A new study has found that even low exposure to light can cause the sleep-inducing hormone melatonin to drop in preschoolers in the hour before bedtime, potentially disrupting their sleep.
The study was published in the Journal of Pineal Research.
The study is the latest in a series funded by the National Institutes of Health examining how unique infants’ central clocks are. It suggested that preschool children are very vulnerable to the physiological effects of light at night, and some children may be even more sensitive than others.
“Our previous work has shown that a fairly high intensity of bright light before bedtime reduces melatonin levels by about 90 percent in young children,” said first author Lauren Hartstein, a postdoctoral researcher in CU Boulder’s Sleep and Development Laboratory.
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“In this study, we were very surprised to find high melatonin suppression at all light intensities, even low ones,” she added.
Light is the body’s most important zeitgeber and influences the circadian rhythm, which regulates everything from when we feel tired or hungry to our body temperature throughout 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 the nightly production of melatonin. When this exposure occurs in the evening, since melatonin naturally increases, it can slow or stop it and delay the body’s ability to enter biological night time.
Because children’s eyes have larger pupils and more transparent lenses than adults, light streams in more freely. (A recent study showed that the transmission of blue light through the eyes of a 9-year-old is 1.2 times higher than that of an adult).
“Children are not just small adults,” said senior author Monique LeBourgeois, associate professor of integrative physiology and one of the few researchers worldwide studying the circadian biology of young children.
“This increased photosensitivity can make them even more susceptible to sleep and circadian system dysregulation,” she added.
To quantify how vulnerable they are, researchers collaborated with mathematician Cecilia Diniz Behn of the Colorado School of Mines for a new study.
They recruited 36 healthy children, ages 3 to 5, for a nine-day protocol in which they wore a wrist monitor that recorded their sleep and light exposure. For seven days, the parents kept the children on a stable sleep schedule to normalize their body clocks and get them into a pattern where their melatonin levels spike at around the same time every night.
On day eight, the researchers transformed the children’s home into what they playfully dubbed a “cave” — with black plastic on the windows and dimmed lights — and took saliva samples every half hour from early afternoon until after bedtime. This allowed the scientists to get a baseline on when the children’s biological night began naturally and what their melatonin levels were.
On the last day of the study, the young study participants were asked to play games on a light table in the hour before bed, a pose similar to a person looking at a glowing phone or tablet. Light intensity varied between individual children, ranging 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 of minimal light, melatonin was suppressed between 70 and 99 percent after light exposure. Surprisingly, the researchers found little to no association between the brightness of the light and the drop in the key sleep hormone. In adults, this intensity-dependent response is well documented.
Even in response to light measured at 5 to 40 lux, which is much dimmer than typical room light, melatonin fell an average of 78 percent. And even 50 minutes after the lights went out, melatonin didn’t come back in more than half of the children tested.
“Taken together, our results suggest that in preschoolers, light exposure before bedtime, even at low intensity, results in robust and sustained suppression of melatonin,” Hartstein said.
That doesn’t necessarily mean parents have to throw away the night light and keep kids in total darkness before bedtime. But at a time when half of children are using screen media before bed, the study reminds all parents to turn off devices and keep lights to a minimum to encourage good sleep habits in their children. Remarkably, at full brightness, a tablet held 30 cm from the eyes in a dark room measures up to 100 lux.
For children who already have sleeping problems?
“They may be more sensitive to light than other kids,” LeBourgeois said, noting that genes — along with exposure to daylight — can affect photosensitivity.
“In this case, it’s even more important for parents to pay attention to their child’s evening light exposure,” she added.
This story was published from a wire agency feed with no changes to the text.