Why Do Babies Smile in Their Sleep? A Scientific Explanation

14th Jul 2021


Baby’s first smile is a huge milestone. It’s probably the one parents look forward to the most after their baby is born.

It’s quite common for them to grin goofily while dad tickles them or while mom plays with them. But they also smile while they’re asleep! Definitely a photo-worthy moment.

But why do babies smile in their sleep? Here’s what we know about why babies smile even when they’re out for the count.

The Reason Babies Smile

Parents grin like a goofball when looking at their little champ or princess. When they smile back, it warms our hearts to think that they love us just the same!

But those first few cute smiles are most likely not because your baby is happy to see you. Sorry!

Baby smiles happen for a variety of reasons. Here are the three most common reasons for your little one’s happy face.


For the first three months of your baby’s life, any smiles you glimpse will most likely be a reflex action. Your little one actually begins smiling before it’s even born!

These types of smiles don’t need any reason to occur. They happen simply because the body does it as a reflex, like kicking or squirming.

There’s no external stimulus for these little grins. In the first few months, even if daddy is making funny faces, those smiles will be purely accidental.


By the age of about 3 months, your little one should be able to smile as a response to interaction with others.

By this age, they will have had some time to engage with their environment. They should be learning how to respond to stimuli.

At this stage, when you make a funny face and tickle them, their smile could be a genuine reaction to your attention!


Smiling can be a reaction to having gas. It’s actually more like a wince, as air gets trapped in the stomach or intestine and causes pain or discomfort.

There’s not a lot of scientific research to back this up. But it’s got the nod from countless parents who’ve experienced it!

This is particularly common amongst colicky babies. But it can happen to all babies. 


Why Does This Happen During Sleep?

There are conflicting thoughts about whether or not babies dream like adults do. Large parts of our dreams are based on memories.

Some research suggests that infants dream about their life in the womb. This makes sense, as it's the dominant memory that they have.

Here are some reasons why your little one may be grinning while they’re sleeping. 

REM Sleep Movement

If you aren’t familiar with sleep cycles, here’s a quick overview:

  • Stage 1 & 2: Non-REM sleep. The body slows down and falls into a state of deep relaxation.
  • Stage 3: Non-REM sleep, also called deep sleep or slow-wave sleep.
  • Stage 4: REM sleep, also known as “active sleep” in babies and children.

Some research suggests that REM sleep is stage 5 and stage 4 is another deep sleep cycle. But regardless, REM sleep is when we dream.

It’s still not entirely clear whether babies dream like we do or not. But there’s a high chance that they go through the same (or very similar) sleep cycles.

REM sleep can cause involuntary movements of the body. Usually, this is in response to something happening in a dream.

If babies dream of being in the womb, there’s a chance that they may smile as they’re dreaming.


A baby’s emotional development starts from the moment they’re born. As they interact with the world, they learn how to respond to both physical and social stimuli.

The brain processes emotion during sleep. This starts from a young age! If your baby smiles or laughs in his sleep, it could be because some happy emotions are being processed.

Medical Causes 

It’s rare, but medical conditions can sometimes cause your little one to smile as they sleep. A particular type of seizure disorder can cause uncontrollable laughing or grinning.

These seizures can occur both during wakefulness and while asleep. They last between 10 and 20 seconds, and they’re generally harmless.

These mini fits don’t display many symptoms. A grin, twitch, or kick could be a sign, but it could just as easily be a normal reflex action.

If you’re worried, monitor your baby closely. Things that may warrant concern include:

  • A vacant stare, grunting
  • Unusual movements
  • Extended periods of irritability
  • Sleep issues

If they happen multiple times, a visit to the pediatrician is a good idea. 



Why do babies smile in their sleep? For many reasons!

Usually, it’s nothing to be concerned about. You can gush over your little cutie while they’re grinning in their sleep without worrying.

But pay attention and seek medical help if other unusual symptoms are present.

Don’t worry, though! 99% of the time, when your little one smiles, it’s a lovely and positive sign of their development.