Ninety percent of Americans frequently use electronic devices right before bed. And though it's a fun way to wind down, a new experiment shows that the bright screens are shining sleep problems directly into their eyes.

In the study, participants spent two weeks sleeping in a private hospital room and reading for four hours before bed with either a paper book or an iPad. Those using the iPad reported feeling less sleepy at night and took about 10 minutes longer to fall asleep. They also felt less alert in the morning. And although they didn't sleep less, they spent less sleep in the REM stage, which may have accounted for the morning grogginess (and presumably fewer dreams).

Those using AN iPad took about 10 minutes longer to fall asleep

At night, the iPad users also experienced a 50 percent drop in melatonin — the hormone that makes people feel sleepy. Even more disturbing, their melatonin cycle shifted forward by an hour and a half, essentially creating self-imposed jet lag.

Anne-Marie Chang, a neuroscientist at Brigham and Women’s Hospital in Boston, is the lead author on the paper, which was published today in the Proceedings of the National Academy of Sciences. I talked to Chang about the experiment's results and what lessons we can draw from it to sleep better:

Susannah Locke: What did you show about tablets and sleep that’s never been shown before?

Anne-Marie Chang: Other studies had showed that light in the evening affects the way you sleep. What we showed was [what happens] when individuals use a light-emitting device in the hours before bedtime. It takes them longer to fall asleep. They feel more sleepy the following morning. They have less REM sleep. The light suppresses their melatonin, which is the sleep-promoting hormone, by more than 50 percent. And their melatonin rhythm was more than an hour and a half delayed. So that amount is pretty large.

SL: Were you surprised by any of the ill effects you found?

AC: We had an idea of the effects of light in the evening. [But] we were surprised in the results we saw in sleepiness and alertness the following morning. The [participants] didn't have any difference in total sleep duration that would account for that.

SL: If people were getting the same amount of sleep, then what was causing more grogginess in the morning?

AC: It could have something to do with the shortened duration of REM sleep, which occurs [more] in the later part of the night, closer to the morning. Also, the delay that we see in melatonin may be contributing to the effect that we see in the morning. There’s a significant difference in how sleepy they feel until midday.

"we were surprised in the sleepiness the following morning"

SL: Do you know if the problem is the tablet’s brightness or the bluish tint of its light?

AC: We don't know if it was the brightness or the blue light from the devices. It could be the combination contributing to what we saw. We know that bright light will induce some of the same responses that we saw, and bright light will [make] you feel more awake and more alert. If the light is not as bright, but is short-wavelength enriched [more blue], you can see these effects.

SL: Your study may have underestimated these devices’ effects because participants’ bedtimes were strictly enforced. No one was allowed to get sucked into an iPad rabbit hole until 3 am.

AC: In our study, we had a very stable schedule. The study participants read from 6 pm to 10 pm, and they had to stop, and they went to sleep. And they were in the dark sleeping between 10 pm and 6 am each night.

You may see even more profound differences in a real-world setting, where people are then sleeping for less because they’re staying up due to the light from these devices. When you're using a device which makes you feel more alert, chances are you won't be going to bed just because it’s bedtime. [The light masks] the sleepiness that your body may be feeling, but you don't discern that as being sleepy.

SL: Why did you compare the iPad to a paper book rather than to a non-light-emitting device like the original Kindle?

AC: That’s a great question. We wanted to compare the devices with what typically people had been doing, and still a lot of people prefer to read books. Because at the time [2010 and 2011], they were pretty new and not so many people were using these devices. We wanted to just compare what is the typical condition — which I don’t know if that’s changed since then, but at the time was reading printed books.

Presumably reading on a non-light-emitting device would be the same as reading on a book, but we didn’t specifically test that in the study.

SL: What are some other differences between the experiment and the real world?

AC: One caveat is that the e-readers in our study were set to the very brightest level, and a reader at home may dim the light. In this case, reducing the brightness may reduce the magnitude of the response that we found.

"IF YOU HAVE TO USE DEVICES, TURN DOWN THE BRIGHTNESS"

Another limitation is that the hours of reading before bedtime that we used in the study is relatively long. Most people don't read for four hours. But if you total up the number of hours spent on [any] screen before bedtime [in the real world], it will be very comparable to four hours. And it may be a lot more.

SL: What do you think of software like F.lux that makes screens less blue at night?

AC: Technology is moving so quickly, and science really can’t keep pace with the pace of technology and the pace of commerce. So I think it’s great when you have individuals and companies coming up with technologies to modify and improve the devices that we have, based on evidence.

SL: What do you personally do about screen time before bed, both for yourself and for your kids?

AC: Let me start with what I tell my kids. This is the technology that’s popular with kids. More and more children and adolescents are sleeping with them, using them all the time. I think it’s best to avoid any screen time an hour or two before bedtime. Devices get charged outside of the bedroom so that they’re not anywhere inside or near the bedroom.

I tell my friends — I tell myself — if you have to use devices, be on the computer, or check your phone, to turn down the brightness. And I think installing some of these programs that compensate for the color can be helpful.

SL: Is there anything else you’d like to add?

AC: For the vast majority of people, we don't have so much control or wiggle room in the morning as far as when we wake up. But we have presumably a lot more voluntary control over when we go to bed. So if we can deal with issues of light-emitting devices and when we use them — or how can we improve what we have — I think those would be really great goals.

This interview has been lightly edited and condensed for length and clarity.