Measurements of Light at Night (LAN) for a Sample of Female School Teachers

Abstract

Epidemiological studies have shown an association between rotating shiftwork and breast cancer (BC) risk. Recently, light at night (LAN) measured by satellite photometry and by self-reports of bedroom brightness has been shown to be associated with BC risk, irrespective of shiftwork history. Importance has been placed on these associations because retinal light exposures at night can suppress the hormone melatonin and/or disrupt circadian entrainment to the local 24-h light-dark cycle. The present study examined whether it was valid to use satellite photometry and self-reports of brightness to characterize light, as it might stimulate the circadian system and thereby affect BC incidence. Calibrated photometric measurements were made at the bedroom windows and in the bedrooms of a sample of female school teachers, who worked regular dayshifts and lived in a variety of satellite-measured sky brightness categories. The light levels at both locations were usually very low and were independent of the amount of satellite-measured light. Calibrated photometric measurements were also obtained at the corneas of these female school teachers together with calibrated accelerometer measurements for seven consecutive days and evenings. Based upon these personal light exposure and activity measurements, the female teachers who participated in this study did not have disrupted light-dark cycles like those associated with rotating shiftworkers who do exhibit a higher risk for BC. Rather, this sample of female school teachers had 24-h light-dark and activity-rest patterns very much like those experienced by dayshift nurses examined in an earlier study who are not at an elevated risk of BC. No relationship was found between the amount of satellite-measured light levels and the 24-h light-dark patterns these women experienced. It was concluded from the present study that satellite photometry is unrelated to personal light exposures as they might affect melatonin suppression and/or circadian disruption. More generally, photometric devices calibrated in terms of the operational characteristics of the human circadian system must be used to meaningfully link LAN and BC incidence.

Personal light-at-night exposures and components of variability in two common shift work industries: uses and implications for future research

“…Median light levels of 38 lux between 00:00-05:00 hours were found in one Spanish study of workers from a variety of industries, including hospitals (25). In Canada, researchers reported a mean light intensity of 7.2 lux (SD 6.7) between 00:00-05:00 hours for nurses in one study (21) and a maximum of 37.2 lux for the same time period among nurses in a separate study (26). These results compare fairly well with the current study, where the average median exposure between 23:00-05:00 hours was 22 lux for nurses and 29 lux for hospital workers overall.…”

“…Personal measurements of photopic illuminance were collected using the Daysimeter (18), a small light monitoring device that has been validated (19) and used in prior studies to examine the impacts of light exposures on health (20)(21)(22). This device continuously records optical radiation from ambient light sources using an integrated circuit light sensor array (19,23).…”

Does Current Scientific Evidence Support a Link between Light at Night and Breast Cancer among Female Night-Shift Nurses? Review of Evidence and Implications for Occupational and Environmental Health Nurses

“…For a recent study, a sample of women wore Daysimeters, small, head-worn devices developed by the Lighting Research Center capable of recording photometric measurements at the cornea. After these measurements were compared to photometric measurements taken at women's bedroom windows, the researchers concluded that satellite photometry was unrelated to personal light exposures (Rea, Brons, & Figueiro, 2011). Research shows that wrist-level monitoring (Actiwatch-L) provides an adequate estimate of light exposure for in-hospital circadian studies (Jardim et al, 2011).…”

A Working Threshold for Acute Nocturnal Melatonin Suppression from “White” Light Sources used in Architectural Applications

“…Given our own concerns for health risk and our field measurements of actual light exposures in the home [40,41] we believe that a one-hour exposure to even 60 lux of "white" light is too high as a working threshold. Rather, given (a) the consistency between the empirical results presented here and the model predictions of CS (Figure 2), (b) the field measurements of actual light exposures [40,41], and (c) our own concerns for public health (i.e., a bias against misses), we concluded that our earlier proposal of 30 lux for 30 minutes from "white" light can still be used as a working threshold for lightinduced nocturnal melatonin suppression. This exposure level should limit, on average, nocturnal melatonin suppression to less than 5% (i.e., CS < 0.05), a value within the uncertainty of measuring melatonin concentrations using current radioimmunoassay methods.…”