People who read from an iPad for 30 minutes before going to sleep felt less sleepy and had different electrical activity in the brain during sleep than those who read from a physical book, a recent study found.

But the time it took to fall asleep and time spent sleeping were similar under both conditions.

"Since light has an alerting effect, we predicted a lower sleepiness in the iPad condition at bedtime compared to the book condition," said lead author Janne Gronli of the University of Bergen in Norway. But it was surprising that the iPad light did not delay sleep initiation, she said.

However, "we found a delay of 30 minutes in the generation of the restorative slow waves during sleep in the iPad condition," Gronli told Reuters Health by email.

The study included 16 nonsmokers ages 22 to 33 who were familiar with tablets and had no sleep, medical or psychiatric disorders. For a week before the study began, they were instructed to keep to a regular sleep-wake schedule and to stay in bed at least as long as they needed to sleep.

During the study, in which participants slept in their own beds, the researchers took polysomnographic recordings for three nights of sleep: one to collect a baseline of how each person slept, one night of reading from an iPad for 30 minutes before turning out the light and one night of reading from a book for the same amount of time.

On the night they read from a book, the participants used ordinary reading light in their bedrooms.

The polysomnographic recordings, including electroencephalograms to measure brain electrical activity, collected data on total sleep time, sleep efficiency, percentage of time spent in each sleep stage, and other aspects of sleep quality during the time between lights off and sleep onset as well as the time between sleep onset and first period of REM (rapid eye movement) sleep.

A light meter measured illumination at eye level while the participants were reading in each condition. Illumination was about twice as high while reading from the iPad compared to the book, and the iPad emitted a high level of blue light, the researchers note.

Bedtime and the time at which people got up from bed were similar in both conditions, with an average sleep duration of slightly less than eight hours on both days.

Participants said they felt sleepier when reading the physical book, as reported in Sleep Medicine. After reading from an iPad, EEG readings showed delayed and reduced slow wave activity, representing deep sleep, in the brain after sleep onset compared to when the participants had been reading from a book.

The eye absorbs short wavelength blue light and signals to the brain that it is daytime by triggering waking and alerting active brain areas, Gronli said.

"Slow wave sleep EEG activity is important for the restorative effect of our sleep," she said.

The brain's ability to synchronize cortical activity and generate slow waves with high amplitude when we sleep improves memory and cognitive performance, she said.

"The effects are not completely huge," Samer Hattar of Johns Hopkins University in Baltimore told Reuters Health.

And since the researchers only measured illumination, they could not compare the "structure" of the iPad light to regular bedroom light, Hattar said. Some indoor light bulbs also emit a great deal of blue light while others do not.

"We only examined one night using an iPad," Gronli said. "It is tempting to speculate that daily use of an iPad, and other blue light emitting electronic devices, before bedtime may have consequences for human sleep and cognitive performance."

"To avoid increased activation before bedtime the bedroom should be used to sleep in, not for work or being on social media," she said.

A new Apple OS update includes a nighttime mode, which takes into account the effects of too much blue light in the late evening by filtering it out, said Christian Cajochen, head of the Center for Chronobiology at the University of Basel in Switzerland who was not part of the new study.

"There is also an app called f.lux which does exactly the same," Cajochen told Reuters Health by email.