Urban heat poses significant challenges to public health, as exposure to high temperatures is associated to heat stress, resulting in heat strain, sleep deprivation, and cardiovascular morbidity and mortality. As the frequency of heat waves is increasing due to global warming, urban green spaces are often proposed as a nature-based solution to mitigate urban heat stress. This study investigated the impact of urban green space on perceived heat stress and sleep quality, using questionnaires and detailed land cover data. We surveyed 584 respondents during four heat and four control events in the summers of 2021 and 2022, assessing perceived heat stress, sleep quality, and mental health. Using structural equation models, this study analysed the influence of both tree cover and grass and shrub cover on perceived heat stress and sleep quality, while controlling for risk and vulnerability factors. The outcomes revealed that during heat events, enhanced tree cover was associated with reduced heat stress (B = -0.484, 95% CI [-0.693, -0.275], p = 0.001), while increased grass and shrub cover was associated with both reduced heat stress (B = -0.361 [-0.529, -0.193], p = 0.000) and improved sleep quality (B = -0. 241 [-0.399, -0.083], p = 0.003). Conversely, during control events, stress indicators were more strongly associated with individual vulnerability factors rather than surrounding green space. These results emphasize the importance of combining trees with lower vegetation in urban planning to mitigate heat-related stress and enhance sleep quality, thereby improving overall well-being during heat events.