Babies often smile in their sleep because of brain development, reflexes, and light‑sleep activity, not because they’re “already joking” with you—though it definitely feels that way.

What’s really happening in their sleep?

  • Newborns spend a lot of time in active/REM sleep, when the brain is very busy and the body can twitch, grimace, and smile.
  • During this stage, facial muscles may activate on their own, creating tiny grins, smirks, or even brief laughs.
  • These early “endogenous” or reflex smiles are generated inside the brain, not as a reaction to something you’re doing in the moment.

Think of it like the brain doing “practice reps” with the face while the baby is off-duty.

Key reasons babies smile in their sleep

  • Reflex smiles (newborn phase)
    • Very common from birth through the first weeks.
* Considered a reflex, similar to sucking or rooting, and often appear during REM sleep.

* Not usually linked to specific emotions yet—more like automatic training of the smile muscles.
  • Brain and emotional development
    • Sleep is when babies’ brains process new experiences and build neural connections.
* Some experts think those little sleep smiles may reflect early positive emotional processing as the brain “replays” soothing sensations like warmth, touch, and familiar smells.
  • Comfort and contentment
    • Well‑fed, warm, and secure babies are more relaxed in sleep, which can naturally soften facial muscles into a smile.
* Feeling safe and bonded with caregivers around sleep time is linked with more settled, “smiley” sleep overall.
  • Possible dreams (a maybe, not a certainty)
    • Babies spend far more time in REM than adults, which is when vivid dreaming happens in older children and adults.
* Some theories suggest babies may “dream” of simple, familiar sensations, like being held or feeding, which could trigger smiles.

* Other researchers are cautious, noting that very young newborns may be too immature for complex dreams, so the smiles may still be mostly reflexive.

How smiles change as babies grow

  • In the first 0–6 weeks, most smiles—especially in sleep—are reflexive and not social yet.
  • Around 6–8 weeks, you’ll start seeing more smiles when the baby is awake in response to pleasant sensations, voices, or faces, though it’s still not full “social” interaction.
  • By about 3–4 months, babies usually develop true social smiles, recognizing caregivers and smiling deliberately to connect.

So, those early sleepy grins are like a preview trailer of the real social smiles that are coming.

Is it anything to worry about?

In most cases, smiling in sleep is a normal, healthy sign of brain and emotional development.

You should check with a pediatrician if you notice, along with unusual smiling or laughing in sleep:

  • Repeated episodes that look like seizures or “spells”
  • Persistent irritability, poor feeding, or weight loss
  • Very little smiling at all by around 12 weeks of age

These situations are uncommon but worth a professional look when combined with other concerning signs. TL;DR: Babies smile in their sleep mainly because their developing brains are highly active in REM sleep, triggering reflex smiles and early emotional processing, especially when they feel safe, warm, and cared for.

Information gathered from public forums or data available on the internet and portrayed here.