Time-varying associations of temperature and sleep as a marker of heat adaptation: A case study of Spanish outdoor workers over one summer of continuous monitoring.

INTRODUCTION

Rising global temperatures are increasingly recognized as a threat to sleep health. Outdoor workers, who often endure prolonged exposure to outdoor heat, may also be impacted by indoor nighttime temperatures. However, few studies have examined both exposures in this population, and, little is known about potential adaptation to heat in relation to sleep outcomes.

METHODS

We conducted an intensive longitudinal study with 13 Spanish outdoor workers over a 3-month summer period, during which sleep outcomes (i.e., total sleep time [TST], sleep efficiency [SE], and subjective sleep quality) were measured daily via an activity monitor and ecological momentary assessments. Temperatures were monitored both indoors and outdoors, using bedroom sensors and weather station data. Linear mixed-effects models were used to quantify the daily associations between heat and sleep.

RESULTS

Across the whole summer, each 1 °C increase in outdoor and indoor temperature led respectively to decreases of 2.13 (95% CI: -3.84, -0.42) and 3.18 (95% CI: -5.67, -0.70) minutes in total sleep time (TST); 0.09 (95% CI: -0.17, -0.01) and 0.18 (95% CI: -0.30, -0.07) percentage points in sleep efficiency (SE); and 0.09 (95% CI: -0.14, -0.04) and 0.17 (95% CI: -0.24, -0.09) points in subjective sleep quality. Time-varying analyses revealed that the adverse effects of heat on subjective sleep quality were attenuated in August compared with June and July for similar temperatures.

DISCUSSION: We found that both indoor and outdoor temperatures were associated with poorer sleep outcomes during the whole summer, along with preliminary evidence of reduced heat-related effects on subjective sleep quality later in the summer.