// Excerpt and observasion from Dallas Hartwig – The 4 Season Solution – To understand our (lack of) light exposure further, we need to understand a couple of measurements: lumen and lux. Lumen is a measurement of light intensity (brightness) taken at the source of the light itself. As light travels away from its source, it scatters into the surrounding area and its intensity changes. Think about a bright LED flashlight shining directly into your eyes versus being fifty feet from it. Lux takes the lumens of a light source and factors in the area over which the light spreads, giving an indication of how bright, for example, a light source is in a particular room.
To give you some scale and perspective on lux readings, the light on a clear day in the summer can exceed 100,000 lux; on a dark and cloudy day in the same outdoor space, it can be as low as 1,000 lux. Full daylight but indirect sunlight can measure 10,000 lux. At night, with a full moon, it would be less than 1 lux. Sunrise or sunset on a clear day is around 400 lux. Now let’s compare these natural light scenarios to some typical artificial lighting. Bright office lighting comes in around 300 to 500 lux (comparable to sunset). An office hallway might be around 100 lux. A very brightly lit home living space might come in at a similar reading to the office but is more likely to be under 100 lux. That means that bright natural daylight is one hundred to one thousand times brighter than our typical indoor lighting. That’s a huge difference.
Linda Geddes, author of the book Chasing the Sun: The Astonishing Science of Sunlight and How to Survive in a 24/7 World, set about on an interesting light experiment in conjunction with sleep researchers from the University of Surrey (UK). Following the same argument that I make here, that our preindustrial ancestors lived and slept in tune with the light and dark cycles of the natural world, Geddes set about to live for four weeks with as little exposure to artificial light after sunset as practicably possible (in the context of having a career and family to manage). Part of her experiment involved measuring the intensity of her light exposure during the day.
On one particular morning, sitting in the park after dropping her children off at school, Geddes measured the light intensity at 73,000 lux. She took another reading at her desk once she arrived in her office, 120 lux. That is, the light in the environment she would be exposed to for much of the day was about one-fifth of the light intensity she might get immediately after sunset, and only a tinier fraction of what she would get if she were outside. Even moving to a desk closer to a window where it was sunnier, the light intensity was 720 lux—still over one hundred times less than her light exposure in the park earlier that morning.
Across the duration of Geddes’s four-week experiment, where she attempted to get more light exposure during the day, her average exposure between 7:30 a.m. and 6:00 p.m. was just under 400 lux in the first week of the experiment and as low as 180 lux in the second (but these were still increases from her preexperiment baseline of 128 lux). The experiment did take place in the middle of a UK winter when sunset occurred at 4:00 p.m. Nonetheless, the magnitude of the difference in light intensity between indoors and out is clear, being in the order of at least one hundred times less for the indoor environments, irrespective of the season.