What Causes Toddler Sleep Resistance?

By Tracy Cassels

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 night-time 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. 

But what if the problem isn’t behavioural? What if it’s actually biological?

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 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 recognise 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.   

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