It's incredibly hard to sleep when light seeps through your curtains or your phone glows from the bedside table. But more and more research is finding that artificial light may be more than annoying. It can be seriously unhealthy, too.

The natural 24-hour cycle of daylight and darkness keeps our circadian biological rhythms in alignment. When those rhythms get out of whack, studies have shown a potential link to health issues ranging from insomnia to breast cancer. One of the major culprits may be melatonin, the main hormone that controls sleep and wake cycles. Light at night suppresses the secretion of melatonin, and a lack of melatonin has been linked to diabetes, obesity, heart disease and other health issues.

The link between health and nighttime light has been a hot topic of discussion, especially in recent years. The most damning came from the cancer research arm of the World Health Organization, which declared working the graveyard shift "a probable carcinogen." The American Medical Association Council on Science and Public Health issued a report evaluating the impact of artificial lighting on human health, wildlife and vegetation.

Most recently, an international team of researchers analyzed data on the impact of light pollution on melatonin formation in humans and vertebrates and found that even low levels of urban skyglow can suppress production. They looked at hundreds of studies, looking for evidence of how light affects melatonin production — and it doesn't take much.

To get an idea of the impact, the researchers spelled out some variables. The skyglow of a city, a form of light pollution, can reach illuminances of up to 0.1 lux, and outdoor lighting can reach about 150 lux, according to a press release from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, which was part of the bigger team. But the thresholds for various creatures isn't that high: in fish the threshold is 0.01 lux, in rodents it's 0.03 lux and in sensitive humans, it's about 6 lux. Their results were published in the journal Sustainability.

The man who first raised the flag

Cancer epidemiologist Richard Stevens from the University of Connecticut School of Medicine is a pioneer in the field, having studied the subject for 25 years. Stevens, who talks about his work in the video above, is credited as the first to hypothesize that the increased use of artificial light at night may be linked to the higher risk of breast cancer.

In a opinion paper published in the journal Philosophical Transactions B, Stevens posed the question: "Electric light, particularly at night, disrupts human circadian rhythmicity: Is that a problem?"

Stevens looked at the link between artificial light at night and its potential short-term and long-term impact on health. He points out that studies point to the importance of sleep at night, but now we should focus on the importance of darkness at night.

"The point of emphasis to all this is that while sleep is deeply important to well-being, so too is exposure at night to dark," he writes. "The importance of sleep has finally entered mainstream thinking and practice; however, the importance of dark is still greatly underappreciated."

Stevens points to evidence that has linked ambient light in the bedroom at night and the risk of depression and obesity, as well as many studies that have examined a similar connection with breast cancer. If those associations are causal, he says, there would be "obvious and easy interventions."

He suggests blackout shades and eliminating all light sources in the bedroom, no matter how small. If you need a night light, he says a dim red light would cause the least disruption to your body's circadian system.