For Earth's first 4 billion years of existence, light and dark followed a predictable 24-hour cycle. Across an ever-increasing amount of Earth's surface, that's no longer the case. With the advent of artificial lighting came the ability to transform night — inside buildings, under streetlights and neon signs, and in those vast areas where night's simply not so dark as it used to be.

For journalist Paul Bogard, author of The End of Night: Searching for Darkness in an Age of Artificial Light, one such place his family's lakeside camp in rural Minnesota. Thirty years ago the nights were pitch-black, the starscapes incandescent. Now there's a glow at the edge of the horizon, a growing dullness to the stars.

"That firsthand experience of a child, standing out on a dock and staring at the Milky Way, stays with you," said Bogard. "That's one of the biggest things we're losing, or have lost, for our kids. More and more people have no idea what it's like."

It's not only lost starscapes that he laments, but darkness itself. WIRED talked to Bogard about what this could mean for humanity's existential and even physical health.

WIRED: In the book, you argue that the loss of darkness isn't just a sentimental or aesthetic loss, but has health consequences. Can you explain?

Bogard: In terms of the physical effects, there's been a real explosion of research in the last 10 or 15 years on the subject of artificial light at night. Its effects on the human body fall into three areas: One, it affects our sleep. Sleep disorders are now tied to many major diseases. Secondly, it disrupts our circadian clock — the internal 24-hour rhythms that coordinate biological processes in our body, and are tuned to natural cycles of light and dark. Finally it impedes production of the hormone melatonin.

The lack of melatonin has been tied to increased risk of cancer, especially prostate and breast cancer. Researchers told me you can't say light at night gives you cancer, but it seems to be costing our health. A few years ago, the World Health Organization declared that the night shift is probably a carcinogen. Other studies are linking it to increased diabetes and cardiovascular disease risks.

WIRED: How much of that is a consequence of artificial light, and how much is just not sleeping enough?

Bogard: The changes caused by artificial light at night are not caused by exposure to starlight, candlelight or moonlight. If you wake up in the middle of the night and there's no other light, my understanding is that it doesn't have an effect on the production of melatonin. It's only artificial light.

One of the most interesting discoveries of the last few years is of cells in our retinas that aren't particularly related to sight, but are sensitive to changes in daylight and seasonal light. The wavelengths to which these are most responsive are the blue wavelengths — which makes sense, because the sky is blue, and blue light means waking up. And more and more light in our society is of these wavelengths.

The new LED lights, which are going in all over the place, tend to be really rich in blue-white light. It looks white to us, but is full of blue. Our gadgets — our notebooks, our tablets, our televisions — are very rich in this. So many people like to lie in bed and read their iPads or watch TV before going to bed. They're being drenched in blue light. The message it's sending our brain is, "Wake up!"

'The beauty we have on Earth is tremendous, and there's no other place to go. The night sky makes this clear.'WIRED: Are there effects on non-human animals?

Bogard: Tremendous effects, I think, and we're just starting to understand them. Life on Earth co-evolved with bright days and dark nights. We need both. So many species — roughly 60 percent of invertebrates and 30 percent of vertebrates — are nocturnal. Many more are active at dawn or dusk. These species have evolved to depend on darkness. When our artificial electric light invades their habitat, it could be as destructive as a bulldozer in daytime.

Research is only getting started on this. There's a 2006 book, Ecological Consequences of Artificial Night Lighting. There have been studies in the last few years showing how light at night affects bats, and another showing how new streetlights act as a vacuum for moths and other insects. They're drawn to the light, and initially there's a bonanza for predators, but they're either devoured or they become exhausted and die. It sucks the protein right out of the food chain.

WIRED: In the book, you make the point that we think streetlights make us safer, but that's not necessarily true. Is there research to support that?

Bogard: There isn't much research on the issue in general. People basically rely on what they think is common sense: that light makes us safer. But it's a complex issue. If some light makes us safer, it's not the case that more light makes us safer still.

A 1977 Department of Justice study concluded that sometimes light is good, and sometimes light is bad. There was also a study of alleys in Chicago a few years ago, where the researchers lit one alley brighter than another. There was no correlated rise in crime. There have also been reviews of the literature, such as one conducted by the Pacific Gas and Electric Company, California's electricity utility. They said there's no case that brighter light makes us safer.

Reproduced from ISTIL Report 2001 .

__WIRED: __Can people dim all this light in a meaningful way?

Bogard: I'm optimistic. I think there's lots of things we can do. We can have LEDs with a different spectrum of lighting. And we could shield our lights.

At home, we seldom have lamps that are unshielded. We have lampshades. Yet when you walk outside, there are bare bulbs all over the places, shining in all directions. It would make a significant difference, especially for astronomical light pollution, to shield those lights.

There is a difference, though, between astronomical light pollution and ecological light pollution. You can have shielded lighting that doesn't obscure the stars, but it's still shining downwards and has an effect there. So once you shield them, you use less-bright light, too.

France recently passed a nationwide ordinance requiring shops and offices to turn off their lights after 1am. It's motivated by saving energy and reducing carbon emissions, but I love that it also preserves the nocturnal environment and ecological health. And for our own health, we can also do things like sleep in the dark.

WIRED: Getting back to night skies and visible starscapes, what psychological impacts could their loss hold?

Bogard: When I was a guest on NPR's On Point program, somebody called in to talk about growing up in Oklahoma, and how as a teenager it was so valuable to drive out into the country and look up into the night sky and realize his problems weren't so big, that there was a vast universe out there beyond him. I love that image, and I think it's a really important one. When we can't see the sky, it's tempting to think we're the most important thing, that there isn't a universe out there that dwarfs us.

When you have that firsthand, it can make you feel small, but it can make you grateful for what we have here, too. You realize the beauty we have on Earth is tremendous, and there's no other place to go. The night sky makes this clear.