As climate change progresses, it is causing more frequent and severe heat waves, resulting in higher indoor temperatures. Various temperature thresholds for indicating indoor overheating have been proposed in different contexts, extending from reduced comfort in buildings to subjective heat stress and onset of first or serious health problems. This study reviews these thresholds and identifies threshold values for subjective heat stress of occupants in the city of Augsburg, Germany, distinguishing between vulnerable and non-vulnerable households. Survey data from 427 private households are analysed using unpaired analysis of variances (ANOVA), t-tests and regression analysis to identify factors related to subjective heat stress at home during night-time. The findings imply that health implications during heat waves, age, local climate zones favouring the urban heat island effect and higher indoor temperature represent significant factors for subjective heat stress. A significant difference in subjective heat stress among different groups related to temperature could be identified for thresholds of 24.8 °C (people living alone) and 26.7 °C (people with chronic disease). As WHO threshold for health risk from overheating is 24 °C, people are apparently at heat-related risk without feeling that they are at risk, especially when they have chronic diseases; thus they may not see the urgency of taking adaptation measures.