In the modern age of technology it is not uncommon to come home after a long day at work or school and blow off steam by reading an e-book or watching television. Lately, however, scientists have been cautioning against using light-emitting devices before bed. Why? The light from our devices is “short-wavelength-enriched,” meaning it has a higher concentration of blue light than natural light—and blue light affects levels of the sleep-inducing hormone melatonin more than any other wavelength.

Changes in sleep patterns can in turn shift the body’s natural clock, known as its circadian rhythm. Recent studies have shown that shifts in this clock can have devastating health effects because it controls not only our wakefulness but also individual clocks that dictate function in the body’s organs. In other words, stressors that affect our circadian clocks, such as blue-light exposure, can have much more serious consequences than originally thought.

To discuss the growing concern Scientific American MIND consulted with Thomas Jefferson University neuroscientist George Brainard, who was among the first researchers to investigate how different wavelengths of light affect the release of melatonin, and Harvard University neuroscientist Anne-Marie Chang, who recently discovered that the effects of light-emitting devices on circadian systems extend beyond evening and into the following morning.

[An edited transcript of the conversation follows]

How did you become interested in the effects of light on sleep?
Brainard: I was interested in the effects of light on animals as a teenager. I never planned to be a scientist—I wanted to be a writer! So I learned more about the topic out of pure curiosity. When I began my career as a journalist, I interviewed researchers on the topic who encouraged me to pursue a career in science. So I returned to school to get my doctorate and studied the effects of different wavelengths and intensities of light on rodents. I have exclusively studied the effects of light on humans for the past 30 years.

Chang: As a graduate student, I researched circadian rhythm disorders resulting from different human sleep patterns, particularly those of early and late sleepers. I became interested in the effects of various aspects of light—such as time of day and duration of exposure—on circadian rhythm, sleep and performance.

How exactly does light affect our circadian rhythms? And how is melatonin involved?
Chang: We have known for quite awhile now that light is the most powerful cue for shifting the phase or resetting the time of the circadian clock. We also know that melatonin is present at low levels during the day, begins being released a few hours before bedtime and peaks in the middle of the night. Past studies have shown that light suppresses melatonin, such that light in the early evening causes a circadian delay, or resets the clock to a later schedule; and light in the early morning causes a circadian advancement, or resets the clock to an earlier schedule.

So how did scientists become interested in blue light in particular?
Brainard: In the 1990s my team performed more than 700 experiments over seven years to measure how different wavelengths of light regulate acute melatonin production. Unexpectedly, we found that humans display a peak sensitivity to light in the blue wavelength region of the spectrum.

Rods and cones [photoreceptors in the eye] could not account for this differential regulation of melatonin production, so we postulated another type of photoreceptor was responsible for mediating such physiological responses. These wavelength-sensitive photoreceptors were identified soon after and are known as melanopsin-containing ganglion cells.

What happens in the body when our eyes are exposed to blue light on electronic devices?
Chang: Recent studies have shown that short-wavelength [blue] light has a greater effect on phase shifting the circadian clock and on melatonin suppression. In 2014 my colleagues and I examined the effects of reading on a light-emitting device compared with reading a printed book. Participants who read on light-emitting devices took longer to fall asleep, had less REM sleep [the phase when we dream] and had higher alertness before bedtime [than those people who read printed books]. We also found that after an eight-hour sleep episode, those who read on the light-emitting device were sleepier and took longer to wake up. In the study all participants had to stop reading and turn off the lights at exactly 10 P.M., even if they did not feel sleepy. At home, I would expect people do not have the motivation to turn off their devices and go to bed, so they would stay up longer and experience even more circadian delay and shorter sleep times. The effects in the real world could actually be even greater.

What about extreme environments in which the body does not experience a normal daily cycle of sunlight and darkness?
Brainard: My research into the clinical applications of light got NASA interested in applying these findings to spaceflight scenarios. When an astronaut leaves Earth, his or her body is operating on a 24-hour light/dark cycle, but the space station orbits Earth every 90 minutes, and astronauts see the sun rise and set each time. This shift in the lighting environment, known as slam shifting, can have many health consequences and inspired my work with NASA on creating light-exposure schedules specifically for astronauts. [Editors’ Note: As a result of this work, NASA will begin implementing a new lighting system onboard the International Space Station next year, designed to improve astronauts’ sleep and waking performance.]

Can the effects of light on melatonin ever benefit the body?
Brainard: My research in the 1980s showed that the effect of light on melatonin secretion has clinical benefits. Since then, light therapy has been shown to be effective in treating several other conditions, including depression, sleep disorders, eating disorders and age-related dementia.

Chang: Short-wavelength light can be applied in different circumstances where you actually want to shift the clock. For example, it could help in the mornings when we need to be at peak alertness or in cases of jet lag when we change time zones abruptly and our circadian clocks get thrown off. People with variable sleep patterns, such as shift workers, could also benefit from using a schedule of short-wavelength light exposure to help realign their circadian clocks.

Finally, the question everyone is wondering about: Do you have any suggestions as to what we can do to reduce our blue-light exposure before bed?
Chang: For those who just cannot turn off digital devices, here are a few suggestions: You can dim the brightness of your devices or you can make use of programs that filter out short-wavelength light in the evening. I’ve also heard of modern technologies that use different settings, such as reversing the print so the page is dark and the text is light, which, though untested, are probably beneficial if they reduce the amount of emitted light. But the best and least popular answer would be to simply avoid your devices before going to sleep!