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If you’re reading in bed using a tablet, you may feel sleepy tomorrow morning.

A study has found that reading from a light-emitting device, such as an e-reader, before bedtime can shift your body’s natural clock and delay the onset and characteristics of your sleep. And that could leave you feeling groggier the morning after, according to the study published online Monday in the journal Proceedings of the National Academy of Sciences.

A dozen people at the sleep lab at Boston’s Brigham and Women’s Hospital spent five consecutive evenings reading a book for four hours under reflected light, and five evenings viewing an iPad for the same duration (the order of the five-day blocks was randomly assigned). All had a mandatory bedtime of 10 p.m. and a 6 a.m. wake-up.

Blood tests showed those who read from the computer tablet had suppressed evening levels of melatonin, and on the next day, the increase in that hormone occurred 90 minutes later, an indication of a shift in their body’s circadian cycle, according to the study. Long-term suppression of melatonin by nighttime light exposure has been linked to increased risk of certain cancers, the study noted.

The study subjects using e-readers also took 10 minutes longer to fall asleep and had nearly 12 minutes less rapid-eye-movement sleep, a stage that has been linked to memory consolidation, the study found.

Tablet-reading participants also rated themselves as less sleepy in the evenings, a subjective measure that also correlated with weaker electroencephalogram, or EEG, readings that are associated with transition to sleep, the study found.

Those who used the e-readers also described themselves as more sleepy the morning after, and needed more time to feel fully alert.

“We really didn’t anticipate it would have an effect the following morning, especially after an eight-hour sleep opportunity — and study subjects slept the same amount of time,” said neuroscientist Anne-Marie Chang, a bio-behavioral health expert from Penn State University who works at Brigham’s division of sleep and circadian disorders.

Although the magnitude of the melatonin shift was large, the smaller REM and sleep delays also may contribute to the morning-after effects, Chang said.

Researchers suspect the quality of the light from computer devices matters more than its overall intensity relative to reflected light cast on a book.

“These devices are enriched for short-wavelength light, which is in the blue range,” Chang said.

Previous studies have shown that exposure to such wavelengths strongly affects the body’s circadian clock, compared with exposure to light of longer wavelengths.

Researchers warned that the effects they measured in a laboratory, with a mandatory lights-out, may understate the problem in the real world.

“If you’re in a home environment and you’re reading on a light-emitting device and you’re not feeling sleepy, chances are you’re not going to stop and go to sleep at the time that you’re supposed to,” Chang said.

There are alternatives, however — some devices don’t emit enriched light, and there are ways to alter such emissions. The study did not examine whether there is a rebound period after putting down the devices that might lessen their effects on sleep.

“There are so many things we don’t know about how these devices affect our health and our sleep,” she said. “How much time before sleep do you need to kind of wind down? Is there a window when this light won’t have that effect? Those are very interesting questions that need further investigation.”