Study design and participants
The cross-sectional National Health and Nutrition Examination Survey (NHANES) uses a complex, multistage, probability sampling methodology to estimate the health status of the United States population. These cross-sectional surveys collect data on diet, nutritional status, demographics, health and disease with nationally representative cohorts children and adults in the United States. All participants provided written informed consents, and all NHANES data is available in the public domain.
This analysis used data from 2007–2008 cycle of NHANES, because only this cycle contained a comprehensive Sleep Disorders questionnaire and question about a history of nephrolithiasis. First, we excluded participants who did not answer questions for the sleep questionnaire and a history of nephrolithiasis. Additionally, participants with missing information on a priori covariates adjusted for in the multivariate analyses were excluded from our analysis. Flowchart of the sample selection is outlined in Figure 1.
Sleep-Related Disorders
We investigated the following self-reported prevalent outcomes that are related to sleep disorders: sleep duration, Obstructive sleep apnea(OSA), sleep difficulty. These outcomes were defined in the following ways:
- Sleep duration: categorized as insufficient (< 7 hours/night) and sufficient (≥ 7 hours/night)[10].
- Obstructive sleep apnea (OSA): defined as any of the following: doctor diagnosed sleep apnea; or snoring 3 or more nights per week; or snorting, gasping or stopping breathing 3 or more nights per week; or (feeling excessively sleepy during the day 16–30 times per month despite sleeping around 7 or more hours per night on weekdays or work nights)[8].
- Sleep difficulty: have told a doctor or other health professional having trouble sleeping, or considered frequent if self-reported “often” or more(≥5 times/month) in response to any of the following questions from the NHANES sleep questionnaire: “Have you ever told a doctor or other health professional that you have trouble sleeping?”; “In the past month, how often did you have trouble falling asleep?”; “In the past month, how often did you wake up during the night and had trouble getting back to sleep?”; or, “In the past month, how often did you wake up too early in the morning and were unable to get back to sleep?”[11].
Kidney Stone
Our primary outcome was nephrolithiasis. This was a self-reported dichotomous variable from the kidney condition data file of the questionnaire component, stated as “Have you ever had a kidney stone?”. A similar study in the Nurses’ Health Study examined medical records from a random sample of ninety women who reported kidney stone. The records confirmed the diagnosis for all except 1 participant (98 %)[12].
Covariates
Potential covariates were identified based on previous studies[13, 14]. The following covariates were included: age, sex, BMI(under/normal weight: <25 kg/m2, overweight: 25 to 30 kg/m2, obesity: ≥30 kg/m2), race(Mexican–American, Other Hispanic, Non-Hispanic White, Non-Hispanic Black, Other Races), education level(less than high school, high school or equivalent, college or above), income (ratio of family income to poverty), diabetes(self-reported doctor-diagnosis or HbA1c≥6.5%), hypertension(self-reported doctor-diagnosis or systolic blood pressure≥140 mmHg or diastolic blood pressure≥90 mmHg), high cholesterol level(self-reported doctor-diagnosis or TC≥240 mg/dl), smoking (whether smoked at least 100 cigarettes in life), alcohol consumption (whether had at least 12 alcoholic drinks in one year) and recreational activities(Yes/No). For covariate for which >5% of survey participants were missing data, a separate level was coded for the missing values to retain these large samples in analyses. Details of all study variables acquisition processare available at www.cdc.gov/nchs/nhanes/.
Statistical analysis
A sample weight was assigned to each person participating in the NHANES. Hence all estimates were calculated accounting for NHANES sample weights. Statistical differences were determined using a weighted t test for continuous variables, while a weighted Chi-square test was used for categorical variables. We used multiple regression analysis to examine whether sleep-related disorders is associated with kidney stones. We estimated the crude model(model 1) as well as model 1(gender; age and race were adjusted). In the final model(model 2), we further adjusted for BMI; poverty income ratio; education level; diabetes; hypertension; high cholesterol level; alcohol; recreational activities and smoking. Data were analyzed with the use of the statistical packages R (The R Foundation; http://www.r-project.org; version 4.0.3) and Empower (R) (www.empowerstats.com, X&Y solutions, inc. Boston, Massachusetts). A P value less than 0.05 was accepted as indicating statistical significance.