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THE QUESTION As technology becomes more a part of everyday life, it’s not uncommon to check e-mail, exchange text messages and do some reading on a laptop or tablet before going to bed. Might doing such things on devices that emit light affect your sleep?

THIS STUDY involved 12 healthy adults (average age, 25). For five nights in a row, they read an electronic book for about four hours before bedtime in an otherwise dimly lit room. On five other consecutive nights in the same room, they read a printed book for about four hours.

When reading from the light-emitting e-book, compared with reading a book in print, participants were less sleepy at night and took longer to fall asleep, had lower levels of melatonin in their blood at night and were less alert the next morning, based on blood samples, recordings from sensors that tracked such things as brain waves, heart and breathing rate, oxygen levels and eye and leg movements and information from the participants.

WHO MAY BE AFFECTED? Adults who use electronic devices before bedtime. Light is considered one of the strongest factors affecting the body’s circadian clock, which regulates the sleep-wake cycle. Normally, natural levels of the hormone melatonin are highest at night, helping to ease the body into sleep and maintain the normal cycle. However, light can suppress melatonin secretion and disrupt the circadian cycle.

CAVEATS The number of participants in the study was small. Data on sleepiness came from the participants’ ratings, using standardized scales. The researchers noted that many newer models of light-emitting devices are brighter than those used in the study and might have a stronger effect. Study participants held their devices 12 to 18 inches from their eyes, exposing the retina to less light than if devices were held closer to the eyes, as is common, especially with small devices.

FIND THIS STUDY Dec. 22 online issue of the Proceedings of the National Academy of Sciences (www.pnas.org; click on “All Early Edition”).

LEARN MORE ABOUT sleep at nhlbi.nih.gov (search for “healthy sleep”) and www.mayoclinic.org.

The research described in Quick Study comes from credible, peer-reviewed journals. Nonetheless, conclusive evidence about a treatment's effectiveness is rarely found in a single study. Anyone considering changing or beginning treatment of any kind should consult with a physician.