If you’ve done any sort of research about baby sleep, you’ve probably heard of overtiredness and the “dangers” of overtiredness.

I chat with families all the time who are so concerned about overtiredness. And as I’m delivering their sleep training plan and we’re going over the schedule, they express their concern about it!

When we had our first baby, I knew so little about sleep. But do you know what I did know about? Overtiredness. And I thought the reason her naps were always so short and she woke up all night long was because of overtiredness.

Could there have been some overtiredness in there? Absolutely, we also had no clue what we were doing schedule-wise. But I knew overtiredness was such an enemy of sleep and figured it had to be our issue.

And yes, being overtired can cause lots of challenges when it comes to your baby or toddler’s sleep.

But do you know what else can cause lots of challenges when it comes to your baby or toddler’s sleep, yet very few families are concerned about? UNDERTIREDNESS!

In fact, as a sleep coach of 6.5 years and a mom of four, I see way more undertiredness than overtiredness. But it’s just not talked about enough.

I had no clue that undertiredness was the reason my oldest struggled to fall asleep at bedtime. Even though she woke up from a nap at 6:15, I thought 7 pm was supposed to be bedtime and truly didn’t understand why it took so long for her to fall asleep or actually stay asleep.

I had no clue she would need bedtime pushed later as a toddler, because of undertiredness – I figured we’d set a bedtime and that would be bedtime!

In this post, I’m doing a deep dive into overtiredness and undertiredness, including:

• What it means when we say a baby or toddler is overtired or undertired
• How overtiredness and undertiredness impact baby and toddler sleep
• How to know if your little one is overtired or undertired
• How to use scheduling to balance overtiredness and undertiredness

And more!

Sleep Pressure Explained

In order to understand overtiredness and undertiredness, you need to first understand sleep pressure!

Sleep pressure is essentially what builds up between your little one’s naps, as well as all day leading up to bed, to help your child both fall asleep and stay asleep. So that when you lay them down, they’re at the “just right” level of tired to fall asleep.

Sleep pressure is actually a thing for adults, too! You know when you take that nap from 4-5 pm and still try to go to bed at 10:30 pm, what happens? Probably not sleep! Because that sleep pressure that had been building all day long got relieved a bit during that nap, and you don’t have enough to fall asleep at 10:30 pm. That would be considered undertired.

Our goal is to find the “tired enough” but not “too tired” sweet spot of sleep pressure – for both ourselves, but also our kids! Which is, of course, who we’re talking about in this post.

So when we’re talking about overtiredness and undertiredness, we’re essentially talking about way too much sleep pressure and not enough sleep pressure, both of which can make sleep really hard for our little ones.

Overtiredness Explained

When a baby or toddler is overtired, here’s what’s happening in their physical body…

Their stress hormones, namely cortisol and adrenaline, start producing, so instead of their body saying, “It’s time to rest,” their body is signaling, “Stay alert”, and, “Fight or flight.”

So what does overtiredness look like?

When a baby is overtired, they often start getting fussy. When a toddler or older child is overtired, they might start getting fussy, or whiny, or complainy, or they might trip more or get hurt more.

Or they might actually hit a second wind and get extra goofy or slap-happy!

And then when it comes to sleep, an overtired baby or toddler often has a harder time falling asleep and/or staying asleep. Your baby or toddler might have more night wakings, and/or wake up earlier in the morning, and/or take a shorter nap.

That deep, restorative sleep we’re going for gets interrupted more.

Undertiredness Explained

Undertiredness, on the other hand, is when your baby or toddler’s brain just isn’t ready for sleep, due to a lack of that sleep pressure.

We’re not going to get too scientific here, but if you’re curious to dig in even more, look up adenosine. Sleep pressure is essentially an accumulation of adenosine, so when we haven’t built up enough sleep pressure, our kids’ bodies just aren’t physically ready for sleep!

Unlike overtiredness, undertiredness doesn’t really have “a look.” So an easy trap to fall into is, “Well, because my baby doesn’t look tired – he’s not fussy, he’s not rubbing his eyes, he’s not staring into space – he must not have enough sleep pressure yet. I have to wait for the tired cues.”

DON’T hear me say you therefore need to follow tired cues! Because some babies don’t show tired cues, or not until it’s way too late! We’ll talk more about this in the scheduling section.

But just because undertiredness doesn’t really have a “look” during awake time, it certainly has “a look” in regard to sleep, and as I said, I think it can often be more frustrating than overtiredness.

So what does undertiredness look like in sleep?

An overtired baby or toddler often has a harder time falling asleep and/or staying asleep. Your baby or toddler might have more night wakings, and/or wake up earlier in the morning, and/or take a shorter nap.

Now you might be scratching your head, saying, “Great Lauren, I came to learn the difference between overtiredness and undertiredness, but you just said they look the exact same when it comes to sleep.”

And they do often look the same on the surface, or when we’re talking in general terms, but now I’m going to let you jump inside my brain a bit to see how, as a sleep coach and mom of four, I differentiate the two and figure out what a baby or toddler actually needs!

How to Know if Your Baby or Toddler is Overtired or Undertired

Before we talk about differentiating overtiredness and undertiredness, I want to start with a quick caveat…

As I describe the ins and outs of how I differentiate between being overtired and undertired, I’m doing so with a sleep-trained baby or toddler in mind, or a baby or toddler in the process of sleep training.

That’s not to say sleep pressure doesn’t impact kids who aren’t sleep trained; it absolutely does! But there are also other factors likely impacting how they fall asleep or stay asleep, so it’s often not just a schedule issue.

Whereas once your little one is sleep trained, there might be boundary brush-ups as they get older, or the crib-to-bed transition, or pulling milk out of the bedtime routine, but schedule adjustments are the main changes you’ll make after sleep training.

So if your little one isn’t sleep trained, this will still be helpful for you to know, but sleep pressure might not be the only issue.  

Bedtime: Overtired or Undertired?

Let’s jump in,  starting with bedtime. Both overtiredness and undertiredness can make it harder for a baby or toddler to fall asleep, but what that actually looks like usually varies.

An overtired baby or toddler might cry really hard or fight really hard when you put them in their crib. And then essentially crash and burn. So being overtired doesn’t mean it will take them forever to fall asleep, but it’s usually much harder or louder or fightier to fall asleep – and then they pass out.

A baby who usually falls asleep within 5-10 minutes with little to no peep might cry really hard for 8 minutes, and then pass out. – that’s an overtired baby.

Whereas an undertired baby or toddler will likely have a longer fight at bedtime. And it could either be really quiet or really loud. An undertired baby or toddler might roll around in the crib, chattering to themselves, blowing bubbles, or playing with their stuffy for 25 minutes before falling asleep. They’re not tired enough to fall asleep, so they’re just playing.

OR an undertired baby might cry on and off for 20 minutes because they’re mad they’re in bed – they want to play! “I’m not tired. What do you expect me to do?”

An undertired toddler might jump up and down, yelling for you, or scream and cry for 30 minutes, because they’re just not tired enough. “How dare you make me sleep? I’m not tired!” Their physical body just isn’t ready for sleep!

So many toddler parents come to me saying their toddler has been a great sleeper, but now all of a sudden, bedtime is such a battle. And do you know what the solution so often is? Push bedtime 30 minutes later to build up that sleep pressure some more – there’s a good chance they’re undertired!

Nights Wakings: Overtired or Undertired?

So what about the staying asleep part? Overtiredness and undertiredness can often make staying asleep harder.

For an overtired baby or toddler, they might have a “false start”, meaning 30-45 minutes after bedtime, you hear them cry out. Or within the first three hours of bedtime, you hear them cry out a few times. But it’s often short-lived, and they’re usually able to fall back asleep without needing the extra help.

For an undertired baby or toddler, they might wake up within the first few hours of the night, but what’s more likely to happen is they wake up in the middle of the night, and it takes an hour or two to fall back asleep. This is often referred to as a “split night,” and can be due to a lack of sleep pressure.

Early Morning Wakings: Overtired or Undertired?

Early morning wakings are definitely tougher to differentiate. Because again, they could be due to overtiredness or undertiredness.

These early mornings wouldn’t necessarily look different, as your baby or toddler could be bright-eyed and bushy-tailed at 5 am for either overtiredness or undertiredness (though overtiredness could certainly make them grumpier or sleepier).

So I’d have to dig into the schedule more here and look at overall awake time and also nap length. Did they barely nap that day? Or were their wake windows super long? Or did they get too much daytime sleep, or wake windows not long enough?

Short Naps: Overtired or Undertired?

Short naps are similar to early wakings, when we’re trying to differentiate between overtiredness and undertiredness.

Naps can be shorter due to either, so I’d be digging into the schedule more there to really feel more confident about what’s going on.

This might sound crazy, especially if you’ve been hearing about the dangers of overtiredness for so long. But if I had to choose between overtiredness and undertiredness, I think I’d choose overtiredness. Because you just can’t make a baby or toddler sleep when they’re not sleepy – how frustrating for you, when you just want them to sleep, and how frustrating to them when they just can’t do it.

Now again, stepping back for a moment, I want to remind you that I’m doing this deep dive with a sleep-trained baby in mind. Not because being overtired or undertired doesn’t impact everyone, but because it gets murkier when they’re not sleep trained.

If your baby is relying on you to rock them or feed them to sleep, they could be taking longer to fall asleep because of a schedule issue, or it could be because they’re stimulated by you. They could be waking up a lot because of a sleep pressure issue, or because they don’t know how to connect sleep cycles without your help. The split night could be undertiredness, but it could also be the pacifier.

I also want to clarify that just because your baby cries hard for 8 minutes before falling asleep doesn’t mean they’re always overtired. Or just because your toddler always takes a short nap doesn’t mean they’re always undertired. Baby and toddler sleep is more nuanced than that! I’m just trying to help break down overtiredness and undertiredness, and how I think through it.

Schedules and How to Balance Overtiredness and Undertiredness

Now let’s switch gears a bit and talk about schedules, because scheduling is how we find that sweet spot of sleep pressure.

This is why we talk about wake windows so much with babies! Wake windows are the age-appropriate length of time babies can handle being awake before becoming overtired. There are average wake windows based on age, and then, depending on the actual child, those might be adjusted a bit.

But starting with wake windows (NOT tired cues) and then finding a baby’s sweet spot is how we walk this balance of not being overtired or undertired.

It’s why having set schedules for babies and toddlers on two-nap and one-nap schedules is so helpful. Those schedules are based on wake windows, but also on body clock consistency. And it’s all about sleep pressure!

If you don’t know what schedule would be appropriate for your baby, make sure you download our FREE baby schedule guide.

That’s not to say your baby’s schedule has to be perfectly aligned to our guide, as all babies will be a bit different, but if you find your baby’s schedule is pretty far from what’s there – maybe their wake windows are 30+ minutes shorter or longer than what I have listed – and you’re having some sleep challenges? Sleep pressure could be a big factor!

The importance of scheduling for sleep pressure is also why I have individual blog posts on all of the nap transitions, because sleep pressure and what’s going on with sleep big picture is often what clues us into needing to drop a nap!

Sleep pressure also dictates how to go about that nap transition! It’s why an early bedtime is so necessary in the transition to one nap, or when transitioning to no more naps, because overtiredness gets so real so fast.

Sleep pressure is also why there’s such a thing as “too much daytime sleep”; we only need so much sleep in 24 hours! So for a 6-month-old who gets four hours of total daytime sleep, chances are high that the baby is going to wake up super early the next morning – they just don’t need any more sleep! That’s undertiredness.

Remember how I said I’d choose overtiredness over undertiredness pretty much any day?

Sometimes when we work with families, on early wakings, for example, I tell parents we’re going to be pushing their little one a bit and dancing with overtiredness. Maybe that’s a longer wake window before their first nap. And that it’s necessary in the short run for the sake of the schedule, and therefore sleep, as a whole.

But I’d never tell a parent to cut their baby’s final wake window 45 minutes shorter simply for the sake of getting in bed by 7:30 because that’s our ideal schedule. That undertiredness would be no bueno.

In all of this, I also need to say that some kids seem to be a lot more sensitive to overtiredness or undertiredness! So if you’ve been listening to this and you’ve never really been crazy about your baby or toddler’s sleep schedule and they’ve been totally fine, they probably just aren’t super sensitive to sleep pressure being off, and that’s amazing!

Our second daughter was really sensitive to overtiredness as a baby. Even 30 minutes “off schedule,” and we knew her nap would be short, or she’d wake in the night, or wake up extra early the next day. We just knew to expect it!

Thankfully she grew out of that in her toddler years. Whereas our third and fourth have been champs with us needing to push their schedule here and there.

Or our oldest stopped napping at 2.5 because, although she’d nap, she then would just not be tired enough at bedtime, and she’d fight it! She was undertired and mad about it.

Whereas our currently 3.5 year old still takes a 1.5-2-hour nap every day and still goes to bed around 7:45. And sometimes she takes 20-30 minutes to fall asleep, I’m sure due to undertiredness, but she’s totally content! And she has no problem overnight or early in the morning, so we’re just letting it be.

So don’t let this conversation scare you into thinking you have to be schedule crazy – that’s absolutely not my goal here. Some kids really do need more consistency for the sake of their body clocks, and some kids can be more flexible.

Conclusion

The goal of this post is to define overtiredness and undertiredness for you, and to hopefully bring undertiredness to your attention a bit more. The goal is also to help you differentiate between an overtired or undertired baby or toddler. So if you’re struggling with your little one’s sleep and you think it might be a scheduling issue, this info, paired with our free schedule guide, will help!

Also remember that sleep can be really hard. Finding the sweet spot of your little one’s schedule can be really tough. Knowing if your baby is overtired or undertired might be making your head spin.

I also want to remind you that if you had so many light bulb moments while reading this and you think undertiredness is what’s been wreaking havoc on your toddler’s sleep, you’ve not been doing anything wrong or messing things up. Sleep challenges are not your fault!  

Please remember that no one is expecting you to be the expert here. That’s why we, at Via Graces, and so many other pediatric sleep coaches exist, so we can help you navigate the wild world of baby and toddler sleep. So never hesitate to reach out if you need help!

With Grace,

Lauren