What’s causing your preschooler to lose 88 percent of a sleep-promoting hormone before bedtime?

Reading a storybook right before bedtime may seem like a gold-star worthy parenting routine, but a new University of Colorado Boulder study is calling for lights out long before kids’ heads hit the pillow.

A first-of-its-kind study published in Physiological Reports determined bright light before bedtime decreases young children’s natural melatonin levels by 88 percent and keeps them lower for at least 50 minutes.

“We know from a lot of research studies in adults that the body clock is very sensitive to light, specifically in the evening,” said lead author Lameese Akacem, a CU Boulder instructor and researcher in Boulder’s Sleep and Development Lab. “But there haven’t been any studies at all looking at this for young children.”

The study consisted of getting 10 children aged 3-to-5-years-old on a strict, matching bedtime schedule over the course of five days. On the sixth day, the kids’ bedrooms got a black-out treatment, covering any windows with black plastic and switching light bulbs to a low-watt variety, regulating their light-intake. Researchers playfully referred to the dark spaces as “the cave.”

Akacem and her team took saliva samples from the kids to find out their melatonin levels at different times of the day. After a day romping around in “the cave,” researchers introduced a table top that put off as much light as a bright room — 1,000 lux — for the kids to color or play on.

When the kids’ melatonin samples were taken again after the light interaction, their sleep hormone levels were down 88 percent.

“The study sample size was small and it used only one intensity of light, 1,000 lux, which is far greater than the intensity of a typical handheld electronic device,” researchers noted.

The light wasn’t the right intensity to determine how screen time before bed affects sleep, but it did find that room brightness is a big factor in how easily a child will give in to counting sheep.

Similar studies in adults found light 10 times as bright as the table top the kids were exposed to decreased the adults’ melatonin by only 39 percent.

The dramatic decrease in melatonin surprised Akacem, who said the nearly 90 percent drop has motivated her team to continue studying the issue with varying intensities of light.

Why are little ones’ eyes so much more sensitive to light than their parents? Children have larger pupil sizes, and their lenses are more clear than adults, Akacem said.

Her advice: “If a kid’s in bright light right before bed, that bright light could still be affecting melatonin levels almost an hour after the lights are turned off, so avoid light exposure an hour before bedtime.”

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