You know that family. The one where everything looks perfect on paper. The bedtime routine is solid. The room is dark. The schedule is consistent and appropriate. The child naps very well. But every single night? Battle time.
The child won’t settle. They take 1 or 2 hours to fall asleep. Or they resist going to bed at all.
Here is what might surprise you… Sometimes this is not a behavior problem. It is a biology problem.
What the Research Found
Researchers from the University of Colorado and the University of Zurich studied something most of us cannot see with our eyes. They looked at the internal body clocks of 14 healthy preschoolers aged 30 to 36 months (LeBourgeois, Wright, LeBourgeois, & Jenni, 2013).
They measured something called dim light melatonin onset. We call it DLMO for short. Think of DLMO as the exact moment when your child’s brain flips the “getting ready for sleep” switch. Melatonin starts rising. Body temperature drops. The sleep window opens.
The average DLMO for these preschoolers? 7:40 PM. But there was a big range between children.
Here is the interesting part… On average, parents were putting their children to bed about 30 minutes after their DLMO started. But this varied a lot between families. Some children were put to bed very close to their DLMO. Others had more time between their DLMO and bedtime.
Here are the patterns
The researchers found 2 important patterns.
Pattern One: Later DLMO = Longer time to fall asleep
Some children naturally have a later DLMO than others. One child might have a DLMO at 7:30 PM. Another child might have a DLMO at 8:30 PM. Both are normal.
But here is what the researchers found: children with later DLMOs took longer to fall asleep once they were in bed. Parents also reported these children had more trouble falling asleep.
What does this mean for you? A child with a naturally later DLMO (say 8:30 PM) will struggle more at bedtime than a child with an earlier DLMO (say 7:30 PM) – even if both are put to bed at the same time.
Pattern Two: Small gap between DLMO and bedtime = More bedtime battles
Now here is the second finding. And this one is really important for your consultations.
The researchers looked at the time gap between each child’s DLMO and their actual bedtime. Remember, some children were put to bed very soon after their DLMO started. Others had more time.
What they found: children who were put to bed very close to their DLMO had two problems:
- They took longer to fall asleep (measured objectively)
- They fought going to bed more – calling out, coming out of the room, refusing bedtime
Think about it this way. Imagine a child whose DLMO starts at 8:00 PM. If parents put this child to bed at 8:10 PM, there is only a 10-minute gap. This child will likely resist bedtime and take a long time to fall asleep.
But if the same child goes to bed at 8:45 PM, there is a 45-minute gap. This gives the melatonin time to rise and the body time to prepare for sleep. Bedtime goes more smoothly.
Why does this happen?
When you try to put a child to bed right when their DLMO is starting, their body is just beginning to prepare for sleep. The process is not complete yet. The child does not feel sleepy enough. So they fight it.
This is not about the child being difficult. This is biology.
The Science Behind
Sleep researchers talk about something called the “forbidden zone for sleep.” This is the time right before your natural sleep window when your brain actively fights sleep.
You know this feeling yourself. Try going to bed at 6:00 PM when you normally sleep at 11:00 PM. Your body resists, even if you are tired.
For preschoolers with a DLMO at 8:00 PM, a 7:45 PM bedtime lands them right in this forbidden zone. No amount of rocking or singing will override their biology.
Why Preschoolers Are Different
Here is something surprising. Regularly napping preschoolers have much earlier circadian phases than older children and adults.
The researchers found that these preschoolers have an average DLMO at 7:40 PM. They compared this to previous studies showing prepubertal children at 8:33 PM, adolescents at 9:29 PM, and adults aged 22-38 at 8:35 PM (LeBourgeois et al., 2013).
Preschooler internal clocks run earlier. But many parents set bedtimes that work for older siblings or their own schedule.
The researchers also found that the time between DLMO and bedtime was much shorter in these young children (about 30 minutes) compared to adolescents (71-116 minutes) and adults (129 minutes).
What This Means for Families
You cannot measure DLMO in your practice. The protocol needs dim lighting, saliva samples every 30 minutes for 6 hours, and lab analysis. Not exactly practical for a home visit.
But you can use this research to change how you talk with parents.
When a family tells you their 2-year-old “just will not go to bed” at 7:00 PM, ask this: Is this bedtime working with or against their child’s biology?
Look for these signs that bedtime might be too early:
- Takes more than 30 minutes to fall asleep consistently
- Shows increased activity at bedtime – not tired behavior
- Settles quickly on nights when bedtime is later
- Naps well but bedtime is a battle
- No signs of being overtired at bedtime
If you see these patterns, the solution might not be an earlier bedtime. It might be a later one.
The Age Factor You Need to Know
The researchers note that bedtime resistance peaks in early childhood. According to the studies they reviewed, settling difficulties increase dramatically between infancy and age 3 years (LeBourgeois et al., 2013).
Why does this happen?
The researchers suggest that as children develop, their circadian rhythms change. If parents do not adjust bedtimes to match these shifts, the mismatch grows. The child’s biology drives what looks like “resistance.”
This is not about poor parenting. It is not about weak boundaries. It is about biology bumping up against family schedules.
Every Child Is Different
The study showed big individual differences. DLMOs varied across children. This means there is no magic bedtime that works for all preschoolers.
Some preschoolers genuinely are ready for sleep at 7:00 PM. Others are not ready until 8:00 PM or later. Both can be healthy and normal.
This explains why generic advice fails. You already see this in your practice when one-size-fits-all approaches do not work.
How to Use This in Your Consultations
When you meet a family with bedtime battles, add these questions:
“What time does your child fall asleep on nights when you skip the battle?”
“Does your child seem genuinely tired at bedtime, or energetic and playful?”
“If bedtime runs late, do they fall asleep quickly?”
The answers tell you if this is behavioral or circadian.
For circadian timing issues, you might recommend:
- Shifting bedtime 15-30 minutes later
- Watching for natural sleep cues instead of clock time
- Using bright light in the morning to shift the rhythm earlier if needed
- Starting the bedtime routine when the child shows biological readiness
Why This Research Matters
The research indicates that about 25% of young children experience sleep disturbances. Many of these problems continue into later childhood.
When you help parents match bedtime with their child’s circadian rhythm, you do more than solve tonight’s battle. You might prevent years of sleep struggles.
The researchers point out that sleep problems in young children connect to attention issues, behavior problems, and emotional difficulties (LeBourgeois et al., 2013). Early childhood is when children learn important skills for school success. If bedtime battles lead to shorter sleep, children miss out on the rest they need for learning.
Reference
LeBourgeois, M. K., Wright, K. P., LeBourgeois, H. B., & Jenni, O. G. (2013). Dissonance between parent-selected bedtimes and young children’s circadian physiology influences nighttime settling difficulties. Mind, Brain, and Education, 7(4), 234-242. https://doi.org/10.1111/mbe.12032