Purpose of Review <p>Sleep is a critical determinant of physical and mental health, and substance use may affect sleep. Despite rising nicotine-cannabis co-use among adults, its impact on sleep remains poorly understood. This narrative review synthesizes recent evidence on nicotine-cannabis co-use and sleep-related outcomes in adults, highlighting patterns, methodological limitations, and future research directions.</p> Recent Findings <p>Although substantial literature has examined nicotine or cannabis use and sleep independently, few studies have evaluated their combined effects. Only six studies published between 2015 and 2025 examined nicotine-cannabis co-use and sleep outcomes in adults. Co-use definitions varied (e.g., concurrent or dual use) and included multiple forms of nicotine and cannabis use (e.g., vaping or smoking). Sleep outcomes were primarily assessed via subjective measures (self-reported scales), with only one study using an objective measure (actigraphy-derived rest-activity rhythms). Findings were mixed: three studies linked co-use to poorer sleep, including short or long sleep duration, sleep disturbances, and disrupted rest-activity rhythms, and three studies reported null associations, potentially reflecting differences in populations, sample sizes, co-use definitions, or heterogeneity in sleep assessments.</p> Summary <p>Nicotine-cannabis co-use is prevalent and may confer greater sleep-related risks than single-substance use, with nicotine may drive most effects. However, evidence from the past 10 years is limited by a small number of studies with heterogeneous co-use definitions, small or non-representative samples, reliance on self-report, and cross-sectional study designs. Future research should standardize co-use definitions, incorporate validated objective sleep measures (e.g., polysomnography), assess timing, frequency, dose and use longitudinal designs to clarify causal relationships.</p>