Reading an iPad or a Kindle before bed instead of a printed book can cause sleep deprivation and increase the risk of cancer, scientists say.
They claim it disrupts normal sleep patterns and can lead to waking up earlier and drowsiness the next day.
The light from the devices suppresses the hormone melatonin, low levels of which have been linked to increased risk of breast, prostate, and bowel cancer, scientists in Boston say.
Dr Anne-Marie Chang, who led the study, said the body's normal rhythms were interrupted by the short-wavelength enriched light, otherwise known as blue light, from these electronic devices.
"Participants reading a light-emitting e-reader took longer to fall asleep and had reduced evening sleepiness, reduced melatonin secretion, later timing of their circadian clock and reduced next-morning alertness than when reading a printed book."
Dr Charles Czeisler, head of the Sleep and Circadian Disorders unit at Brigham and Women's Hospital, where the study was carried out, said sleep length and quality has been declining for 50 years.