Anyone who's raised teenagers knows how tough it can be to get them to go to bed on time \u2014 and how difficult it is to get them out of bed when they don't. This was a problem long before young people curled up at night with a smartphone or tablet, but the gadgets may be making the problem worse.\nThat's the conclusion of a Norwegian study on the effects of late night screen time on teenagers' sleep. The researchers, who deemed sleep deprivation "a major public health concern,\u201d looked at the sleep habits of 9,846 men and women between the ages of 16 and 19.\n"There are probably many possible pathways between screen time and sleep, some of which are direct," says lead author Mari Hysing of Uni Research Health in Bergen, Norway. "The light from the screens may directly affect our circadian rhythms, and teenagers may be especially sensitive."\n[Related: Not Getting Enough Restful Sleep? Try ResMed S+ Device and App]\nThe villain here appears to be the blue light emitted by most digital screens, which reportedly disrupts the production of melatonin, a substance that makes you feel sleepy. The American Chemical Society last year produced a short video called "How Smartphones Keep You Awake," which demonstrates the idea quite effectively.\nUsing any device in the hour before bed led to a 13 percent to 52 percent increase in the likelihood of needing more than an hour to actually fall asleep, according to the Norwegian researchers. More than four daytime hours of screen time was associated with a similar increase in risk of "sleep latency," or an increase in the time it takes to fall asleep.\nLast year,\u00a0a similar studyfrom\u00a0researchers at Pennsylvania State University's Department of Biobehavioral Health also hypothesized that sleep is disrupted by blue-light devices. However, that report suggests that devices like Amazon's older Kindle Readers, which aren't backlit and therefore don't emit blue light, don't disrupt sleep patterns.\nThere's an increasing amount of evidence about the link between sleeplessness (and other disorders) and blue light. If you do a quick search using terms like "blue light" and "melatonin" on the U.S. National Library of Medicine's PubMed website\u00a0you'll find many examples.\nOne such study that caught my eye suggests an unconventional solution: "blue blocker glasses." Swiss and German researchers outfitted a group of 15- to 17-year-old males with glasses that were specially treated to block light in the blue wavelength, then had them spend evening hours using devices with LED screens. Compared to a control group that didn't wear the glasses, their sleep patterns were much better.\nIt's unclear if blue blockers will ever be widely available, and even if they are, do you really think a typical teenager will wear them?