compared with those who slept longer. The results obtained from this study are consistent with the findings of two previous studies (15, 16).

A possible explanation for the association is that long-term inadequate sleep may lead to an accumulation of sleep debt and, in turn, both subjective and objective sleepiness (18–20). It has been well established that sleepiness and fatigue are major risk factors for injury, particularly motor vehicle crash-related mortality and morbidity (21–24). Hence, a shorter duration of sleep can potentially be considered a primary risk factor for injury. Another possible explanation is the effect of sleep debt on the physiologic rhythms of the body and cognitive functioning. It has been reported that sleep debt manifests a physiologic response that mimics the state of depression (25). Furthermore, sleep deprivation has been shown to impair the waking neurobehavioral functioning of healthy adults and to cause a significant deficit in cognitive performance in experimental tasks (26). Because of these deficits, it is easy to understand why short-sleeping adolescents are more likely to be injured.

Sleep duration is a potentially modifiable risk factor. Adequate sleep is positively associated with health status and many health-related behaviors among adolescents, such as adherence to a healthy diet, effective stress management, and regular exercise (27). Hence, getting adequate sleep can be considered part of a cluster of healthy adolescent behaviors that are modifiable and can be implemented via proper health education. Early identification of sleep problems, including short duration of sleep, can provide opportunities for early treatment and preventive counseling and thus prevent potential injuries and their consequences.

This study had several strengths. Data analyses took into consideration the effect of the cluster sampling. The random sampling used provided confidence that the sample was representative of the population under study. The use of a standard protocol and training for all persons involved in data collection further minimized biases. However, some potential limitations have been identified. First, there may have been reporting bias in the outcome variable due to self-reporting, although information on injuries was cross-checked with medical records. Minor episodes of injury may not have been reported to school health centers. Second, recall bias may have occurred in the assessment of sleep duration, also because of self-reporting. This may have been more severe with variables such as snoring, where the information came from a third party. Finally, the evidence provided by a study with a cross-sectional design is not strong enough for drawing any causal inferences (28). In order to confirm the relation between short duration of sleep and risk of injury, a prospective cohort study should be conducted.


   ACKNOWLEDGMENTS
 
The authors acknowledge Yanyan Zheng, Chong Ruan, and Zhenlian Lei for their assistance.

Conflict of interest: none declared.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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