People who pass many waking hours exposed to bright light may not experience sleep problems even when they read on a tablet every night before bed, a small experiment suggests.
Plenty of previous research has linked reading on tiny screens to disrupted or abbreviated sleep, especially when people use tablets and smartphones right before bed.
But after 6.5 hours of constant bright light exposure during the day, there were no differences in sleep between the 14 participants in the current experiment, whether they read a traditional book or on a tablet for two hours before bedtime.
"One plausible explanation for these discrepant results across experiments, in our view, is that bright light exposure during daytime - similar to that employed in the present study has previously been shown to attenuate the suppressive properties of evening light exposure on melatonin levels," said lead study author Frida Rangtell, a neuroscience researcher at Uppsala University in Sweden.
Melatonin is a hormone that helps control the body's sleep and wake cycles, and bluish light emitted by tablets and other electronics is thought to disrupt sleep by lowering melatonin levels.
"Our results could therefore suggest that light exposure during the day, e.g. by means of outdoor activities or light interventions in offices, may help combat sleep disturbances associated with (light from electronic devices)," Rangtell said by email.
For the experiment, Rangtell and colleagues assessed participants' sleepiness before bed and after waking up and measured concentrations of melatonin in saliva each night.
Starting in the afternoons, participants were exposed for six and a half hours to a constant level of light about equivalent to that indoors next to the window on a sunny day.
Each person in the study started either by reading a book or a tablet before bed, and then repeated the experiment a week later by using the opposite format to read.
Researchers didn't find any significant differences in sleep or melatonin levels before bedtime between tablet and book reading, the authors report in the journal Sleep Medicine.
The experiment may not have included enough participants, however, to detect statistically meaningful differences between reading tablets and books, the authors note. They also lacked data on light exposure prior to the start of the experiment, which might have influenced the results.
"There is a lot of research in both human and animal models that shows the timing and intensity of prior light exposure can cause a sort of 'lighting history' after-effect on the brain clock," said Ilia Karatsoreos, a neuroscience researcher at Washington State University in Pullman who wasn't involved in the study. "Thus, previous exposures to different lighting cycles, and light intensity, can change the way this clock works."
Because the current study looked at just one night of bedtime reading with a tablet and one with a book, and had participants sit at a desk to read, it's possible the results would look quite different in a real-world setting where people might read most nights in bed before going to sleep, Karatsoreos added by email.
"As most of us know, reading on tablets, checking emails on phones, or watching late night TV are a chronic issue, and unfortunately become part of our pre-sleep habits," Karatsoreos said. "So while this study seems to argue a single exposure to a tablet does not have a major effect on sleep if one was also exposed to bright light beforehand, it doesn't really address what has become a common ritual for so many of us."