It's long been known that using a screen right before bed can make it harder to sleep - and, now, the reason for this has become clearer.
Scientists at the Rensselaer Polytechnic Institute have found that just two hours' exposure to a device with a backlit display causes melatonin suppression.
"Our study shows that a two-hour exposure to light from self-luminous electronic displays can suppress melatonin by about 22 percent," says associate professor Mariana Figueiro.
"Stimulating the human circadian system to this level may affect sleep in those using the devices prior to bedtime."
The team gave 13 people a tablet to read, play games, and watch movies.They discovered that the duration of exposure and the distance between the eye and the display, which determines the amount of light reaching the back of the eye, affects melatonin levels.
While an hour's exposure had little effect, after a two-hour exposure there was significant suppression.
"We recommended dimming these devices at night as much as possible in order to minimize melatonin suppression, and limiting the amount of time spent using these devices prior to bedtime," says Figuero.
Suppression of melatonin by light at night has been implicated in sleep disturbances and an increased risk for diabetes and obesity. The risk of more serious diseases, such as breast cancer, also rises if circadian disruption occurs for many consecutive years, for example in night shift workers.
"Technology developments have led to bigger and brighter televisions, computer screens, and cell phones," says research specialist Brittany Wood.
"To produce white light, these electronic devices must emit light at short wavelengths, which makes them potential sources for suppressing or delaying the onset of melatonin in the evening, reducing sleep duration and disrupting sleep. This is particularly worrisome in populations such as young adults and adolescents, who already tend to be night owls."