The bed must be reserved as a place for sleep and sex only. That was the decree of psychologist Richard Bootzin in his influential 1972 proposal for a "stimulus-control" approach to better sleep. One central tenet was operant conditioning: The bed must be a sanctuary, such that the brain is trained to sleep when it is in the bed.

The allowance for sex—which, in its ideal form, is one of the most stimulating things a human body can experience—always seemed to me at odds with the stimulus-control approach. A note to aspiring self-help writers: The book For Better Sleep, Kitchen Sex remains unwritten. Otherwise, though, the idea of stimulus control made sense to a lot of people. It also included advice to avoid bedroom behaviors that abet anxiety, like clock-watching. And Bootzin's approach to insomnia endured, proving itself in several studies.

So last year when Charles Czeisler, a professor of sleep medicine at Harvard Medical School, found that around 90 percent of Americans use some kind of electronic device within the hour before bed—and correlated the degree of use with ever-poorer sleep—one of his first theories of the case was overstimulation. That's because, Czeisler and colleagues wrote in the Journal of Clinical Sleep Medicine in December of 2013, "In addition to making phone calls, cell phones now allow the user to instant message, listen to music, send emails, play games, and surf the Internet." So they do. And all of that stimulation, the researchers proposed, may "impede the natural withdrawal of sympathetic nervous system activity necessary for sleep onset." Or, they preclude chill.

A more vague hypothesis raised by Czeisler at the time was that the detrimental effects on sleep might be due to "light and electromagnetic transmissions from technological devices." Other researchers, too, have lately blamed the light exposure element, which is well understood to influence sleep patterns through photoreceptors in the retina that tell the brain to release the hormone melatonin. It flows forth from the pineal gland into the blood and around the body. Normally, melatonin levels rise before bedtime, peak in the middle of the night, and decrease in the early morning. That is why daily light-dark cycles are important for normal levels of alertness and sleep in most people. Abnormalities in these cycles are the basis for shift-work sleep disorder. But to what degree is the light from a device, not the cognitive stimulation it provides, responsible for the known sleep effects of screens? And is that light worse than, say, the light from a light bulb? In research published today in the journal Proceedings of the National Academy of Sciences, Czeisler and colleagues made an interesting advance in understanding that idea, and just how screens themselves tell our brains to stay awake.

It began with the mundane observation that people read to unwind—often at night, often in bed. As far as sleep is concerned, Czeisler wondered, does it matter whether those people read books on iPads, Kindles, or paper?

Czeisler and colleagues, including lead researcher Anne-Marie Chang, an associate neuroscientist in the division of sleep medicine at Harvard Medical School, recruited a small group of people for an intensive experiment. For two weeks, participants took up residence in a monitoring unit at the clinical investigation center at Boston's Brigham and Women's Hospital. The subject group was only 12 people, but for a trial that requires so much commitment, that is not an unimpressive number. Though it was an enviable situation, in some ways: The people were allotted an idyllic eight hours for sleep, from 10:00 p.m. to 6:00 a.m.—a schedule they kept for three weeks prior, as well, in preparation. They were also allotted four hours of undisturbed reading time each night.

For five days in a row, the people read either a book or an iPad for four hours prior to sleep. Both were kept at constant, preordained distances from the person's face. Chang and colleagues monitored the people's sleep patterns with electroencephalograms all night. Each person did one stint with the iPad, and one with the book. Before and after each trial period, the people underwent hourly blood tests to paint a day-long picture of just how much melatonin was in their veins at any given time.

When subjects read on the iPad as compared to the paper books (hereafter referred to as "books"), the people reported feeling less sleepy at night and less alert the following morning. Of course, this is not a blinded study, the kind with a placebo—the participants could have been biased in reporting how they felt. But, empirically, they also took longer to fall asleep on the iPad nights and spent less time in the REM phase of sleep. And the blood tests showed that on average, the brain's melatonin secretion on those nights was delayed by an hour and a half.

Though this experiment dealt exclusively with iPads, the researchers also tested the light-emission patterns of other tablets, as well as phones and laptops. Chang told me they all emitted similar patterns, enriched for short-wavelength blue light, and at a similar intensity. So she would expect similar clinical findings for any such device.

Chang and her team also measured a Kindle—the original, non-light-emitting Kindle—and it reflected the same wavelengths and intensities as a book, meaning it shouldn't have negative effects on sleep and circadian rhythms in the same way as a lighted screen.

The blue shift is really what seems to distinguish the effect of screen light from that of typical ambient light. Our circadian pacemakers are in the hypothalamus, specifically the suprachiasmatic nucleus, which influences the release of melatonin from the pineal gland. And while we know that light exposure is critical to this process, research like this is a contribution to a still-basic understanding of how light affects that pacemaker: How much light, at what wavelengths, and by what source, has what effects on sleep cycles. Czeisler's work has previously shown that even blind people experience a resetting of their circadian cycles in response to changes in light exposure, because the circadian-regulation pathway is distinct from the visual pathway, contingent only on the light-sensitive cells in the retina containing a pigment called melanopsin that reacts to light.

The researchers conclude in today's journal article that the effect of light-emitting screens on circadian cycles "has important implications for understanding the impact of such technologies on sleep, performance, health, and safety." Expanding on the safety claim in a press statement, Czeisler said that given the rise of e-readers, and their increasingly widespread use among children and adolescents, more research into the "long-term consequences of these devices on health and safety is urgently needed."

"We introduce these devices that have medical and biological effects without requiring any health studies on their impact," he elaborated in The Washington Post, noting the absence of a safety evaluation process like what the FDA might do for a medication. "I think it's time to rethink that."

Czeisler and colleagues go on, in the paper, to raise the specter of cancer, even. They note that chronically suppressed melatonin has been linked to increased risks of breast, colon, and prostate cancers—an implication that invited headlines like the not-inaccurate but too-worrisome "Reading an iPad in Bed May Increase Cancer Risk."

Lest this all sound too technophobic, remember that the messages we get from the screens also bring us a lot of good feelings. Primarily in the form of little bursts of dopamine, the sort that comes from a Facebook like or an effusive tweet or a well-pinned pin on someone's board, or however that works. That dopamine surge mimics the process of eating something delicious, or, in an extreme and tired comparison, a drug. In that way, too, Czeisler's proposal may be apropos.

There is software that can attenuate out some of the blue light from the screens of phones and computers according to time of day, and there are also glasses that are made to filter short wavelengths. While they seem like a logical solution for the nighttime tech user, which is apparently almost all of us, Chang was hesitant to endorse those products at this point. More research, as the saying goes, is needed. The only way to be sure to avoid the adverse effects on sleep is to fully power down. The exact amount of screen-free time required isn't clear, though Chang doesn't think four hours is necessarily necessary or practical.

"Perhaps an hour or two hours is really sufficient," she said. "But perhaps it's difficult for people to even do that."