Why some bedtime struggles start before the routine begins

Routines, overstimulation, and daily habits all shape how easily a child falls asleep. Many bedtime challenges can be improved just by working on consistency, transitions, and calming activities. But in some cases, even when everything seems in place, the child still resists sleep.

That is when we have to look deeper – at how light interacts with the body’s sleep system. Evening light can delay melatonin release. Without that internal signal, the brain does not fully switch to nighttime mode. The child may lie in bed, physically tired but still alert.

A 2018 study by Akacem and colleagues looked at this very effect in preschoolers. What they found was measurable, immediate, and easy to miss in daily life.

The one-hour test 

Ten healthy preschoolers, around age four, followed a regular bedtime for 6 nights. Lights were kept low at home – below 15 lux, about as dim as a soft nightlight.

Then came the test night. One hour before their usual bedtime, researchers introduced a bright light table – about 1000 lux, like a fully lit kitchen. The children sat near it for one hour. Every 10 to 30 minutes, researchers collected saliva to measure melatonin.

What happened to melatonin

Measure	Dim-light night	Bright-light night
Average melatonin during the hour	Expected increase under dim light	87.6% lower (mean suppression)
Melatonin 50 min after lights off	Expected post-bedtime peak	Still below 50% of dim-light levels in 7 of 10 children

Melatonin stayed suppressed throughout the full hour of bright light exposure. Even 50 minutes after the light ended, levels had not recovered to 50% of baseline in 7 out of 10 children.

Why the signal is stronger in preschoolers

Younger children absorb more light. Their pupils are wider. Their lenses are clearer. That means more light reaches the part of the brain that controls melatonin. This single exposure flattened the evening rise in melatonin. 

In real life, many families repeat this kind of exposure daily – screens, strong overhead lights, TV. The effect does not just happen once. It happens night after night.

Melatonin takes time to recover. Dimming the lights 10 minutes before bedtime is too late. The brain still thinks it is daytime. That makes bedtime resistance more likely, even when the routine is well planned.

Older children show similar patterns, but preschoolers are more sensitive (Higuchi et al., 2014; Crowley et al., 2017).

How to use this with families

Create a one-hour low-light zone
Use soft, warm lighting under 50 lux. This could mean table lamps, string lights, or dimmable bulbs. No ceiling lights.

Remove screens during the last hour
Even at arm’s length, tablets and phones give off enough blue light to delay melatonin. Swap this time for books, coloring, or calm play.

Work in steps
If change feels overwhelming, suggest small steps. Dimming lights is the easiest place to start. Replacing screens can follow. A low nightlight is fine- but keep it close to the floor and very dim.

Use language that helps parents act
Try this: “That last hour before bed is when the brain decides it’s night. If it sees bright light, it does not start preparing for sleep. If we dim the room, the body starts doing the work on its own.”

This study gives you a clear way to explain why bedtime can feel harder than it should. You can explain how to protect the signal that makes the routine work.

References

Akacem, L. D., Wright, K. P., Jr., & LeBourgeois, M. K. (2018). Sensitivity of the circadian system to evening bright light in preschool-age children. Physiological Reports, 6(5), e13617. https://doi.org/10.14814/phy2.13617

Crowley, S. J., & Eastman, C. I. (2017). Human adolescent phase response curves to bright white light. Journal of Biological Rhythms, 32(4), 334–344. https://doi.org/10.1177/0748730417713423

Higuchi, S., Nagafuchi, Y., Lee, S. I., & Harada, T. (2014). Influence of light at night on melatonin suppression in children. Journal of Clinical Endocrinology & Metabolism, 99(9), 3298–3303. https://doi.org/10.1210/jc.2014-1629