Relationship Between Wearable-based Measures Of Physical Activity, Sedentary Behavior And Light Exposure

PURPOSE: Wearables have created new opportunities to objectively quantify digital biomarkers related to the duration and intensity of both physical activity and daily light exposure—both of which are important indicators of circadian health. The purpose of this analysis is to describe the relationship among these exposures, which is not well understood. METHODS: Study participants (N = 39, 74% female, age (mean[SD]) 46.4 [10.1] y, BMI 25.2 [4.1] kg/m2) completed a 10-day study wearing an accelerometer on their non-dominant wrist and a light sensor on their outer clothing. We estimated time spent in sedentary (<40 mg) and moderate-vigorous physical activity (MVPA: >125 mg, ≥1-min bouts) using GGIR (v2.8.2). Active days were categorized as days with ≥30 minutes of MVPA. Daily light exposure was categorized as dark (<10 lux), indoor (10-1000 lux) and outdoor (>1000 lux). We used Pearson correlations to examine associations between daily time spent in sedentary behavior, MVPA, light exposure categories and average light exposure (lux/day). We also tested for differences in light exposure duration on days where participants were active vs. less active using linear mixed models. RESULTS: Average time spent in MVPA was (mean[SD]) 21.8 1(5.1) min/day, and sedentary 11.2 (2.3) hrs/day. Mean overall light exposure was 735.7 (598.6) Lux, with variability among participants in time spent across light categories: indoor (8.8 [2.9] hrs/day), outdoor, (2.1 [1.5] hrs/day), and dark (11.3 [2.8] hrs/day). Participants spent an additional 60 min/day exposed to outdoor light on active vs. less active days (174.8 vs 112.5 min/day), and had higher daily average light exposure (1059.3 vs. 639 lux; p < 0.05). There were no statistically significant differences on active vs. less active days in time spent in indoor light or darkness (p > 0.05). Time spent in MVPA and average daily light exposure had a correlation of r = 0.33. Sedentary time was positively correlated with indoor light (r = 0.22), and negatively correlated with outdoor light (r = -0.22) and average daily light exposure (r = -0.26), but not time spent in darkness (r = 0.11). CONCLUSIONS: Accumulating 30 min/day or more of MVPA was associated with greater exposure to bright daylight outdoors - an important contributor to circadian health. Supported by NIH/NCI contract 75N91019C00001