New research shows children are very sensitive to bright light before bedtime, adding to concern about night-time digital technology use.

One hour of exposure to bright light before bedtime almost completely shuts down the production of the sleep-promoting hormone melatonin in preschoolers and keeps it suppressed for at least 50 minutes, a US study has found.

In response to the proliferation of digital technology, researchers assessed the physiological impact night-time light exposure has on the circadian system of children, the body's internal clock.

"It is really important to study the effects of light in young children and how it can effect sleep especially because sleep problems that develop in childhood can persist later on in life and have a multitude of negative consequences, said lead author Lameese Akacem, a researcher in the Sleep and Development Lab at the University of Colorado Boulder.

"In this study we found that these kids were extremely sensitive to light," she said.

For the study, published in journal Physiological Reports, researchers enrolled 10 healthy children aged three-to-five in a seven-day trial.

After five days of following a strict bedtime schedule - to normalise their body clocks - the children were then exposed to bright light by playing at a 'light table' before bedtime on days six and seven.

Analysis of saliva samples taken from the children throughout the week found melatonin levels were 88 per cent lower after bright light exposure compared to no light exposure.

Melatonin is a hormone that helps to regulate sleep and wakefulness.

It's thought structural differences of the eye during childhood may make the young more vulnerable to night time use of digital devices, said co-author Monique LeBourgeois, an associate professor in the Department of Integrative Physiology.

"Light is our brain clock's primary timekeeper," Professor LeBourgeois said.

"We know younger individuals have larger pupils, and their lenses are more transparent. This heightened sensitivity to light may make them even more susceptible to dysregulation of sleep and the circadian clock," she said.

Professor LeBourgeois explained that when light hits the retina in the eye in the evening, it produces signals to the circadian system to suppress melatonin and push back the body's entrance into its "biological night".

For preschoolers this may not only lead to trouble falling asleep one night, but to chronic problems like not feeling sleepy at bedtime.

It's hoped the research will help parents and clinicians make more informed decisions about a child's use of digital technology at night.

"The preschool years are a very sensitive time of development during which use of digital media is growing more and more pervasive," Professor Le Bourgeois said.