Basking in the blue glow of tablets, smartphones, and other devices before bedtime could be messing up our sleep patterns more profoundly than we realize, a new study says.

"It may be having a greater impact than we previously thought," said Anne-Marie Chang, a professor at Pennsylvania State University and a study co-author.

During a two-week inpatient stay for 12 adults, some were asked to read on an iPad for four hours each night before bedtime for five nights. Others read printed books in dim light. After a week, the groups switched.

Researchers found that those using iPads had reduced levels of melatonin, a hormone that typically rises in the evening and helps induce sleepiness. They took longer to fall asleep and spent less time in restorative REM, or rapid-eye movement, sleep.

The iPad readers were also sleepier and less alert the following day even after eight hours of sleep.

"A lot of people think this is psychological," said Charles Czeisler, director of the Division of Sleep Medicine at Harvard Medical School. "But what we showed is that reading from light-emitting, e-reader devices has profound biological effects" that are not unique to iPads. Other products produce similar blue-light emissions, including tablets, e-readers, smartphones, laptops, and LED monitors. Czeisler said the standard Kindle e-reader, which does not emit light, was an exception and was more like a printed book. -L.A.Times