These data demonstrate that dual use of e-cigarettes with conventional tobacco is associated with increased cough as well as increased sleep latency. Historically, conventional tobacco use is known to cause both cough and increased sleep latency^34,35. Dual users have been found to have higher levels of nicotine exposure relative to use of either inhalant alone¹⁰, and the increased time to fall asleep may be directly due to the activating effects of nicotine in the central nervous system (CNS). It is also possible, however, that the higher incidence of cough may alone result in the longer sleep latency found in this cohort³⁶. Our study did not find any significant associations between overall sleep quality, as measured by PSQI scores, and inhalant usage.

While previous studies have highlighted the associations of sole e-cigarette³⁷ or conventional tobacco¹¹ use and adverse sleep outcomes, there is little research on the impact of dual usage. For example, while Brett et al. found that e-cigarette use worsened sleep quality in young adults, the study excluded dual users³⁸. There are two studies that sought to address the impact of dual usage on sleep health; however, each had limitations such as they either focused on specific age groups, assessed only one type of inhalant use, or used generic questions (non-validated questionnaires) for evaluating sleep^39,40. Moreover, the articles also concluded that further investigation was required to properly assess the impact of e-cigarette or dual usage on sleep. As a result, our study contributes to the current literature by robustly assessing sleep quality using the validated PSQI questionnaire in all age groups.

Recognizing the altered sleep patterns in adult e-cigarette users, in particular dual users, raises concerns since impaired sleep or sleep disturbance has been associated with worse outcomes across disease states. Studies have found that sleep disruption adversely impacts outcomes in cardiovascular disease, cancer, and infectious disease⁴¹. Furthermore, increased sleep latency in dual users is concerning as difficulty falling asleep can be indicative of autonomic dysfunction³¹.

As dual users reported the highest use of drugs such as cocaine and methamphetamine relative to all other groups, the gateway hypothesis may be a possible explanation⁴². Since dual users have been found to have higher levels of nicotine exposure¹⁰, it is possible that dual use may create an increased priming effect for drug use and increased facilitation of stimulant addiction compared to sole inhalant use of tobacco or e-cigarettes. Another explanation may be that dual users believe that other drugs are either less risky or more pleasurable than e-cigarettes and conventional tobacco. Further studies are needed to explain the causal mechanism of dual use of e-cigarettes and conventional tobacco leading to other drug use.

Our study has several limitations. First, we recruited our study sample through social media platforms, which can cause convenience sampling bias and limit our conclusions on the population studied. Due to social-media recruitment, we were unable to calculate an accurate response rate since a denominator, or the number of people the survey was sent to, could not be identified. Also, survey research relies on self-reporting, such that some inaccuracy is likely, but misclassification should be random and biased towards the null hypothesis. Furthermore, the low number of observations from underrepresented minorities (Alaska Native, Pacific Islander, Middle Eastern, Hispanic, African American and Mixed ethnicities) means that the results of our study may not be applicable to these groups. Since our study design was cross-sectional, it is difficult to understand the temporal relationship between inhalant use and sleep. For instance, some inhalant users might utilize e-cigarette devices or conventional tobacco products as a coping mechanism when having trouble sleeping. Moreover, while IP addresses were recorded to prevent more than one response per device, the same participant could have used multiple IP addresses to increase their chance of winning the lottery. However, each survey response was reviewed thoroughly by the research team to assess for valid entries to each question.

Another limitation is that our survey did not include questions regarding comorbidities; consequently, significant associations found within the study regarding sleep latency or cough may be due this confounding variable. Our survey was also designed for English-based participants and was posted on American-based social media platforms. We posit that the international respondents who accessed and took the survey are likely to be English speaking. However, we still include this as a limitation to our results since our survey has not been validated for use in different countries and across cultures. Finally, while we hypothesized that vaping would lead to higher incidence of cough, which would impact sleep quality, our study was not sufficiently powered to perform formal mediation analyses. Mechanistic studies would be required to draw any rigorous causal inferences. However, we still view cough as an important patient-reported outcome even though our data fall short of allowing us to define definitive causal pathways.

While this study has several limitations, our findings validate those of our smaller pilot study in 2018, further suggesting that dual usage affects sleep latency⁹. Dual use was also associated with increased cough, such that cough may be the primary contributor to increased sleep latency. Finally, dual use was associated with the highest use of cocaine, marijuana, DMT and opiates across inhalant users. This work suggests that dual usage of e-cigarettes with conventional tobacco is associated with adverse outcomes on sleep, including increased amounts of time to fall asleep. Further studies are needed to determine the mechanisms underlying these findings.