Tablets may be great for catching up on your reading, but new research suggests they may be disturbing your sleep.

It may take longer to fall asleep if you use an iPad or similar device, such as a laptop, before bedtime, according to a study in the Proceedings of the National Academy of Sciences.

"It may be having a greater impact than we previously thought," said Anne-Marie Chang, a professor at Penn State University and a co-author of the study, which was conducted at Brigham and Women's Hospital in Boston.

The neuroscientists say people using e-readers tended to feel less sleepy.

In the 14-day study,12 young people read for four hours before they went to bed in a dimly lit room.

The participants first read for four hours using an iPad before bed, for five consecutive nights. They then read from a printed book before bed for another five nights.

After the e-book nights, readers reported that they took an average of 10 minutes longer to fall asleep. They also spent less time in REM sleep – which is connected to dreaming and deep, restorative sleep.

Blood was also drawn from all the participants.

When a participant read from an e-book, there was a delay in the natural nightly surge in melatonin levels by more than an hour and a half, compared to when a printed book was read.

Melatonin level affected

The bright light from these devices seem to suppress melatonin, the sleep-promoting hormone that normally rises during the evening and reaches its highest levels in sleep, noted Chang.

"This light has serious consequences on sleep and on our alertness, not only while we're using these electronic devices, but the following morning as well, even after eight hours of sleep," Chang said.

In addition, the research team discovered that iPads emitted heavy doses of blue-wavelength light, which has been shown in previous research to suppress melatonin and increase alertness.

Blue-light emitting devices include laptops, cellphones, LED monitors and other electronic devices, the researchers added.

Chang said anyone wanting to read before bedtime should use a printed book, or use an e-book reader such as a Kindle that does not emit light.