Overtired vs undertired baby: how to tell the difference

By Angelica Videla — Certified Baby and Toddler Sleep Consultant, London | Supporting families across the UK, Europe, US, and Australia

Why this is happening

I see this quite a lot with families I work with 🤍

Sleep works best when there's the right balance between sleep pressure and rest. When that balance is off, sleep can start to feel unpredictable.

An overtired baby has been awake too long, which increases cortisol levels and makes it harder to settle and stay asleep.

An undertired baby hasn't built enough sleep pressure, so even if they fall asleep, they may wake quickly or stay awake for long periods.

The tricky part is that both can look quite similar on the surface, which is why it often feels confusing.

What's making it worse

• Stretching wake windows too much
• Letting naps run too long
• Inconsistent nap timing
• Adjusting one day at a time without a clear plan

What actually helps

The key is looking at your baby's full day, not just one moment.

This usually means:

• Adjusting wake windows to suit your baby’s age
• Balancing total daytime sleep
• Creating a more consistent daily rhythm
• Watching patterns over a few days rather than reacting to one night

Once that balance is right, sleep tends to become much more predictable.

How this might look in real life

The signs can look similar, which makes it tricky. Here is how to spot the difference.

• Baby is crying at bedtime and you are not sure if they need more or less awake time
• Nap refusal could mean the wake window is too short or too long
• Baby seems wired at bedtime — overtired babies often look wide awake
• Baby lies quietly in the cot for a long time before falling asleep, suggesting undertiredness
• You have been adjusting wake windows but are not sure which direction is right