Depression affects one out of five Americans and is poised to become the biggest source of worldwide disability. Why is depression reaching such epidemic proportions?

One important, and overlooked, factor: the dreadful effects of modern lighting conditions.

Our capacity for mood evolved in the context of a rotating Earth, with its predictable 24-hour cycle of light and dark phases. Our species is diurnal. As hunter-gatherers, we spent hundreds of thousands of years being active during the daylight hours. Why? Because the best chance of our finding sustenance and other rewards was in the light phase. Just try to find edible berries by moonlight!  As a result, we’re configured with a strong 24-hour biorhythm. This rhythm, driven by cues of light and dark, enables us to be alert during the day and sleepy at night.

About 10,000 years ago, we abandoned our nomadic hunter-gatherer ways and took to permanent dwellings. Village living, by itself, did not change our lightscape. But in the last 150 years, as we increasingly traded the outside lifestyle of the farmer for the inside lifestyle of the urbanite, we began to get less and less daylight.

Recent data show dramatic light deprivations, even in very sunny places. When small devices that measure light exposure and duration were attached to adults in San Diego, it was discovered that the average person received only 58 minutes of sunlight a day. What’s more, those San Diegans who received less light exposure during their daily routines reported more symptoms of depression. But what about the light we receive from light bulbs? It’s no substitute for the sun; artificial light is fainter and provides fewer mood benefits. Our newfound reliance on indoor light has effectively turned most people into cave dwellers.

Not only are we not getting enough light during the day, we’re getting too much at night. For this, we can blame Thomas Edison and, more recently, Steve Jobs. For millions of people, consumerelectronics—particularly laptops, smartphones and  iPads—are shining light into our eyes until just moments before we doze off. According to the National Sleep Foundation, more than 90 percent of Americans regularly use a computer or an electronic device of some kind in the hour before bed. The light exposure from light bulbs, TVs, iPads, and phone screens is enough to fool the brain, tricking our our 24-hour biological clock and delaying sleep. Plus, the games, shows, texts, and emails on these devices often provide intense stimulation just when we should be winding down. Is it a big surprise when survey data show that the average American sleeps 1.5 hours less now than in 1900?

Modern lighting conditions are out of sync with how our moods evolved. We’re getting light in all the wrong places and times. Rather than feeling alert during the day and sleepy at night, millions feel like the walking dead, insufficiently alert during the day, insufficiently tired at night.

It’s never wise to ignore eons of evolution. So we are paying the price for disregarding our natural cycles of light and dark. The cost: Millions are chronically experiencing low mood, which is not only an unpleasant feeling, but also a state that primes people for more serious depression. The wrong light sets the stage for darkness. For many, all it will take is one more blow from the environment—a  romantic breakup, a pink slip, or the death of a loved one—to set in motion a full downward spiral.