Light Exposure during Days with Night, Outdoor, and Indoor Work.
Level 3 - non-randomized controlled study
Prospective observational comparative cohort study
PubMed 30865270 · doi:10.1093/annweh/wxy110
What was done
Personal light recorders continuously recorded light exposure for 7 days across the year among indoor (n = 170), outdoor (n = 151), and night workers (n = 188) in Denmark (55–56°N). The study measured white light intensity, duration above 80, 1000, and 2500 lux, and the proportions of red, green, and blue light across time of day and season on work days and days off.
What was found
Indoor workers' average light exposure intermittently exceeded 1000 lux during daytime working hours in summer and never in winter. During daytime working hours, most outdoor workers exceeded 2500 lux in summer and 1000 lux in winter. Night workers spent on average 10–50 min >80 lux during night shifts. On days off, indoor and night workers experienced higher light intensities than on work days, with few differences between indoor, outdoor, and night workers. Spectral composition was similar across all worker groups on work days and days off.
Why it matters
This study provides objective measurement of occupational light deficits in indoor workers at northern latitudes and documents light levels during night shifts capable of biological disruption.
Limits
Geographically limited to Denmark (55–56°N), restricting generalizability to other latitudes. Clinical and biological outcomes—such as melatonin concentrations, sleep quality, and mood—were not directly measured and were inferred from physical light thresholds.
Cited by
- supports Indoor light is typically around 500 lux and rarely exceeds 1,000 lux, whereas bright direct outdoor sunlight can reach 200,000 lux and overcast daylight reaches at least 10,000 lux.