Seasonal variation in human illumination exposure at two different latitudes

The authors measured ambient illumination exposure in healthy volunteers in San Diego, California (latitude 32 degrees 43' N, n = 30), and Rochester, Minnesota (latitude 44 degrees 1' N, n = 24), during each of the four quarters of the year, which were centered on the solstices and equinoxes. Subjects wore photosensors on their wrists and lapels (or foreheads while in bed) 24 h per day for an average of 5-6 days per quarter. The maximum of the two illumination readings was stored each minute. Annual average time spent per day in outdoor illumination (greater than or equal to 1000 lux) was significantly higher in San Diego than it was in Rochester (p <.04). Daily durations of illumination at or exceeding thresholds of 1, 10, 100, 1000, and 10,000 lux were highly seasonal in the sample as a whole (p <.01 at 1 lux, p <.0001 at other thresholds). Seasonal variation in outdoor illumination was far more pronounced in Rochester than it was in San Diego (interaction p <.001) but remained significant in San Diego (p less than or equal to.03). Seasonal variation in indoor illumination was generally similar in the two cities. The median Rochester subject experienced illumination greater than or equal to 1000 lux for 2 h 23 min per day during summer and 23 min per day during winter. The corresponding times in San Diego were 2 h 10 min and 1 h 20 min. Neither age nor gender predicted illumination duration at any level. Both season and geographic location strongly influenced human illumination exposure, and behavior (choice of indoor vs. outdoor environment) was the most important mediating factor.