Using light-emitting tablet devices in the evenings can delay bedtimes, interfere with a sleep-regulating hormone, and impair alertness in the morning, a study has found. The study, published in the journal Physiological Reports, compared nine healthy adults for five consecutive evenings of unrestricted use of light-emitting tablet computers versus evenings reading from printed materials. "These findings provide more evidence that light-emitting electronic devices have biological effects," said Jeanne Duffy, from Harvard Medical School in the US. "Using light-emitting electronic devices in the late evening can postpone our decision to go to sleep, and make us more sleepy the next morning," said Duffy. On evenings when using light-emitting tablets, participants' self-selected bedtimes were on average half an hour later, and they showed suppressed melatonin levels, delayed timing of melatonin secretion onset, and later sleep onset. Melatonin is a hormone that regulates sleep and wakefulness. When using the tablets, participants rated themselves as less sleepy in the evenings and less alert in the first hour after awakening on the following mornings.