#1 - I'm not convinced we should be saying that screenlight affects young people's sleep: Here's Why.
In 2010, I was one of the first researchers to theorise how technological devices could affect sleep. But a theory is only as good as the data supporting it. Now that we're coming up to a decade of research on this popular topic, I've changed my mind. And I'll come back to the following statement at the end of this article - "I don't believe blue-enriched screenlight directly affects sleep." Here's why.
In 2011, Dr Michelle Short and I were asked to write a chapter about 'Sleep Hygiene' for the Oxford handbook of infant, child, and adolescent sleep and behaviour. Given we'd already published a review, laboratory experiment, and that I served on the National Sleep Foundation's Task Force for their 2011 Sleep in America Poll - all which focused on the link between technology use and sleep - it was clear that technology should be a major focus in this chapter (in fact, the chapter title ended up being "Sleep hygiene and environment: Role of technology").
It had only been a couple of years since we published our first review, yet there had been a steep rise in the number of publications in this area (and this continued years later). Not only were we interested in summarising the consistent significant associations found between various technological devices and aspects of sleep (eg, bedtimes, sleep latency, total sleep time), but we were also interested in the How question - that is, how does technology use affect young people's sleep? After reviewing the evidence, there were a few likely candidates. The first was the 'arousal' hypothesis - that using technology caused an increase in both physical and mental arousal, which then made it harder to fall asleep. Another mechanism, originally proposed by Prof Jan van den Bulck, was the displacement hypothesis - that using technology for longer than the user intended, ended up displacing sleep. Yet, arguably the most popular hypothesis was that bright screens were alerting and delayed sleep and circadian timing.
By the time Michelle and I wrapped up the final draft of the chapter, we were really dumbfounded by one glaring omission. Despite so many media reports that screenlight was a cause of poor sleep, there were very few mentions of this in the scientific literature. In fact, because our chapter was focused on pediatrics, we did not find a single study that produced any data to support this idea the bright screens from technological devices affected young people's sleep!
How come the world knew about this phenomenon yet the scientists hadn't even proved it?
So in 2012, we set about testing this. Our group of 4th-year Honours students, Melanie Heath, Cate Sutherland, and Kate Bartel, banded together in our sleep lab to design a study testing the alerting effects of light from a bright iPad 2 screen on teenagers' sleep. We were inspired by a 2011 study of young adults, who used either a bright or dim laptop screen for 5 hours in the evening. This study showed increased alertness due to the bright screen, but for some reason didn't measure sleep.
In preparing for this 2012 study, we had to seriously think of a few design issues. First, we wanted to control the type of technology teenagers would use in the lab. We were really surprised by the results of our 2010 study, where we compared playing 'Call of Duty 4' (that was considered a violent videogame back then) to watching 'March of the Penguins' (a much less stimulating documentary about penguins and narrated by that smooth-talking Morgan Freeman). Well, one-third of the teenagers fell asleep watching the documentary (p.s. none of them fell asleep playing the videogame). So we needed teens to do something more-or-less stimulating in this iPad study. This meant reading was off the cards. We decided to mix it up, and have them play games, and watch some videos.
Next, we had to measure how bright the iPad 2 screens were going to be. I say 'screens' (plural) because we had 3 conditions. One was an iPad on full brightness. One was an iPad on the lowest brightness. And like goldilocks's choice of beds, one was right in the middle - it was using an app called f.lux that lessened the emission of blue-light from the iPad (this was prior to Apple's Nightshift mode). There was just one problem ...
When we measured the lux - that is, how bright the light is when received near the eye - from the bright iPad screen, it was lower than we expected. Low enough that we didn't think it was going to be bright enough! For context, a bright sunny day is around 100,000 lux - we've done studies using bright light lamps (1,000 lux) and bright light devices (500 lux), and read that room light as low as 150 lux can have an effect. We were getting readings <50 lux. So we did a bit of our own experimenting, and we managed to get the brightness of the screens to 80 lux. But. Not only did the iPad need to be on full brightness, but it needed to have a predominantly white background. And, it needed to be an uncomfortably 40cm away from the face. The only way to make it comfortable was for the teenagers to be sitting up on the bed, knees bent, and resting the iPad on their knees. I don't see teenagers using technology this way.
Finally, I learned something I didn't realise. If using a within-subjects research design (i.e., where the same person comes into the sleep lab and gets tested under different conditions), then you need to have a period of darkness prior to screen exposure, as this helps to 'reset' the eye, and reduce pre-screenlight exposure. Said another way, if the young person has been receiving light by going outside during the day, then under normal lighting conditions indoors, then this light exposure during the day reduces the effect of subsequent screenlight in the evening. Again, what teenager goes into the dark for an hour prior to getting on their screens at night? Nevertheless, we were ready to give a bunch of teenagers a bright iPad screen for that classic hour before bed!
When we ran the analyses on a host of pre-sleep and post-sleep variables, we found nothing to write home about. There weren't any meaningful differences in measures of evening alertness (self-report, reaction times), sleep (time taken to fall asleep, various stages of sleep) or how the teens felt the next morning. And around this time, a 2012 study was published showing the rise in melatonin was not attenuated after 1 hr of bright iPad 2 screenlight (somewhat concurring with our findings), yet, 2 hrs of exposure did - this was great because these 3 studies now showed 1 hr of bright screenlight didn't do much, but something happens between 2-5 hrs.
But still, this 2012 study didn't measure sleep, and therefore we still did not have data supporting the notion that bright 'screenlight affected sleep'.
In 2013, we took a side-step with our investigations into this link between technology use and sleep - and it's THE path I'm following still to this day. But it has nothing to do with screenlight. You can read that story in another post (TBA).
In 2014, we published our iPad study - the first one to test whether a bright screen could affect sleep. It didn't receive any noteworthy media attention, and did not really get cited in the early days. 2014 was the same year that I stumbled upon a terrific poster at the European Sleep Research Society meeting in Tallinn, in Estonia. Stephanie van der Lely was the lead author, and it was the same Swiss group that published the 2011 paper. This new study (which was later published in 2015) not only tested pre-sleep alertness for 30-min intervals prior to sleep, BUT they also measured sleep! They found continuous use of a bright screen for at least 1.5 hours increased alertness (self-report, reaction times), and attenuated the rise in melatonin. Fantastic, and a wonderful aspect of science where you have independent research groups looking at the same phenomenon and their findings showing either convergent or divergent evidence of what we hope is 'the truth'. So what happened with the teenagers' sleep? Nothing meaningful.
A story was starting to emerge. Now that there had been a number of studies testing the 'arousal' or 'screenlight' hypotheses, it was suggesting these were not the mechanisms that would explain How technology use affected sleep.
Nevertheless, there were still some great studies to emerge that added to our knowledge of this topic. One of the ones needed was performed by Anne-Marie Chang et al in 2014. What they added to this field was that they tested consecutive nights of technology use. See, in the previous studies, technology use was tested on a single night. But this is not what young people do. We knew how ubiquitous technology use was across all age groups with the data obtained from the National Foundation's 2011 Sleep in America Poll (which we later published). Chang et al provided college students with either an e-reader for 5 consecutive nights or a printed book. Their results were really interesting. The bright screen from the reader delayed the onset of the rise of melatonin (an indicator of circadian rhythm timing) by 90 min! That's truly meaningful. What was also interesting is that the time taken to fall asleep in the e-reading condition was statistically significantly longer than the printed book condition. This was the first evidence of bright screens having an affect on sleep.
And this is where the term 'meaningful' comes in again. The difference in the time taken to fall asleep between the two conditions was ... (drum roll) ... 10 min. A 10-min difference isn't much. Let's put this into some context. When I've worked with adults experiencing insomnia, and they tell me they had a bad night of sleep versus another, I use cognitive therapy to help evaluate the interpretation of their own data. I'll use a question like, "Does taking 10 min longer to fall asleep matter?". Inevitably, it doesn't. And when we ask teenagers in our research studies or in my sleep clinic about what they think is a long time to fall asleep ... I'll summarise for you, "10 min doesn't mean sh#t".
Thus, my position on interpreting the Chang et al (2014) study has been to minimise the meaningfulness of this 10-min difference in sleep - even if the title of their study is "Evening use of light-emitting e-readers negatively affects sleep, circadian timing, and next-morning alertness". The same group from Harvard have since published another great study in 2018 titled "Unrestricted evening use of light‐emitting tablet computers delays self‐selected bedtime and disrupts circadian timing and alertness." This study differed from all the ones above. You see, if you want to examine a direct influence of screenlight on sleep, you provide the bright screenlight right up to the time the person would usually attempt sleep, and then see if they take longer to fall asleep.
In Chinoy et al.'s (2018) study, they instead allowed college students in each condition to choose when they wanted to attempt sleep after either using a bright tablet or reading a printed book. And what they found was when using the bright tablet, the students fell asleep 30-min later - and that's meaningful. What's great* about this study is that it summarises nicely what likely happens in the real world. That is, people retire to their bed, get on their screens, the light from the screens keeps them alert, and thus people use their screens beyond their bedtime - aka the displacement hypothesis. That is, screenlight doesn't directly affect sleep - it indirectly affects it.
Yet, keep in mind that the effect of screenlight on sleep is further minimised when people have been exposed to outdoor and indoor light during the day - as this laboratory study found when it did not rest participants' eyes to pre-screen darkness. Even an interview with Prof Jeanne Duffy, one of the authors from the Harvard studies, discussed this 'mitigation' of evening screenlight due to prior light exposure.
So to sum it up "I don't believe blue-enriched screenlight directly affects sleep".
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*worth noting is that in the tablet condition, users were allowed to choose their own activity, whereas it was fixed to reading in the printed book condition. Because the content of the pre-sleep activity and the lighting were both manipulated, it makes causal conclusions that the screenlight was the cause of the later sleep onset time less convincing.