At last week’s Apple launch event, the company unveiled Night Shift, a new feature of the iOS operating system that changes the color of the light emitted by iPads and iPhones as evening falls. “Studies have shown that exposure to blue light in the evening can actually make it harder for you to fall asleep at night,” Apple Vice President Greg Joswiak said. “Night Shift automatically shifts the colors of the display to the warmer end of the spectrum, which may help you sleep better at night.”
But blue light is only part of the reason why using devices before bedtime may be affecting your sleep. If you wake up groggy the morning after you used your iPad before bed, you’re probably going to have to do more than upgrade your operating system.
A gadget’s tech specs can include a lot of iffy science, but Apple is right about how blue light affects our sleep. Special cells in the retina are most sensitive to short wavelength radiation, i.e., blue light. Those cells communicate with a part of the brain called the suprachiasmatic nucleus, or SCN, a small part of the hypothalamus also known as the “master clock.” It’s the SCN that controls circadian rhythms, which influence a range of bodily processes, including sleeping and waking.
When the SCN gets a dose of blue light, it tells the brain’s pineal gland to stop producing melatonin, the hormone that primes the body for sleep by signaling that it’s nighttime, said Mariana Figueiro, a professor at the Lighting Research Center at Rensselaer Polytechnic Institute. If you get a large enough dose of light from your iPad,1 it can push back the time you’d otherwise feel sleepy enough to turn in. (And it may not just be blue light. Research by Figueiro and her colleagues has also suggested that red light, which does not affect melatonin, can increase alertness at night.)
That’s not to mean you need to put on horse blinders the moment the sun goes down. Self-luminous displays can reduce melatonin production depending on the size of the screen, how close your face is to the device and how long you’re exposed. So, for example, a big-screen TV might not be problematic if you’re watching it from across the room. Figueiro and her colleagues have shown that tablet displays can suppress melatonin in young adults after two hours, but not one. The effects were more pronounced in teenagers, with a one-hour exposure to self-lighting devices suppressing melatonin by 23 percent and two-hour exposure by 38 percent. (This line of inquiry could have consequences beyond sleep, because research has linked exposure to artificial light at night to an increased risk of certain cancers among night-shift workers.)
So, yes, Apple’s doing you a favor by offering to filter out some of the blue light — not all of it, or you wouldn’t be able to read blue type on your screen — in its new operating system.
But light isn’t the only way that devices and computers might affect sleep. Most simply: It’s hard to sleep while staring at a screen. If you have your iPad in bed, bingeing on your third episode of “House of Cards,” you’re not sleeping. “The urge to stay up late is so much more enticing because we have these handheld devices,” said Lauren Hale, an associate professor of family, population and preventive medicine at Stony Brook University School of Medicine and editor in chief of the journal Sleep Health. “You don’t even have to work very hard to stay up.”
And your screen activities — watching a horror movie, getting into a Bernie vs. Trump Twitter slap-down — might be stimulating enough to keep you alert when you’re supposed to be winding down, the same way an in-person fight with your partner might.
Hale led a 2014 review of the research on screen time across different devices — TV, video games, computers and mobile devices — and sleep among school-aged children and teens. Screen time2 was associated with poorer sleep outcomes, mostly shorter duration of sleep and a later bedtime, in 90 percent of the studies. That’s “pretty strong and consistent evidence that there’s an association,” she said.
(There were, as always, limitations to many of these studies: If someone asks you how much time you or your children usually spend sleeping and using devices with screens, your estimate probably won’t be as accurate as it would be if you kept a careful diary. Journals may also be biased against studies that show no effect, so those results aren’t published as often.)
People are also doing a range of things on their devices, and not all of them may have the same effect. Browsing Facebook, say, might be different from watching a movie. “One of the ways we’ve started to think about this is in terms of more interactive versus more passive media,” said Jessica Levenson, a postdoctoral scholar in the translational research training in sleep medicine program at the University of Pittsburgh. She and her colleagues used a 1,788-person survey of young adults to look specifically at the association between the volume and frequency of social media use and sleep disturbances. Their study, published this year, found that the heaviest users3 had twice the risk of disturbed sleep compared with the lightest users, and the most frequent users4 had three times the risk of the least frequent ones.
Research hasn’t yet concluded whether those circumstances are causing sleep problems. Screen time’s relationship with sleep can be a tricky subject to study, in part because technology keeps changing. (You can study iPads today, but by the time the research is published the hottest thing might be virtual-reality headsets.)
Randomized studies, where groups are assigned to screen time or not, can be difficult and expensive to conduct, but they aren’t impossible. In late 2014, for example, researchers published a small study that looked at what screen exposure did to sleep. They took 12 young adults with an average age of 25 and had them either read an iPad in a dimly lit room for about four hours before bed for five consecutive nights or read a printed book for the same amount of time. (Then each person did the experiment again, under the other circumstances.) The results showed that the e-readers didn’t take much longer than the print-book readers to fall asleep — just 10 minutes more. But they reported feeling less sleepy before bedtime and less alert in the morning, and they had less REM sleep during the night.
Because the vast majority of the research on the topic is still observational, though, it’s not yet known whether screens cause the problem. “Are screens keeping [people] up and then they’re excited, stimulated and alerted by light?” Hale said. “Or is it just that people who are anxious or suffering from insomnia for some other reason choose to watch a movie?” It’s also possible that both are correct: Insomniacs turn to screens late at night, which in turn leads to more sleep problems.
Given the current research, there’s no proven way to address the association between screen time and sleep. To avoid all the potential disruptive effects of screens, Figueiro advises avoiding them as much as possible for two hours before bedtime.
The simplest advice Hale gives out, even without strong evidence behind it, is to use phones and devices during the day. “At night, store and charge them in another room,” she said. “Don’t use them as alarm clocks. I’m all for reducing blue light exposure at night. But I doubt it will systematically solve the problem.”