Study population
The data in this study were obtained from the National Health and Nutrition Examination Survey (NHANES) 2015–2018 cycle. NHANES is a nationwide cross-sectional survey in which participants are selected using a stratified multistage probability design, and it is thus representative of the US population. NHANES is unique because it combines interviews and physical examinations. The interviews included demographic, dietary, and health-related questions. The examination component consisted of medical, dental, and physiological measurements as well as laboratory tests administered by highly trained medical personnel [35] (more details via http://www.cdc.gov/nchs/nhanes.htm). Figure 1 shows the flowchart for selecting the study participants in the current analysis. Adults aged 18–64 years who participated in the 2015–2018 NHANES surveys and had complete data on sleep duration, e-cigarette use, and marijuana use were included in the analysis. The NCHS Research Ethics Review Board approved this study. The approval number is Continuation of Protocol #2011-17 for the 2015–2018 cycle. All the methods were carried out in accordance with relevant guidelines and regulation.
Study procedure
Sleep duration was assessed through household interviews using a computer-assisted personal interviewing system. Participants were asked, “How much sleep (in hours) do you usually get at night, on weekdays, or workdays?” According to the National Sleep Foundation, the recommended sleep duration for people aged 18–64 years was 7–9 h; thus, we categorized weekday sleep duration as <7, 7–9 h (reference group), and >9 h per night, which we defined as short, normal, and long sleep durations, respectively.
E-cigarette users were asked, “Have you ever used an e-cigarette, even once?” and “During the past 30 days, how many days did you use e-cigarettes?” Participants who had never used an e-cigarette even once were defined as “never users”; those who had used e-cigarettes before but had not them in the last 30 days were defined as “former users,” whereas those who had used e-cigarettes in the last 30 days were termed “current users.”
Marijuana users were defined using the questions: “Have you ever, even once, used marijuana or hashish?” and "How long has it been since you last used marijuana or hashish?” Participants who had not used marijuana or hashish even once were defined as “never users”; those who had used marijuana or hashish before but had not in the last 30 days were defined as “former users,” whereas those who had used marijuana or hashish in the last 30 days were termed “current users.”
Cigarette users were asked: “Have you smoked at least 100 cigarettes in your entire life?” and “Do you currently smoke cigarettes?”. Participants who had not smoked at least 100 cigarettes in their entire lives were “never users.” Participants who had smoked at least 100 cigarettes in their lives, but did not smoke now, were defined as “former users,” in contrast, those who still smoked were “current cigarette smokers.” Smoking status was categorized as “never users”, “former users” and “current users” of e-cigarette or traditional cigarette depends on the model.
“Dual use of e-cigarette and marijuana ” refers to participants who had used both e-cigarettes and marijuana in the last 30 days. Participants who did not use e-cigarettes or marijuana in the last 30 days were defined as “neither users”; “e-cigarette only users” were those who used e-cigarettes but not marijuana, whereas “marijuana only users” were participants who used marijuana but not e-cigarettes in the last 30 days.
Overweight/obese was defined when the body mass index was ≥25. Snoring or difficulty in breathing was a symptom of sleep disorders. Participants were asked, “In the past 12 months, how often did {you/SP} snort, gasp, or stop breathing while {you were/s/he was} asleep?” Individuals were categorized as “never,” “1–4 nights/week,” or “≥5 nights/week” according to the frequency of occurrence. Depression severity was calculated using the Physical Health Questionnaire (PHQ) score. PHQ score <10 classified as minimal to mild depression, PHQ score 10–15 as moderate depression, and PHQ score ≥15 as severe depression. Chronic diseases such as diabetes, cardiovascular diseases, and asthma also affected sleep duration and were considered [36-40].
Statistical analysis
The chi-square test and analysis of variance were used for analyses of binary and continuous variables, respectively. Multinomial logistic regression models were used for univariate and multivariate analyses of e-cigarettes, marijuana use, and sleep duration. All multivariable models were adjusted for age, sex, race/ethnicity (non-Hispanic white, non-Hispanic black, Mexican American, other Hispanic, and others), annual household income (<$20,000 vs. ≥$20,000), overweight/obese, serum cotinine level, cigarette use (current user and former user vs. never user), education (more than high school and high school graduate vs. less than high school), marital status (married/living together and divorced/separated vs. widowed/never married), the severity of depression (severe and moderate depression vs. minimal to mild depression), occupation (with a job vs. jobless/looking for a job), snoring or difficulty breathing (frequently, occasionally, and rarely vs. never), and chronic diseases (yes vs. no). A two-tailed P value <0.05 indicated a statistically significant difference. The R Program version 3.6.2 (The R Foundation for Statistical Computer, Vienna, Austria; www.r-project.org) was used for all analyses.