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What determines whether we’re night owls or morning larks?

Scientists in the University of Colorado Boulder recently found that 4- 5-year-olds who go to bed later and are exposed to brighter nighttime light experience delays in the timing of their brain’s central timekeeper—the biological clock. That, in turn, could lead to night-owl schedules that are associated with a host of health problems.

Your central clock is a tiny—smaller than a half-peanut—but essential region housed deep in the brain, behind the area between your eyes. Our clocks keep us in sync with day and night; they send signals throughout our brains and bodies that encourage us to wake up and be active during the day and lull us to sleep at night. When our clocks become delayed, we can stay up later in the evening.

Scientists have studied the impact of light at night on the timing of the central clock in school-age children, adolescents and adults, but not in preschoolers. Monique LeBourgeois, senior author, lab director and professor in the Department of Integrative Physiology, is a pioneer in investigating the timing of the circadian clock and factors that influence it in young children.

“To our knowledge, we’re still the only group in the world that’s doing this work,” says LeBourgeois. “We know very little about the primary fundamental systems that influence sleep, especially pediatric sleep. There’s a huge knowledge deficit.”

Why is it important to understand what factors influence the clock in preschoolers? Poor sleep patterns commonly emerge in childhood, explains Lameese Akacem, lead author on a published in November in the journal Neurobiology of Sleep and Circadian Rhythms.

Children who experience delays in the timing of their clocks have more trouble falling asleep and are more likely to resist bedtime, previous work from LeBourgeois’ laboratory has found. “When kids don’t sleep well, parents tend to be sleepy and feel more stressed,” says LeBourgeois.

Also, early life sleep habits can persist. Importantly, late sleep patterns are associated with health problems later in life; night owls are more likely to suffer from mental illness, heart disease and obesity.

Both bedtime and light can be modified to shift sleep habits, Akacem says. These findings may help parents prevent night-owl sleep patterns as well as the associated health risks down the road.

“One of the things that really drives me as a researcher and excites me about what we are doing in our lab is that I truly believe, by studying the basic characteristics of the clock, we can translate the work to the real world and help parents whose kids are having sleep problems,” says LeBourgeois.

In the study, 21 healthy children followed their regular sleep schedules for four days. They wore wrist devices that tracked body movements, and donned light meters—on Disney-themed lanyards—that monitored light intensity in their surroundings. On the fifth day, researchers made house calls and, under dim-light conditions, collected saliva samples, which were used to measure the hormone melatonin.

Melatonin release is orchestrated by your clock and is used as a measure of your internal timing. Our clocks boost melatonin at night; when this hormone surges, we feel tired. During the day, specialized cells in our eyes sense light, then send signals to our clocks to halt melatonin. Light at night delays the evening melatonin surge, hijacking the natural timing of our clocks and keeping us awake.  

The researchers found that children who went to bed later experienced more delayed melatonin release on the fifth day. The impact of bedtime on the clock is partially due to light, because, naturally, going to bed late makes it more likely that a child will see light at night. When researchers set aside the influence of bedtime, they found that children exposed to more light at night also showed more delayed melatonin secretion.

Thanks to this work, “We know that light, over and above a child’s bedtime, is super important in influencing the timing of the clock,” says LeBourgeois.

Scientists think children are more sensitive to light than adults because their eyes are different, Akacem explains. Kids have larger pupils and clearer lenses. LeBourgeois’ research provides the first clues to understanding how light uniquely affects the biological clock of young kids.

LeBourgeois and her team are continuing to better understand how light at night impacts children’s clocks. In an unpublished follow-up study, researchers found that exposing children to a light at night, equivalent to a brightly lit room, can suppress their melatonin levels by about 90 percent.

Here, researchers tracked melatonin release over time from saliva samples, similar to the previous study. On the second day, the researchers repeated the same procedure, but instead, they exposed the children to bright light for an hour until bedtime.

Getting children to stare at a bright light for an hour can be tricky, so Akacem got creative. She and her team fashioned a light table by placing a light source inside of a storage bin, then a covering it with a clear top. Kids colored pictures on transparencies atop the brightly lit table.

Can bright light at night eventually program children into becoming night owls for life? “We don’t know for sure, but it is certainly possible,” says Akacem.

However, we do now know that nighttime light has a significant impact on children’s clocks. This work helps us understand how sleep problems develop and can help us improve our children’s overall health and well-being.  

For LeBourgeois, this research lays the foundation for more studies. She will soon receive a National Institutes of Health grant to study how different intensities of light can affect children’s clocks. This study begins soon, so if you have a healthy, good-sleeping preschooler, call the lab to find out how to get involved, at 303-492-4584.

LeBourgeois’ team is also investigating if extending sleep in young children with behavioral sleep problems can improve their emotional processing. If your child battles bedtime, struggles to fall asleep, or doesn’t get enough sleep, he or she may qualify for the study.