This scoping review identified three articles examining the relationship between EHEs, sleep, and CV health. These papers, while not elucidating a causal mechanism linking EHEs to worsening CV measures or CVD, examined associations between these three factors.

Importantly, our review highlighted several current research gaps linking EHEs, sleep health, and CVD health. First, the studies reviewed suggest but do not rigorously examine causal links between heat, sleep, and CVD through physiological mechanisms. Second, while duration and quality of sleep were examined, future research could evaluate multiple dimensions of sleep health and their relationship to EHEs and CVD. Sleep health is a multidimensional process that constitutes important subjective and objective measures [27]. Utilizing these multiple measures, as well as alternative definitions of EHEs (including absolute measures and relative measures) that both represent different aspects of thermal comfort [2], is critical to understanding the relationship between EHEs, sleep health, and CVD. Third, studies lacked the inclusion of participants from at-risk populations including low-income individuals or other socioeconomically vulnerable groups prone to both CVD risk and heat-related illness. Research including these populations could offer perspective on ways to aid those most vulnerable to harm in a warming climate.

Furthermore, the existing studies were limited in their geographic reach and lacked standardization of key terminology such as the EHE definition. Given that all of the studies reviewed were conducted in East Asia, our analysis underscores the need for a broader geographic representation in research on this subject. Further analyses may show important regional variation: given that the impacts and manifestations of climate change are so variable, associations between EHEs, sleep, and CVD should be investigated in different settings to inform interventions and policy, and research conducted in specific countries may find an association between EHEs, sleep, and CVD more likely, particularly where CVD outcomes are more prevalent [28]. The need for a uniform characterization of what constitutes an EHE or heat wave is also important in order to promote coherence and comparability between studies [2]. This would facilitate a more nuanced understanding of the broader implications of EHEs on health on a global scale and allow comparisons across regions and countries. Without such standardization, the heterogeneity in EHE definitions across studies complicates direct comparisons of findings, potentially hindering the development of a cohesive research approach. This inconsistency challenges efforts to draw generalized conclusions about EHEs’ health impacts and may impede the formulation of effective, evidence-based strategies for mitigating CVD risks associated with extreme heat.

Interestingly, the role of air-conditioning (AC) as a potential confounder in the relationship between EHEs, sleep health, and CVD is likely an area for further study. In the studies conducted by Huang [24] and Yan [26], a significant emphasis is placed on the role of interior room temperature, positing it as a more important critical determinant of health outcomes than external temperature. Their findings suggest that regulating indoor temperatures may enhance both sleep quality and CV health, potentially mitigating some of the adverse effects of EHEs. Notably, Huang’s research delineates that daytime exposure to heat does not suffice to induce elevated SBP, suggesting that nocturnal temperature levels particularly may play a pivotal role — an area of future research [29].

Although we conducted a comprehensive review, the final number of results is very small. We argue that this underscores the early stage of research in this area, highlighting the need for more extensive and in-depth studies to build a robust body of evidence that seeks to understand the physiological mechanisms linking the impact of sleep health on CVD through EHEs. Only then can potential interventions be used to address the clear association between CVD and heat-related health risk, not just today but into the future as EHEs become more frequent.

We describe two limitations to our work. First, we did not include non-English literature and therefore may have missed important peer-reviewed manuscripts. This is particularly noteworthy because of the three studies that met inclusion criteria, and all were from countries outside the English-speaking regions. Second, we did not consider gray literature. However, given the nuanced nature of our research question and the conceptual framework we examined (Fig. 1), the peer-reviewed literature is the most likely source of empirical studies on this topic.