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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.

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    The totally non-scientific answer from a mother of two: Heck, yes, it exists. In my experience it’s less about being “too late” on a regular day, but happens when there’s A lot of unusual activity going on - going on a visit, for example - and the mind struggles to process all the impressions. And that is a phenomenon I as an adult have also experienced: The body is tired, the mind is racing. (Which would mean the age bracket is 0-100, with decreasing severity.)
    – Stephie
    Sep 25 at 14:07
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    Same here. Anecdotally it clearly exisits
    – Hilmar
    Sep 26 at 12:43
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    Even adults get overtired. I find there's a period about around 9:30-10ish (my usual bedtime) when I get absolutely exhaused for ~30 min or so. If I push through till 10:30, I will be fine to stay up until midnight (and in fact, going to bed during that time is difficult). I think it has something to do with melatonin levels and when it is released in the body.
    – stan
    Oct 6 at 10:56
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    I experienced this from kindergarten to curent day. For me it exists, my poor mom, my poor wife...
    – Mołot
    Oct 8 at 23:18
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    I'm trying to find articles about this and the name that keeps showing up is Ståle Pallesen (a researcher at University of Bergen). I'm seeing plenty of blog posts that reference him / some study he did, but I'm having trouble finding the study. I'm trying to search through Google's scholarly search but haven't found it yet. scholar.google.com/…
    – Becuzz
    Oct 12 at 15:03
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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.

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    The reason we have the scientific method is because things everybody knows can be wrong. For example, the popular idea that sugar causes hyperactivity is not supported by science. All that said, I don't mean to imply that your answer is incorrect-Your arguments seem plausible to me. Oct 11 at 16:29
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    The scientific method sits uneasily with such anecdotal evidence because it is often subject to various biases, including in this case confirmation bias. People who think overtiredness exists will tend to observe overtiredness (or at least, will think they are witnessing overtiredness). But they haven't tested that rigorously. In my own experience (anecdotally), as one who intuits that the idea of "overtiredness" doesn't make sense, I also don't think I've ever witnessed it. That's either my bias or my honest observation, but I'm not in a position to discriminate the two. Oct 11 at 22:36
  • @alphabetasoup "Overtiredness" is an oversimplified term. If you didn't appreciate the science behind it, you would rightly ask why being more tired wouldn't make someone more sleepy. If you read my answer you'll now know that it does make them more sleepy, but the resulting stress brings other conflicting hormones into play. It would be like an adult drinking very strong coffee because they were extra tired, and then trying to sleep. They are still tired, but now they have an irritant in their system setting their nerves on edge and preventing sleep.
    – Astralbee
    Oct 12 at 15:03
  • @WayneConrad Sure - the only reason I've even talked about the scientific method is that the OP was asking for a scholarly, peer-reviewed paper on this topic. I'm not dismissing it in favour of popular opinion, rather saying that there is no reason to be so sceptical about it when the theory has already been put to the test.
    – Astralbee
    Oct 12 at 17:26
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    I'm not attempting to rip your answer apart: it's plausible, as Wayne Conrad says. It's just that I'd still like to have a citation to a study, that I can read for my own interest. That would actually answer the question. Oct 13 at 20:33

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