Heat Stress in relation to Sleep Health among Farmers

While high temperatures are linked to poor sleep, few studies considered heat stress, especially among outdoor workers. We investigated associations between heat stress and sleep health among farmers. The study included 8,203 male participants from Iowa (78%) and North Carolina (NC, 22%), enrolled in the Agricultural Health Study (2013-2015), who were actively farming when data were collected. Survey data on sleep, demographics and covariates were linked to daily wet bulb globe temperatures (WBGT) from May 2013-September 2015. We calculated absolute heat stress by averaging WBGT over 2, 5, and 7 days before the interview. Relative heat stress (i.e., the difference between absolute heat stress and the 92.5th percentile of WBGT) was also calculated. WBGT was categorized by heat stress risk (low, moderate, high). Sleep measures included short nightly sleep duration (<7 hours), daytime sleepiness (≥3 days/week), daytime napping (yes), and long napping duration (≥30 minutes). Poisson regression with robust variance was used to estimate sociodemographic-adjusted prevalence ratios and 95% confidence intervals (PR[CI]) in Iowa and NC, separately. Farmers had a mean age of 63 years [SD=10.1]; 37.8% reported short sleep, 8.1% daytime sleepiness, 44.6% daytime napping, and 17.1% long naps. Mean absolute WBGT were 70.4°F (SD=6.36) in Iowa and 77.7°F (SD=7.83) in NC. In Iowa, moderate heat stress (2-day average) was associated with higher short sleep prevalence (PR= 1.04 [1.00-1.07]). In NC, higher absolute and relative WBGT (2-/5-/7-day average), as well as moderate (2-/7-day) and high (2-day) heat stress were associated with daytime napping (e.g., PR 2-day absolute WBGT = 1.02 [1.01-1.04]). In both states, high heat stress was associated with a lower prevalence of long naps (e.g., PR Iowa, 2-day heat stress = 0.86 [0.83-0.89]). Among farmers, heat stress was associated self-reported indicators of poor sleep. Future research with objective sleep measures is warranted. This study is among the first to determine associations between daytime heat stress exposure, measured by wet bulb globe temperature (WBGT), with both nighttime sleep and daytime napping among farmers who reside on farms and are actively engaged in farming. Our results suggest that heat stress may be related to short nighttime sleep duration and daytime napping but shorter nap duration. These findings have important implications for improving sleep health, which contributes to favorable outcomes such as injury prevention in agricultural workers.