Modern life and inadequate exposure to natural light during the day combined with overexposure to artificial light at night is believed by some to be harmful to the body's natural sleep/wake cycle.

One of those who contends so is University of Connecticut cancer epidemiologist Richard Stevens. "It's become clear that typical lighting is affecting our physiology. But lighting can be improved. We're learning that better lighting can reduce these physiological effects. By that we mean dimmer and longer wavelengths in the evening, and avoiding the bright blue of e-readers, tablets and smart phones."

Those devices emit enough blue light when used in the evening to suppress the sleep-inducing hormone melatonin and disrupt the body's circadian rhythm, the biological mechanism that enables restful sleep, say Stevens and co-author Yong Zhu from Yale University, who outline the known short-term and suspected impacts of circadian disruption in Philosophical Transactions of the Royal Society B.


 In 2007, Stevens was on an International Agency on Cancer Research committee that promoted the idea that shift work is a "probable carcinogen".  As smartphones and tablets become more commonplace, Stevens recommends a general awareness of how the type of light emitted from these devices affects our biology. He says a recent study comparing people who used e-readers to those who read old-fashioned books in the evening showed a clear difference - the e-readers showed delayed melatonin onset.

"It's a new analysis and synthesis of what we know up to now on the effect of lighting on our health," Stevens says. "We don't know for certain, but there's growing evidence that the long-term implications of this have ties to breast cancer, obesity, diabetes, and depression, and possibly other cancers.

"It's about how much light you're getting in the evening. It doesn't mean you have to turn all the lights off at 8 every night, it just means if you have a choice between an e-reader and a book, the book is less disruptive to your body clock. At night, the better, more circadian-friendly light is dimmer and, believe it or not, redder, like an incandescent bulb."