Overtired or undertired? How to actually tell the difference

There's a particular bedtime that makes no sense. You did everything the same as the night it worked, the room is dark, the routine was calm, and your baby is still wide awake an hour later or screaming the place down. And the advice you find splits straight down the middle, half of it telling you they're overtired and half telling you they're undertired, which is not especially helpful when you're trying to work out which one you've got.

Unfortunately, being overtired and undertired can look almost identical at the pointy end of the day, because both of them end in the same place, a baby who won't settle and struggles to stay aslep.

Why they look so similar

Both states are about sleep pressure, which is just the build-up of tiredness across the time your baby is awake. Get the balance right and they go down fairly easily. Get too much pressure or too little and the wheels come off, and from the outside the wheels coming off can look the same either way. A baby fighting sleep is a baby fighting sleep. You have to look deeper to understand the problem.

I want to be clear, one off sleeps that aren’t great, is absolutely normal and rarely anything to look into. Also, it’s not always that X time awake = great sleep. There are so many different factors that go into sleep pressure, that usually taking the pressure off us as parents (pun intended) is the way we can read their cues better and know how to time sleep well most often.

What overtired actually looks like

Overtired is usually the wired one. They get a second wind, go a bit giddy and hyper, maybe cry in a more frantic way, and they're genuinely hard to wind down even though every part of you can see they're exhausted. Then when they do finally crash, the sleep is often short or broken, an early-morning start or frequent wakes, because they went to sleep with their system still running hot.

The mechanism, roughly, is that when sleep pressure climbs too high, the body stops letting them wind down and starts keeping them alert instead, cortisol kicking back in when it should be dropping away. It's the same thing that happens to us when we push way past our own bedtime, or come off a long-haul flight, that wired, past-it feeling where you're exhausted and somehow still can't drop off. So the more overtired they get, the harder sleep becomes, which feels deeply unfair when you're in it.

What undertired actually looks like

Undertired is usually the calm one that still won't sleep. They're not upset, they're just not drowsy. They'll lie there happily babbling, or take forty minutes of patting to drop, or go down fine and then wake an hour into the night ready to party, or have a long stretch awake in the middle of the night for no obvious reason. None of it reads as distress. It reads as a baby who simply doesn't have enough tiredness in the tank yet to stay asleep. I will say, I have met some babies who are undertired and when they wake overnight they’re very very angry, but it is rare!

This is the one people miss most, because we're so primed to worry about overtiredness that an undertired baby gets misread, and then the fix makes it worse. If you respond to a calm, undertired baby by bringing bedtime even earlier, you will get more of the same.

How I actually tell them apart

This is where a lot of the online advice, and honestly a lot of what the AI tools will tell you, gets it wrong. Almost everything gets labelled overtired, when if you actually look at the full clinical picture it often isn't. So rather than reading the meltdown in front of me, I look at two things.

The first is how much sleep they're getting across the whole twenty-four hours, compared to what's typical for their age. That's usually a clear sign that tells you whether there's genuinely too much sleep in the system or not enough (compared to “average”). A baby getting well above what most babies their age need is a different situation to one getting well below it, even when bedtime looks the same from the outside.

The second is their mood when they wake, and this one is probably the most useful of the lot. A baby who isn't tired enough is usually pretty content, happy to lie there and hang out, happy to be awake, because their body genuinely doesn't need the sleep yet. A baby who's overtired is usually the opposite, unsettled and unhappy, because their body wants the sleep and isn't being allowed to get there. Content and awake tends to point undertired. Distressed and fighting it tends to point overtired.

I also look at how sleep has changed, if they’ve been on the same schedule or awake windows for a long period, usually undertired. As developmentally, we need to expect they will reduce their sleep needs.

Wake windows and the timing of the day all feed into this, and they're worth paying attention to, but they change with age and temperament and the kind of day you've had, so I don't chase a number off a chart. I'm reading this particular baby, on this particular day, through how much they've slept overall and how they seem when they surface.

If you take one thing from this, let it be that the day is your evidence as to what’s happening, and so is the mood they wake up in. How does your little one seem when they wake at the moment, content to hang out, or unhappy and fighting it?