Does overtiredness exist?

Question

I found it difficult to locate any peer reviewed articles that attempted to define this concept, but Whittingham and Douglas (2014) state on p4 that

[B]abies are thought to easily become overtired if parents miss early tired cues, and an overtired baby is thought to have greater difficulty in falling asleep.

In my view this matches fairly closely how the term seems to be used informally in online discussions about parenting. The idea is that if you miss the opportunity to put a baby to sleep when they're mildly tired, they can get "overtired" and then it will be harder to put them to sleep if you had tried earlier. This is sometimes contrasted with adult sleep, where it gets easier to fall asleep the more tired you are.

Whittingham and Douglas seem to state that overtiredness doesn't really exist.

In fact, with increased tiredness comes a greater tendency to fall asleep—the very definition of sleepiness (high homeostatic sleep pressure) is that the individual, given opportunity, falls asleep.

I'm wondering firstly whether overtiredness exists at all, and if it does exist, at what ages does it operate?

Whittingham, K., & Douglas, P. (2014). Optimizing parent–infant sleep from birth to 6 months: a new paradigm. Infant mental health journal, 35(6), 614-623.

Answer 1

The term "overtiredness" somewhat oversimplifies the problem which is probably why you are questioning the science behind it. It seems logical that the more tired anyone is, the more sleepy they are, and the easier they fall asleep. As your quote says, "with increased tiredness comes a greater tendency to fall asleep". But the term "overtiredness" in children is applied to a compound problem and means far more than just being extra tired.

When a baby is 'overtired', they are so physically fatigued that their stress-response system is activated. Stress hormones such as cortisol and adrenaline flood their bloodstream, making it difficult for them to relax and calm down. They are tired and want to sleep, but a conflicting set of hormones is preventing them. It would be rather like an adult drinking a strong coffee before bedtime - their body would be tired and releasing adenosine to promote sleep, but the caffeine inhibits this, so there is a conflict.

This is definitely what I have observed in my own children, particularly my youngest, currently aged two. She loves sleeping and climbs into bed without any fuss, sleeping 12 hours straight most nights. But if she is overtired, perhaps because she missed her daytime nap, she will still fall asleep but then wake after a couple of hours, and often again any number of times through the night. Babies have a much shorter sleep cycle than adults. They come into light sleep more often through the night, so if they are unsettled due to the stress hormones in their system there are more opportunities for them to wake.