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 very 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.

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 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 new report in the journal Philosophical Transactions B, Stevens poses 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 science and studies stress the importance of sleep at night, but now it's key to stress 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 out recent 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.

The link between health and nighttime light has been a hot topic of research and discussion. In recent news:

  • Several years ago, the cancer research arm of the World Health Organization declared working the graveyard shift as "a probable carcinogen"
  • The American Medical Association Council on Science and Public Health issued a report in 2012 evaluating the impact of artificial lighting on human health, as well as noting concerns about energy costs, and the impact on wildlife and vegetation.

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.