A study by CU Boulder researchers finds that a night light may disturb sleep in preschoolers. Even exposure to low intensities of light an hour before bedtime can disrupt melatonin production. As this sleep-inducing hormone decreases, it makes it difficult to fall asleep.

Furthermore, the study found that a night light may negatively impact sleep for hours after bedtime. The findings were published earlier this month in the Journal of Pineal Research.

 Funded by the National Institutes of Health, the research marks the latest in a series investigating the circadian rhythm of young children. The study suggests that preschoolers have a higher sensitivity to night lights, with some showing more physiological impacts than others.

“Our previous work showed that one, fairly high intensity of bright light before bedtime dampens melatonin levels by about 90% in young children,” said first 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 intensities of light, even dim ones.”

Light impacts our circadian rhythm more than any other biological or environmental factors. Our central body clock regulates many functions in the body, from sleep to hunger to body temperature. When light hits the retina, a signal travels to a brain region called the suprachiasmatic nucleus. This coordinates rhythms throughout the body, including melatonin production each night.

If light exposure occurs during the evening when melatonin naturally increases, it can slow or even inhibit the hormone. Then, the body and mind become confused, thinking it’s still daytime, even late at night.

University Explains Why A Night Light May Disrupt Sleep in Preschoolers

night light

This impacts preschool children even more because their eyes have larger pupils and more translucent lenses. Therefore, light can more easily travel into their eyes. In fact, one study found that 1.2 times more blue light streams into a 9-year-old’s eye than an adult’s.

“Kids are not just little adults,” said senior author Monique LeBourgeois, an associate professor of Integrative Physiology. She’s one of only a handful of researchers worldwide studying the circadian rhythm of young children. “This heightened sensitivity to light may make them even more susceptible to dysregulation of sleep and the circadian system.”

To study their vulnerability to night lights, the researchers collaborated with  Colorado School of Mines mathematician Cecilia Diniz Behn for the study. They recruited 36 healthy children between the ages of 3 and 5. The nine-day study required the children to wear a wrist monitor, keeping track of their sleep and night light exposure.

Parents ensured their children stuck to a stable bedtime routine for seven days, so their circadian rhythm remained stable. This way, their melatonin levels would start increasing around the same time each night.

On the eighth day, researchers turned the childrens’ rooms into a “cave,” as they jokingly called them. They covered the windows with black plastic and dimmed the lights to do this.


Then, they took saliva samples from the children every half-hour, beginning in the early afternoon until after bedtime. This allowed scientists to measure the children’s melatonin levels and document when their biological nighttime began.

During the final day of the study, the preschoolers played games on a light table in the hour leading up to bedtime. They sat about a foot from the light table, similar to a person using a phone or tablet. The children were exposed to varying light levels, ranging from 5 lux to 5,000 lux. For perspective, a candle about 3 feet away emits one lux of light.

Children More Susceptible to Sleep Disruptions from Night Lights

As expected, their melatonin production fell drastically compared to the nights with minimal light exposure. After using the light table, the children’s melatonin levels declined by 70 to 99%. However, the researchers didn’t notice a correlation between light intensity and melatonin suppression. On the other hand, studies on adults have found an intensity-dependent response.

Even when children had dim light exposure of 5 to 40 lux – much darker than normal room light – melatonin declined by 78%. What’s more, even after 50 minutes of darkness, melatonin didn’t increase in over half of the children tested.

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

Despite the findings, parents don’t need to shun night lights and transform children’s rooms into “caves.” However, it’s vital to monitor children’s screen time before bed in the age of technology. Have rules in place, so kids know when to shut off electronics and wind down for bedtime.

By dimming the lights an hour or two before bed, children will learn healthy sleep habits and enjoy better quality sleep. Keep in mind that a tablet emits up to 100 lux when turned to full brightness and held 1 foot away in a dark room.

For children who already have a hard time falling and staying asleep, it’s even more crucial to instill sleep hygiene.

“They may be more sensitive to light than other children,” said LeBourgeois, noting that genes — along with daytime light exposure — can influence light sensitivity. “In that case, it’s even more important for parents to pay attention to their child’s evening light exposure.”

night light

Final Thoughts on How Night Lights Impact Melatonin in Preschoolers

A recent study by CU Boulder researchers revealed that preschoolers’ sleep might suffer when a night light illuminates their room. Shockingly, the team found that the children’s melatonin levels declined by 70 to 99% when they used a light table before bed. Light intensity ranged from 5 to 5,000 lux, but even minimal light exposure suppressed melatonin by 78%. Therefore, children need to adhere to a bedtime routine and put away electronics before bed.

Today, many young children use electronic devices at bedtime, but we shouldn’t normalize this practice. As children have vulnerable, developing brains and bodies, they need all the rest they can get. If you’re a parent, make sure to talk with your children about the importance of prioritizing sleep. It’s never too early to teach them about fostering healthy habits.