In less than a decade, reading has shifted from the medium it dominated for centuries—paper—to screens of various sorts. The change in habit has been accompanied by concerns over whether this could be influencing sleep. Exposure to light biased toward blue wavelengths, such as that produced by the screens of tablets, has been shown to alter the circadian rhythms that set the body's clock.

A number of studies have suggested that this is a real problem—enough that the Mayo Clinic's advice on getting better sleep notes that "Some research suggests that screen time or other media use before bedtime interferes with sleep." Now, new research published in PNAS provides some hard numbers to back up these worries. But it's a small study with some significant limitations, so this shouldn't be seen as the final word on the topic.

The study relies on a panel of a dozen volunteers who spent two weeks in a sleep center. During that time, all 12 spent four hours reading before a strictly enforced 10pm bedtime. For the first week, half were given a regular book and dim room light; the other half an e-reader with an active screen. After a week, the roles were switched.

That makes for an extremely controlled study, but it isn't likely to be representative of real life, where reading time is likely to be more limited, bedtime to be more flexible, and room illumination to vary a bit. The results have to be interpreted with a bit of caution.

However, even with that caution in mind there were clearly some results. Evening levels of melatonin, which help modulate circadian rhythms, were down by 50 percent in those using e-readers compared to their book-reading companions. Melatonin levels also suggested the circadian cycle had been pushed back by about an hour and a half among the e-reader users. Those same readers also took an average of 10 minutes longer to fall asleep once lights-out arrived. While they ended up with about the same total amount of sleep, they received 10 minutes less of REM sleep, perhaps as a result of the delayed start.

Although these were relatively minor effects, it appeared that the participants noticed them. When asked how they felt at bedtime, e-reader users said they felt less sleepy than their book-reading peers. They also took much longer to feel fully awake the next day. Given that perceptions can influence how people act, this might cause a wide variety of behaviors

In additional data published with the report, the authors looked at some of the actual hardware used for e-reading (the study itself used only active screens). A Kindle without a backlight, as you'd expect, only reflected about as much light as a book and had its peak omission in relatively red wavelengths. Anything with an active screen—the authors looked at early iPads, Kindle Fires, and Nook Colors—emitted at least an order of magnitude more light, and was biased toward the more energetic, bluer end of the spectrum.

The results line up with previous work that suggests e-readers could be interfering with a normal sleep schedule and back it up with some impressively detailed data. But, because of the artificial circumstances set up to do the testing, it's not clear how precisely the results will translate to real-world use. Still, if sleep is something you're struggling with, the Mayo Clinic's suggested caution about spending less screen-time right before bed seems worth heeding.

PNAS, 2014. DOI: 10.1073/pnas.1418490112 (About DOIs).