Light exposure has a close link with numerous aspects of human physiology including circadian rhythm, mood disorders, metabolism, and eye growth. Here, a lightweight wrist-worn device was employed to continuously measure light exposure and activity across seasons and between refractive error groups to assess objectively measured differences and compare with subjectively reported data.
Subjects, aged 21–65 years (n = 55), wore an actigraph device (Actiwatch Spectrum) continuously for 14 days to quantify light exposure, activity, and sleep. Subjects were classified as emmetropic (n = 18) or myopic (n = 37), and answered an activity questionnaire. Additionally, devices were calibrated against a lux meter and UV sensor for indoor and outdoor settings, and used to measure ambient illumination in various environmental conditions.
Subjects spent 1:52 ± 0:56 hours outside per day, as measured objectively. Subjectively reported measures overestimated objective measures by 0:25 ± 1:19 hours per day (range −1:49 to +4:29 hours, P < .05). Subjects spent 1:04 hours more outdoors in summer and received an increased cumulative light dose compared to winter (P < .005). There were no significant differences in objective measurements of time outdoors between myopic and emmetropic subjects. Ambient illumination measures from the Actiwatch correlated with a lux meter for all locations tested (R2 = 0.99, P < .001). Ambient illumination was highest in the summer at 176,497 ± 20,310 lux and lowest for indoor artificial light at 142 ± 150 lux.
Subjects spent more time outdoors and received an increased light dose in summer, with no differences between refractive error groups in this adult population. Various environmental and seasonal measurements revealed significantly different available light in winter versus summer and indoors versus outdoors. Objective devices such as the Actiwatch can be valuable in studies where quantification of environmental factors is critical.