Thermal and Lighting Housing Environments and Circadian Blood Pressure Variability: Findings from the HEIJO-KYO Cohort

The purpose of this short review is to describe the influence of housing environment temperature and lighting on circadian blood pressure (BP) variability using data from the HEIJO-KYO cohort, a community-based cohort study launched in 2010. Increased excess mortality from cardiovascular disease in winter is a worldwide problem. Previous studies showed higher conventional BP and higher daytime ambulatory BP in winter; however, the relationship between indoor cold exposure and circadian BP variability remained unknown. In our cohort, we found a significant inverse relationship between indoor temperature and morning BP surge, independent of potential confounding factors. In addition, we found the tertile group with the lowest daytime indoor temperatures showed significantly higher urinary sodium excretion than the tertile group with the highest daytime indoor temperatures. Higher sodium intake caused by indoor cold exposure may partly explain the higher BP in winter. Physiologically, light exposure is the most important environmental cue for the circadian timing system and melatonin secretion. In our cohort, we observed that an increase in nighttime short-wave length light exposure and a decrease in daytime light exposure were significantly associated with lower melatonin secretion. Furthermore, lower melatonin levels were significantly related to higher nighttime BPs and parameters of atherosclerosis, which are predictors of cardiovascular disease incidence. Further longitudinal studies of the influence of housing environment temperature and lighting on cardiovascular disease incidence are required.