Contrary to research done on adults, a recent study found that children may be highly sensitive to light exposure of any intensity before bedtime.
Light exposure is known to be a crucial factor in circadian rhythm regularity, and a study published in Journal of Pineal Researchfound that young children may be highly sensitive to light before bedtime. Even exposure to dim light in the hour before sleep suppressed melatonin levels in preschool-aged children, and these levels remained lower after exposure.
Prior to this study, whether different light intensities were linked to different melatonin suppression percentages in children was unknown as far as the authors were aware. In adults, melatonin suppression related to light depends on light intensity.
“Our previous work showed that one, fairly high intensity of bright light before bedtime dampens melatonin levels by about 90% in young children,” said study author Lauren Hartstein, PhD, a postdoctoral fellow in the Sleep and Development Lab at University of Colorado, Boulder, in a statement. “With this study, we were very surprised to find high melatonin suppression across all intensities of light, even dim ones.”
The study, which is part of a series on circadian rhythms in young children, included 36 healthy children aged 3 to 5 years with no parent-reported sleep or neurological disorders. Participants were screened for eligibility through online questionnaires and a phone interview.
Those included in the study wore a wrist monitor to objectively measure sleep and light exposure for a 9-day protocol in their respective homes. For the first 7 days, children followed a strict parent-chosen sleep-and-wake schedule that included at least 10 hours of time in bed per night. Parents completed a sleep diary and confirmed adherence to the schedule with researchers.
On day 8, the participants’ homes were transformed into dimly lit environments with black plastic window coverings, low wattage bulbs, and dimmer switches. The average light level was approximately 1.5 lux, and participants entered the dimly lit space 4.5 hours before the scheduled bedtime on day 8 until 1 hour past the scheduled bedtime on day 9. Participants’ baseline dim-light melatonin onset (DLMO) was measured via saliva samples at 20- or 30-minute intervals starting 3 hours and 20 minutes before their typical bedtime on day 8. Samples were taken until 1 hour past the scheduled bedtime, and light levels were measured at the time each sample was taken.
On day 9, the children remained in the low-light environment and were then set up for 1 hour of light exposure before their scheduled bedtimes. They played at a table with a dimmable flat LED panel set to the desired light exposure between 10 and 5000 lux. They played with items like translucent blocks or colored on transparencies to ensure light exposure. Saliva samples were taken before, throughout, and after light exposure on day 9.
During the 1-hour light exposure period, melatonin levels were 69.4% to 98.7% lower compared with participants’ baseline DLMO measurements. Light intensity and percentage decrease were not directly related in this study group—all intensities of light exposure had high suppression values, although those exposed to 40 lux or lower had less melatonin suppression compared with participants who were exposed to more intense light. But even at low intensities such as 20 or 40 lux, melatonin was roughly 70% lower than baseline. These findings differ from the light intensity–dependent melatonin suppression seen in research on adults.
“Kids are not just little adults,” said senior author Monique LeBourgeois, PhD, an associate professor of integrative physiology who has studied circadian biology in young children. “This heightened sensitivity to light may make them even more susceptible to dysregulation of sleep and the circadian system.”
In 62% of participants, melatonin levels were below 50% of baseline levels even 50 minutes after the end of light exposure. Melatonin recovery was a mean of 45.2% but ranged from 3.8% to 98.7% throughout the group, and there was no significant correlation between melatonin recovery and light intensity.
“These findings highlight the importance of reducing light levels in the home before bedtime in order to support healthy sleep and circadian rhythms in young children,” the authors wrote.
Study limitations included the cohort of healthy children with no sleep issues, meaning the findings may not be generalizable to a broader group. Each participant was also assigned a single light intensity, so differences between individual photosensitivity could not be analyzed. Further studies are needed to explore the effects of prior light history on photosensitivity in children, the authors concluded.
Hartstein LE, Behn CD, Akacem LD, Stack N, Wright KP Jr, LeBourgeois MK. High sensitivity of melatonin suppression response to evening light in preschool-aged children. J Pineal Res. Published online January 8, 2022. doi:10.1111/jpi.12780