Toddler sleep struggles are a pretty normal occurrence for many families.  Many report it taking a long time for children to fall asleep and tantrums around bedtime are frequent in many households.  The common suggestion is akin to earlier sleep training with families insisting on children staying in bed, coming up with hall passes, and so on.  But what if the problem isn’t behavioural?  What if it’s actually biological?  More specifically, physiological?  Research out of the University of Colorado, Boulder suggests that for some, there may be a physiological reason our toddlers struggle with sleep so much.  

Dr. Monique LeBourgeois and colleagues set out to determine if the circadian process influenced nighttime settling difficulties in toddlers, specifically bedtime resistance and sleep-onset delay (i.e., how long it takes the child to fall asleep after being put to bed).  Why look at the circadian process?  Although many people are aware of the sleep-wake homeostatic system (i.e., our sleep pressure), when we sleep, our sleep pressure decreases and in order for us to sleep longer throughout the night, we need our circadian process to be strong enough to overcome the tendency to then wake.  The specific hormone of interest in this research was melatonin, which is a part of the circadian process.  

Melatonin is regulated by the pineal gland and follows a 24-hour cycle, with levels hitting a peak in the middle of the night (to help us sustain sleep) and are lowest in the middle of the afternoon.  Notably, melatonin is suppressed when there is enough light (which is why its apex is in the middle of the night and its lowest point is in the middle of the day when the sun is at its peak), meaning that our light exposure can modify the rhythm that is typically based on this 24-hour cycle (and this will be a topic to cover in depth on another day).  All individuals have a what is called a dim-light melatonin onset (DLMO) which is a time at which their melatonin levels naturally cross a threshold (unless suppressed by light) and sleep onset is closely related to this time, but typically happens approximately 1-2 hours after this stage (depending on age).  

Adults will generally recognize this time as being when we are tired enough we are ready to sleep.  However, younger children rarely have the opportunity to make this selection, with parents often choosing the child’s bedtime, regardless of how this time matches their biology.  Thus these researchers aimed to explore how parent-selected bedtimes interacted with DLMO in their toddlers to predict settling difficulties.

What did they do?

Fourteen aged 30-36 months participated.  There were strict criteria for inclusion including:

  • Toddlers followed a strict sleep schedule with overnights of 10.5 hours or more and one nap opportunity of 45-min or more per day (and of which they fell asleep at least 3 times per week)
  • No regular bedsharing
  • Similar bedtimes and rise schedules on weekdays and weekends (less than 2 hours apart between weekdays and weekends)
  • No travel across time zones within 3 months of the study 
  • No medication used that would affect sleep
  • No sleep problems
  • No developmental disabilities or medical conditions
  • Born between 37 and 42 weeks gestation
  • No low birth weight at birth
  • No family history of sleep or psychiatric disorders

To put in perspective how strict this was, they screened 78 families for the study and only had 32 meet the criteria and 16 agreed to take part (2 lost data).  Of course, this will have effects on the applicability of the study to other individuals, but this can be necessary to avoid confounding factors.  

Prior to the in-house portion, parents completed questionnaires regarding sleep and the struggles they have regarding settling.  The in-house portion of the study took part over 6 days.  On days 1-5, researchers collected saliva samples to get kids used to the process of collecting samples and children went to sleep on the same parent-defined schedule each night and did not eat or drink anything that would impact their sleep.  Sleep was measured using a sleep diary and actigraphy.  On day 6, researchers conducted a DLMO assessment which includes children having to give a saliva sample every 30 minutes for 6 hours, ending one hour past their average bedtimes.  All of these assessments were done in dim lighting, with researchers ensuring children were in dim light for an hour prior to the first assessment and a light meter was used to ensure that children were not being exposed to light that would impact their sleep.  

What did they find?

There were two elements of interest: The DLMO time and the angle of entrainment.  The angle of entrainment is the time between DLMO and bedtime (not sleep time).  A narrow angle of entrainment means that the bedtime was closer to the DLMO.  For example, if one person’s DLMO is 7:30pm and bedtime is 7:45pm, that would be a very narrow angle of entrainment because typically sleep onset is an hour after DLMO.  

In looking at the DLMO time, researchers found that the average time was 7:40pm +/- 48 minutes and this was, on average, 30 minutes prior to bedtime.  In line with previous research on the relationship between DLMO and sleep time, sleep onset was on average around 30 minutes after bedtime, making it around one hour past DLMO.  

With respect to the relationship to bedtime, children with later DLMOs often struggled more as they took longer to fall asleep after lights-out and were rated by their parents as having a harder time falling asleep (settling difficulty).  This relates to the other finding regarding the angle of entrainment: The more narrow the angle (i.e., the closer the bedtime to DLMO), the more struggles getting toddlers into bed at bedtime and the longer the sleep-onset latency.  Interestingly, there was no relationship between DLMO and parent reports of children’s sleep overnight (maintenance and arousals).

What does this mean?

The main finding is that when parents are trying to put children to sleep too early – as evidenced by their DLMO – they also report far more settling difficulties.  Notably, the problems are not the child, but rather the expectation of an early bedtime by the parent.  Simply pushing bedtime back and letting children find their own rhythm may alleviate some of the nighttime struggles parents have.  

The researchers also call out the “limit-setting disorder” that suggests sleep disorders (especially with sleep onset) are actually limit-setting disorders with the parents.  I know I have heard this many times, that the struggle is more to do with limits than anything else.  This research counters that and suggests the struggles children have are due to a mismatch between their internal biology and the external environment, that is parent-set bedtimes.

For parents, the take-home message should be that if you’re struggling with your toddler’s sleep, especially sleep onset times and resistance, consider pushing bedtime back.  It may make all the difference.  

 

Reference

LeBourgeois, M. K., Wright Jr, 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.