Researchers found that after using e-readers near bedtime, people were more tired and took longer to become alert the next morning.
The study, from Brigham and Women's Hospital and Harvard Medical School in Boston, MA, is published in the Proceedings of the National Academy of Sciences.
Study leader Anne-Marie Chang, now assistant professor of biobehavioral health at the Pennsylvania State University (Penn State), says compared with natural light, the light emitted by electronic devices has a higher concentration of blue light - with a peak around 450 nm.
"This is different from natural light in composition, having a greater impact on sleep and circadian rhythms," she adds.
For the study, the team monitored 12 adult volunteers for 2 weeks and compared them reading from an iPad with reading from a printed paper book.
They found after reading from iPads, the volunteers took nearly 10 minutes longer to fall asleep and their sleep had less rapid eye movement (REM) compared with when they went to sleep after reading paper books.
After using e-readers, volunteers were more tired and less alert the next morning
Prof. Chang says the most surprising finding - and one that concerned them - was that after using the e-reader, the volunteers were more tired and took longer to become alert the next morning:
"This has real consequences for daytime functioning, and these effects might be worse in the real world as opposed to the controlled environment we used."
The 14-day study took place under controlled conditions in a sleep lab at Brigham and Women's Hospital.
Each participant read from an iPad for 4 hours before bed - from 6-10 pm - for 5 nights in a row. They did the same with a printed book. Whether they read first from the iPad or from the printed book was randomly decided, but the researchers say the order did not make a difference to the results.
The participants could read what they wanted as long as it only had text - no images or puzzles - and could be classed as "leisure" reading.
Using light-emitting e-readers suppresses melatonin, delays circadian clock
As well as next-morning alertness, the team also monitored the participants' hourly melatonin levels and took other measures of sleep.
They used a polysomnograph to measure brain waves, heart rate, breathing and eye movements. The device also measures how long it takes to fall asleep, the time spent asleep and how long is spent in each stage of sleep.
To assess how sleepy the participants felt, the team asked them to fill in a quick subjective questionnaire known as the Karolinska Sleepiness Scale.
The researchers conclude that using light-emitting e-readers before bedtime "prolongs the time it takes to fall asleep, delays the circadian clock, suppresses levels of the sleep-promoting hormone melatonin, reduces the amount and delays the timing of REM sleep, and reduces alertness the following morning."
The researchers also measured the brightness of light emitted by several types of e-reader, including iPad, iPhone, Nook Color, Kindle, and Kindle Fire. They found the iPad, Nook Color and Kindle Fire emit similar amounts of light; however, the iPad was the brightest of them.
The Kindle e-reader model they included does not emit light - like a paper book, it reflects ambient light.
More studies of light-emitting devices required as we live in a 'sleep-restricted society'
In their background information, the team notes that over the past 50 years, our length and quality of sleep have gradually diminished, "with adverse consequences on general health."
A survey recently showed that 9 out of 10 Americans use some kind of electronic device within 1 hour of bedtime on at least a few nights a week. Prof. Chang says:
"We live in a sleep-restricted society, in general. It is important to further study the effects of using light-emitting devices, especially before bed, as they may have longer term health consequences than we previously considered."
The National Institutes of Health and the National Center for Research Resources funded the study.
Meanwhile, Medical News Today recently learned about a study that found alcohol may disrupt the body's sleep regulator. Researchers showed that while alcohol may induce sleepiness, it can also disrupt it and eventually causes insomnia by disrupting the body's system for regulating sleep.