To Help Children Sleep, Go Dark
Children’s eyes let in more light than adults’ eyes do.
You may think I am speaking metaphorically, perhaps about how open children are to the world and its beauties, or how easily their brains are shaped by what they see and hear. But it turns out that children’s eyes are anatomically slightly different, and they do let in more light, and a new study suggests that exposure to bright light before bedtime can throw their body clocks out of whack.
In an article published this week in the journal Physiological Reports, researchers report on an experiment in which they measured levels of melatonin, the hormone that regulates sleep, in a group of 10 children, ages 3 to 5. First they had the children follow a regular sleep schedule for five days, and checked their saliva several times a day to measure their baseline levels of melatonin. Then, on day six, they turned children’s homes into low-light “caves,” covering the windows with black plastic and swapping in low-wattage light bulbs.
Lameese D. Akacem, an instructor at the University of Colorado, Boulder, who was the lead author on the study, said that the children spent the whole day in the dim light, again with researchers tracking their melatonin levels. Then, the next day, an hour before bedtime, the preschoolers were exposed to bright light for an hour, by playing on a “light table” — a glass-topped surface containing a bright light source.
“We know from a lot of studies done in adults, adolescents, school children, that the body clock is very sensitive to light exposure,” Dr. Akacem said. “Particularly in the evening, it tends to suppress the sleep-promoting hormone melatonin.”
But it had not been studied in preschool age children, and there was reason to believe that they might be even more sensitive.
“We were trying to simulate what would go on at home when you put a child to bed,” said Dr. Monique LeBourgeois, an associate professor of integrative physiology at the University of Colorado, Boulder, and the director of the sleep and development lab; she was the senior author on the study. A child who is resisting being put to bed may come out of the dark bedroom and approach the parents in rooms where lights are on, she said, and “they get blasted with light. Just even a short exposure of bright light may suppress melatonin and shut down that sleep-promoting effect.”
The average bedtime for the children in the study was 8:27 p.m., and in their dim-light “caves,” with no bright light to interfere, the researchers found that the children began secreting melatonin, on average, at 7:47 p.m., marking the beginning of their “biological night.”
And then the next night they played on the light tables. “We found that the bright light exposure suppressed melatonin by almost 90 percent, and the effects persisted even after the kids returned to dim light,” Dr. Akacem said. Fifty minutes after the light was gone, most of the children were still not back to 50 percent of the melatonin levels seen the day before.
Melatonin secretion is usually low during the day, and then rises in the evening, causing the body clock to prepare for sleep. The melatonin comes from the pineal gland, located between the two hemispheres of the brain, which is neurologically connected to body clock central, the suprachiasmatic nucleus in the hypothalamus, which in turn is influenced by how much light gets through to the retina.