Reading a light-emitting ebook before bed is bad for your health, according to a new US study. It warned that use of the devices affected both sleep at night and alertness the following morning.

Researchers from Harvard Medical School’s sleep medicine department put 12 healthy young adults through a two-week experiment, in which the participants would either read a light-emitting ebook for four hours before bedtime or a printed book. Study participants reading a light-emitting ebook took on average almost 10 minutes longer to fall asleep and said they were less sleepy an hour before bedtime than they were reading a paper book.

They also had suppressed evening levels of the sleep-promoting hormone melatonin – readers of print showed no suppression – and significantly less REM (rapid eye movement) sleep than print book participants. The next morning, they took “hours longer to fully ‘wake up’ and attain the same level of alertness”, researchers have reported in a new paper published in the journal of the Proceedings of the National Academy of Sciences (PNAS).

Harvard’s Anne-Marie Chang, Daniel Aeschbach, Jeanne Duffy and Charles Czeisler wrote that sleep quality and duration has declined over the past 50 years, adversely affecting general health. They point to a recent survey which found that 90% of Americans use an electronic gadget at least a few nights a week before going to sleep. The Harvard study participants were reading on an iPad, but researchers said other devices would cause the same effect. (Lead researcher Czeisler told the BBC: “The light emitted by most ereaders is shining directly into the eyes of the reader, whereas from a printed book or the original Kindle the reader is only exposed to reflected light from the pages of the book.”)

In the paper the researchers write: “The use of light-emitting electronic devices for reading, communication, and entertainment has greatly increased recently. We found that the use of these devices 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. Use of light-emitting devices immediately before bedtime also increases alertness at that time, which may lead users to delay bedtime at home.”

They point out that the use of technology before bedtime is “most prevalent” in children and young adults, and call for further studies on the impact of the light exposure on learning and development.

They add that the results of the study are especially concerning given the “recent evidence linking chronic suppression of melatonin secretion by nocturnal light exposure with the increased risk of breast, colorectal and advanced prostate cancer associated with night-shift work”. Another concern of sleep deficiency is the increased risk of cardiovascular disease and metabolic diseases such as diabetes and obesity.

Angus Phillips, director of the Oxford International Centre for Publishing Studies, said previous research had also shown that “the blue light emitted by phones and other gadgets is disruptive to sleep”. “At the end of the day many people want to get away from their gadgets and have a rest from screens, so a printed book is perfect,” he said. “Although some heavy hardbacks can be quite unwieldy.”

Phillips noted that previous research has found that most readers prefer print, but when reading performance such as accuracy and speed is compared, some screens have shown higher levels. “This may explain why the highest takeup for digital reading is of genre fiction – linear reading often at some pace,” Phillips explains. “Also, when you reach the end of the second part of a gripping trilogy, you can simply download the next part and carry on reading through the night.” He points to other benefits of ebooks, including the option to adjust type size and contrast on a back-lit screen, and that ereading may appeal to those who don’t want to disrupt a partner who wants to sleep.

“But other types of book should be more contemplative reads,” he says, noting that in his book Turning the Page, he writes: “If books in digital form enable us to read faster, this may not necessarily be a good thing. To relax, to engage in deep thought are not encouraged by rushing through at speed. Just as we have a movement for slow food – in reaction to fast food – we should be advocating slow books – read aloud to children, broadcast on the radio, or taken at a leisurely pace in whatever format.”