Wed. Aug 17th, 2022

Colorado: Researchers have found that exposure to even dim light can cause the sleep-promoting hormone melatonin to plummet in preschoolers in the hour before bedtime, disrupting their sleep.

The study has been published in the ‘Journal of Pineal Research’.

The study is the latest in a series, funded by the National Institutes of Health, that examines how young children’s central body clock is unique. It suggested that preschoolers are very susceptible to the physiological influences of light at night, and some children may be even more sensitive than others.

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“Our previous work showed that a fairly high intensity of bright light before bedtime attenuates melatonin levels by about 90 percent in young children,” said lead author Lauren Hartstein, a postdoctoral fellow in the Sleep and Development Lab at CU Boulder.

“With this study, we were very surprised to find high melatonin suppression across all light intensities, even faint ones,” she added.

Light is the body’s primary time signal, affecting circadian rhythms that regulate everything from when we feel tired or hungry until our body temperature is during the day.

When light hits the retina, a signal is sent to a part of the brain called the suprachiasmatic nucleus, which coordinates rhythms throughout the body, including nocturnal production of melatonin. If this exposure occurs in the evening, as melatonin is naturally rising, it may slow or stop it, delaying the body’s ability to transition to biological night time.

Because children’s eyes have larger pupils and more transparent lenses than adults, light flows more freely into them. (A recent study showed that the transmission of blue light through a 9-year-old’s eye is 1.2 times higher than in an adult).

“Children are not just small adults,” said senior author Monique LeBourgeois, an associate professor of Integrative Physiology and one of the few researchers in the world studying the circadian biology of young children.

“This increased sensitivity to light can make them even more susceptible to dysregulation of sleep and the circadian system,” she added.

To quantify how receptive they are, the researchers collaborated with Colorado School of Mines mathematician Cecilia Diniz Behn for a new study.

They hired 36 healthy children aged 3 to 5 years for a nine-day protocol in which they carried a wrist monitor that tracked their sleep and light exposure. For seven days, parents kept the children on a steady sleep schedule to normalize their body clocks and arrange them in a pattern where their melatonin levels rose at about the same time each night.

On the eighth day, researchers transformed the orphanage into what they playfully described as a “cave” – ​​with black plastic on the windows and the light dimmed – and took saliva samples every half hour from early afternoon until after bedtime. This allowed the researchers to get a baseline for when the children’s biological night naturally began and what their melatonin levels were.

On the last day of the study, the young subjects were asked to play games on a light table in the hour before bedtime, a posture similar to that of a person looking at a glowing telephone or tablet. The light intensity varied between the individual children, from 5 lux to 5,000 lux.

(One lux is defined as the light from a candle 1 meter or about 3 feet away).

Compared to the previous night with minimal light, melatonin was suppressed anywhere from 70 percent to 99 percent after light exposure. Surprisingly, the researchers found little to no correlation between how bright the light was and how much the main sleep hormone dropped. In adults, this intensity-dependent response has been well documented.

Even in response to light measured at 5 to 40 lux, which is much weaker than typical room light, melatonin decreased by an average of 78 percent. And even 50 minutes after the light was off, melatonin did not return in more than half of the children tested.

“Taken together, our results indicate that exposure to light before bedtime in preschool children, even at low intensities, results in robust and sustained melatonin suppression,” Hartstein said.

This does not necessarily mean that parents should throw off the night light and keep children in absolute darkness before bedtime. But at a time when half of children use screen media before bedtime, the research serves as a reminder to all parents to turn off gadgets and keep light to a minimum to promote good sleep habits in their children. It is noteworthy that a tablet with full brightness kept 1 foot from the eyes in a dark room measures as much as 100 lux.

For those kids who already have sleep problems?

“They may be more sensitive to light than other children,” LeBourgeois said, noting that genes – along with daylight exposure – can affect light sensitivity.

“In that case, it is even more important for parents to be aware of their child’s exposure to evening light,” she added.


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