Indoor heat and sleep quality: building and occupant modifiers in the HeatwiseSweden cohort

Background

Rising summer temperatures increasingly expose residents to indoor heat, with consequences for sleep and health. We investigated whether building and occupant characteristics modify the association between indoor heat exposure and sleep quality in a population-based cohort.

Methods

Data from 2,209 adults aged 30–85 in six Swedish cities were analyzed during summers 2024-2025. Participants’ bedroom temperatures and humidity were recorded at 15-minute intervals and summarized for afternoon, evening, and early morning exposure windows. Sleep quality was self-reported daily using a visual analogue scale (VAS 0–100) over three study weeks. A within-person case time series design with distributed lag non-linear models estimated window-specific associations, adjusting for humidity, day of week, and date. Effect modification by building type, ventilation system, bedroom window openability, and noise barriers to window opening was assessed.

Results

Participants contributed 50,735 sleep-quality observations averaging 78.9 VAS units (SD 17.4). Higher indoor temperatures were associated with non-linear declines in sleep quality that accelerated at higher temperatures, particularly during the evening window and consistently across both years. Per 1 °C increase in evening temperature (20-28 °C), average declines in sleep quality were greater in apartments vs small houses (−1.26 vs −0.39 VAS units), with non-openable vs openable bedroom windows (−1.86 vs −0.81), with extract air vs natural ventilation (−0.93 vs −0.42), and where nighttime noise limited vs permitted window opening (−0.96 vs −0.75). Modifier-specific estimates showed non-overlapping 95% confidence intervals.

Conclusions

Indoor heat-related sleep disruption appears to be amplified by housing and environmental conditions that limit passive nighttime cooling during the pre-sleep period, supporting the integration of building and behavioral factors into heat-health risk assessments and adaptation strategies.

Key messages

• Indoor heat impaired sleep most in apartments, homes with non-openable windows, and where nighttime noise restricted window opening, suggesting limited pre-sleep cooling increases heat vulnerability.

• Per 1 °C rise in evening temperature, sleep-quality declines were more than twofold greater in homes with non-openable bedroom windows, highlighting passive cooling as a heat-health adaptation issue.