Title : Sleep as an early warning signal of heat-related climate vulnerability: A public health framework for urban surveillance

Background: Heat-health surveillance still relies predominantly on severe and late outcomes, including mortality, emergency visits and hospital admissions. However, before these outcomes occur, heat exposure may first become visible through disrupted sleep, nocturnal discomfort, fatigue and impaired daytime functioning. This is particularly relevant in dense urban settings, where high night-time temperatures, poor housing quality, urban heat islands, noise, overcrowding and limited access to cooling may converge.

Objective: To propose sleep disruption as an early warning signal of heat-related climate vulnerability and to outline an operational public health framework for incorporating sleep indicators into urban heat-health surveillance.

Methods: This conceptual paper integrates perspectives from sleep medicine, environmental epidemiology, urban health and public health surveillance. The proposed framework is organised into three levels: environmental exposure, sleep-mediated vulnerability and downstream health risk. Night-time heat, humidity, housing conditions, artificial light, noise, overcrowding and lack of cooling resources are defined as upstream determinants. Insomnia symptoms, nocturnal awakenings, non-restorative sleep, circadian disruption and daytime fatigue are interpreted as intermediate indicators of reduced adaptive capacity.

Results: The framework suggests that sleep disruption may identify vulnerable individuals and communities before severe heat-related outcomes become visible in conventional epidemiological systems. Older adults, people with chronic diseases, shift workers, low-income households and residents of dense urban areas may be especially susceptible. A minimum sleep-surveillance set should include perceived sleep quality, nocturnal awakenings, difficulty returning to sleep, non-restorative sleep and next-day fatigue during heat events. These indicators could be incorporated into heat-warning systems, primary care screening, occupational health programmes, community surveys and digital public health platforms. This approach would shift heat-health surveillance from a reactive model based on severe outcomes to a preventive model capable of detecting early physiological and social strain.

Conclusion: Sleep should not be treated only as a private symptom or individual lifestyle issue. In the context of climate change, disrupted sleep may function as a population-level signal of environmental exposure, social inequality and reduced adaptive capacity. Incorporating sleep into heat-health surveillance may strengthen early detection, risk stratification and equity-oriented climate adaptation, particularly in vulnerable urban communities.

Dr. Victor de Oliveira Liberale is a Brazilian physician with training in Internal Medicine and Sleep Medicine. His work focuses on sleep medicine, neurobehavioural and psychiatric aspects of sleep, public health, cardiovascular prevention and medical education. He has additional training in Psychiatry, Cognitive Behavioural Therapy, Audit and Compliance, and is expanding his academic background through studies in International Relations and International Law. He is currently involved in teaching and research activities in Sleep Medicine, with a particular focus on the interface between sleep, population health and social vulnerability.