Study population and overall design
This cross-sectional study was embedded in the second wave of the Seven Northeastern Cities study between April 2012 and January 2013. The sampling strategy was developed as follows: a representative sample of Liaoning province located in Northeastern China was generated by randomly selecting half of the 14 cities in this province. One elementary school and one middle school was randomly chosen in 24 urban districts from the selected seven cities. From each grade level of the included schools, we invited students of one or two classrooms to participate in this study, who lived in the study area for at least 2 years before the start of this study. Finally, a total of 48,612 eligible children and adolescents aged 6 to 18 years have participated in this survey, and 45,562 of them have completed the assessments of sleep behaviors. This study was approved by the Ethical Review Committee for Biomedical Research, Sun Yat-sen University. We declared that we followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline for cross-sectional studies.
In each school, we organized face-to-face appointments for the teachers and the principals to introduce the aims, proposed methods and the procedures of our study. We aimed to incentivize permission for parents by using active communication techniques with the help of the teachers and the principals. We provided standard procedures for the school teachers. School teachers were required to explain the study aim, obtain the informed consent, and distribute the questionnaires in regular parent-teacher conferences. Parents could fill out the questionnaires during the conference or take it home and return it in a sealed envelope. Parents had the right to decline to consent and refuse to join the study.
Parent-reported sleep disturbance and related problems of their children
We asked parents to fill out the Sleep Disturbance Scale for Children (SDSC) to measure sleep quality in all children. The SDSC was a 26-item parent-rated scale developed by Oliviero Bruni in 1996 . Each item was rated on a 5-point Likert scale: 1 = never; 2 = occasionally (once or twice a month); 3 = sometimes (once or twice a week); 4 = often (three to five times a week) and 5 = always (six or seven times a week). The SDSC provides a total score of sleep disturbance and six domain scores including: (1) disorders of initiating and maintaining sleep (DIMS), such as sleep duration, sleep latency, night awakenings, and anxiety falling asleep; (2) sleep breathing disorders (SBD), such as snoring and breathing problems; (3) disorders of arousal (DA), such as sleepwalking, sleep terrors, and nightmares; (4) sleep–wake transition disorders (SWTD), such as rhythmic movements, hypnic jerks, sleep talking, and bruxism; (5) disorders of excessive somnolence (DOES), such as difficulty waking up, morning tiredness, and inappropriate napping; and (6) sleep hyperhidrosis (SHY), such as nocturnal sweating. The total score and the subscale scores can then be converted to a T-score so that we can compare them among children in different age groups. We also used validated cut-offs to yield proxies for sleep disturbance and increased problems in the six domains within the clinical range (i.e., T-scores ≥70). In addition, both sleep duration and sleep latency could be derived based on two of the items in the SDSC. We defined inappropriate sleep duration (i.e., < 7 h) and sleep latency (i.e., > 45 min) according to the international consensus recommendations [13, 14].
We have already conducted a validation study to revise the Chinese version of SDSC in the first wave of the Seven Northeastern Cities study, and it is reliable in screening parent-reported sleep problems in Chinese children (Cronbach’s α = 0.81). The detail of the study is described elsewhere .
Assessment of SHS exposure
We collected information on SHS exposure via questionnaires. We defined having exposure to early-life SHS based on an affirmative answer to the two questions: (1) Did anyone who lived with the mother during her pregnancy smoke anywhere inside the house? and (2) Did anyone who lived with the child during his or her first 2 years smoke anywhere inside the house? Therefore, exposure to early-life SHS was a category variable encoded as (1) unexposed; (2) ever exposed during pregnancy or the first 2 years of life; and (3) ever exposed during both pregnancy and the first 2 years of life.
We also collected information on the current number of cigarettes smoked inside the house per day during weekdays and weekends by all family members who lived with the child, and therefore we defined having current SHS exposure if any family member who lived with the child smoked cigarettes.
We conducted data analyses from April 32,021 to May 3, 2021. We calculated means and standard deviations for continuous variables and percentages for categorical variables. The differences across different SHS exposure groups were determined using ANOVA test for continuous variables and chi-square tests for categorical variables.
We analyzed the associations of exposure to early-life SHS with sleep problems in children by fitting generalized linear mixed models with an identity link (continuous outcomes with gaussian distribution) or logit link (binary outcomes with binomial distribution) function. We fitted crude models with school as random intercept. We fitted adjusted models for each outcome, adjusting for covariates collected from questionnaires (child age, sex, only child, preterm birth and low birth weight, parental educational levels, yearly household income, maternal age during pregnancy, maternal smoking and alcohol consumption during pregnancy). We conducted stratified analyses by sex and by child age, and whether the associations varied with sex or child age were assessed from the heterogeneity of effect across strata and the significance of interaction terms.
To verify the robustness of the results, sensitivity analyses were performed: (1) we examined the associations by excluding children with parent-reported asthma (n = 2257, 4.95%) since previous studies have found strong associations between SHS exposure and sleep problems in asthmatic children ; (2) we grouped SHS exposure into more detailed categories of exposure by considering pregnancy and the first 2 years of life separately and re-analyzed the data; (3) we used current SHS exposure derived from the questionnaires to confirm the associations between current SHS exposure and simultaneous sleep problems since most of previous studies have confirmed the above association.
Statistical analyses were conducted with the statistical software R 4.0.3 (R Core Team 2020). We presented the results as estimates (β) and odds ratios (OR) with the 95% confidence interval (CI). A P value < 0.05 for two-sided test was considered statistically significant.