Reading has long been suggested for those who can't sleep, but it looks like it does not apply on all kinds of reading, with researchers saying that reading on a device actually has the opposite effect of what is intended.

Researchers from Harvard Medical School published a study in the journal Proceedings of the National Academy of Science detailing the effects of using devices around bedtime and in the hours leading up to it. According to their findings, exposure to light in the evening suppresses the release of melatonin, a hormone that facilitates sleep, and disrupts the body's circadian rhythm which makes it more difficult to fall asleep.

Corresponding author Anne-Marie Chang, an associate neuroscientist from the Division of Sleep and Circadian Disorders at the Brigham and Women's Hospital, said that short-wavelength enriched light, more commonly referred to as blue light, is particularly to blame.

"Participants reading an LE-eBook took longer to fall asleep and had reduced evening sleepiness, reduced melatonin secretion, later timing of their circadian clock and reduced next-morning alertness than when reading a printed book," she explained.

Aside from taking almost 10 minutes more to fall asleep, subjects also had dramatically lower levels of REM sleep on days they used iPads.

Twelve subjects participated in the study, each one admitted to the Brigham and Women's Hospital to allow researchers to control certain factors and observe more closely. Subjects were made to use an iPad as an e-reader for five straight nights four hours before going to bed. The same was repeated with printed books. Whether subjects started with iPads or printed books was determined randomly. Subjects were also allowed to choose whatever material they wanted to read as long as it fell under "leisure reading" and didn't include puzzles or any images.

During the course of the study, researchers took blood samples every hour from the subjects to measure melatonin levels. Polysommography was also employed, recording eye movements, breathing, heart rates and brain waves to see how long subjects took to fall asleep, the overall amount of sleep achieved and how much time was spent in every stage of sleep. Sleepiness was measured using the Karolinska Sleepiness Scale.

"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," added Chang.

The study was supported by the National Center for Research Resources and the National Institutes of Health. Other authors include Charles Czeisler, Jeanne Duffy and Daniel Aeschbach.