A reader asks:

I am looking into light therapy, not so much because the days are short, but because I work in a place with no windows. It is light out (and usually sunny) when I get up in the morning, so I am considering simply going outside in the mornings. How does 10,000 lux for 30 minutes compare to exposure to outdoor light? In other words, how much time in sunlight would be equivalent? What about a cloudy day?


Your suggested strategy — an outdoor walk before work — makes a lot of sense, but there is no simple way to know how much is equivalent to light therapy. There are some relevant observations, however. 10,000 lux is measured on a horizontal surface of the earth — from global clear skylight illumination (i.e., not directional from the sun) — about 40 minutes after sunrise. In one case study, where a volunteer attached a light meter/data logger to a sweatband over his forehead, he received far more light from 30 minutes of light therapy than he ever received outdoors. Almost surely, that is because people don’t look up at the sky consistently. It may be possible to relieve the daylight deprivation of windowless indoor environments using a light box designed for work stations, with a variable intensity control. An example is the DayLight XL (see the CET Store page).