The house is quiet. The baby has finally fallen asleep after a long feed, tiny fists curled near the face, breathing slow and soft. And then, out of nowhere, it happens: the corners of the mouth lift, the cheeks round up and a perfect little smile spreads across that sleeping face. A second later, it is gone.
Every parent who has witnessed this moment has asked the same question: why do babies smile in their sleep? Are they dreaming of something happy? Is it just gas, as grandmothers often say? Or is there something deeper going on? In this article, we look at what science really knows about newborn sleep smiles, from the way babies sleep to the evolutionary origins of the smile itself.
Newborns Sleep Very Differently From Adults
To understand sleep smiles, we first need to understand how babies sleep. And newborn sleep is very different from ours.
A newborn sleeps a lot, typically somewhere between 14 and 17 hours a day, spread over many short periods both day and night. But it is not only the quantity that differs. The structure of their sleep is different too.
Adults move through cycles of light sleep, deep sleep and REM sleep, the phase associated with rapid eye movements and vivid dreams. In adults, REM sleep represents only about a fifth to a quarter of the night. Newborns, on the other hand, spend roughly half of their sleep in "active sleep", the infant equivalent of REM sleep. Their sleep cycles are also much shorter than ours.
Active sleep is easy to recognize once you know what to look for. The baby's eyes may flutter or move under the eyelids, breathing becomes irregular, and the face and body come alive with small movements: twitches, sighs, frowns, sucking motions, little grimaces... and smiles. Unlike adults, whose muscles are mostly paralyzed during REM sleep, babies still move a lot during this phase. That is why a sleeping newborn can look like a tiny actor running through an entire repertoire of expressions.
Reflex Smiles: The First Smiles of Life
Most of the smiles you see on a sleeping newborn are what researchers call spontaneous smiles or reflex smiles. They are not a response to anything in the outside world, such as a voice or a face. Instead, they seem to be generated from within, linked to the baby's internal state and brain activity.
Research using video recordings and sleep monitoring has shown that the vast majority of these newborn smiles occur during active sleep. They often appear only on the lower part of the face, around the mouth and cheeks, without the crinkling around the eyes that comes with a fully expressed adult smile.
These reflex smiles can appear very early. Some are visible in the first days of life, and there are even ultrasound images of babies appearing to smile in the womb. This suggests that the ability to produce a smile is built into us long before we have anything to smile about.
Is it just gas?
It is a classic piece of family wisdom: "Don't get too excited, it's just wind!" There is a grain of truth in it. Specialists explain that internal physical sensations, such as digestion, a full tummy or passing gas, can sometimes trigger reflexive facial movements, including smiles. But gas is far from the whole story. Most sleep smiles appear simply as part of the natural activity of the brain during active sleep, whether or not the baby's digestion is involved.
Do Babies Dream When They Smile?
This is the most romantic question, and the honest answer is: we do not know.
Adults often smile during REM sleep when they have pleasant dreams, and studies on adult sleep have linked facial expressions to the emotional content of dreams. Since newborns spend so much time in active sleep, it is tempting to imagine them dreaming of milk, warmth or a parent's voice.
However, dreaming as adults experience it involves memories, images and stories, and a newborn has had very little experience of the world to draw from. Most researchers believe that the very intense active sleep of newborns has a different main purpose: it is thought to play an essential role in brain development, helping to build and strengthen connections in a brain that is growing at an extraordinary pace. In this view, the smiles, twitches and grimaces are part of the brain "testing" and organizing its circuits, including those that control facial expressions.
Some scientists suggest that these early sleep smiles may even be a kind of rehearsal, a way for the baby to practice the muscle movements that will later be used for real, social smiles. Whether or not there are dreams behind them, the smiles may be preparing babies for the moment they will smile at the people they love.
Babies Are Not the Only Ones: What Monkeys Tell Us
One of the most fascinating discoveries about sleep smiles comes from the animal world. In 2016, researchers at Kyoto University's Primate Research Institute, led by Fumito Kawakami, observed newborn Japanese macaques during routine health checks, often while the infants were napping.
They recorded 58 spontaneous smiles in seven baby monkeys, and every single infant smiled at least once. Just like in human babies, the smiles appeared during irregular, REM-like sleep. Earlier work by the same team had already shown that newborn chimpanzees also produce spontaneous smiles. You can read the Kyoto University announcement of the study for more details.
The implications are remarkable. If humans, chimpanzees and macaques all produce spontaneous sleep smiles, the researchers suggest that the origins of the smile go back at least 30 million years, to the common ancestor of Old World monkeys and humans. In other words, when your baby smiles in their sleep, they may be performing one of the oldest facial expressions in our family tree.
From Sleep Smile to Social Smile
For parents, the most exciting milestone comes a little later: the first social smile, a smile directed at someone, in response to a face, a voice or a game.
This usually appears around six to eight weeks of age, and by about two months most babies smile in response to the people around them. Unlike reflex smiles, social smiles happen while the baby is awake and attentive, often with eye contact, and they tend to involve the whole face, including the eyes.
As the weeks go by, spontaneous sleep smiles become less frequent, while social smiles multiply. The baby begins to smile at familiar faces, then to coo and "chat" in response to smiles, and eventually, at around three to four months, to laugh out loud. Smiling becomes a true conversation, one of the first ways a baby communicates with the world.
This transition is not only adorable, it is also an important developmental milestone. The social smile shows that the baby's vision, brain and emotional systems are maturing and that they are starting to connect with others. Pediatricians often ask about it during routine checkups. If a baby has not started smiling socially by around three months, parents may want to mention it to their doctor, keeping in mind that every baby develops at their own pace and that premature babies may reach milestones a little later.
Why a Baby's Smile Affects Us So Deeply
Whether it is a reflex smile in sleep or a beaming social smile, a baby's smile has a powerful effect on adults. Brain imaging studies have shown that seeing their own baby smile activates reward regions in parents' brains, the same circuits involved in other deeply pleasurable experiences. A smiling baby is, quite literally, rewarding to look at.
This makes sense from an evolutionary perspective. Caring for a newborn is exhausting, and the first social smile often arrives right when parents are most sleep-deprived. That smile acts as a powerful reward, strengthening the bond and motivating parents to keep going. We explore this broader phenomenon in our article on why babies look so cute, which explains how baby faces trigger our brain's caregiving instinct.
The sight of a peacefully sleeping baby has its own special power too. The relaxed features, the closed eyes, the slow breathing and the occasional fleeting smile convey a deep sense of calm and safety. It is no coincidence that artists and photographers so often choose to portray babies asleep, and that sleeping baby dolls are among the most loved by collectors of lifelike dolls. Others prefer to capture the joy of the awake, smiling baby that comes a few weeks later.
Sleep Smiles in Folklore and Tradition
Long before scientists studied active sleep, people around the world had their own explanations for sleep smiles. In many cultures, folk beliefs say that a baby smiling in their sleep is being visited by angels, playing with invisible friends or remembering heaven. Other traditions say the baby is hearing a blessing or dreaming of a guardian spirit.
These beliefs reflect something universal: the sense that a sleeping baby's smile is a little window into a mysterious inner world. Science has given us a better understanding of the mechanisms, but it has not made the moment any less magical.
When to Mention Unusual Movements to a Doctor
Sleep smiles, twitches and grimaces are completely normal and are signs of healthy active sleep. In very rare cases, however, repeated episodes of laughing or unusual movements may be related to a medical condition. Health experts advise parents to talk to their pediatrician if laughing or smiling episodes come with stiffening, jerking movements, unusual eye movements, changes in breathing or color, or if they seem to happen in clusters. In the vast majority of cases, though, a smiling sleeping baby is simply a baby in the middle of a very busy night of brain development.
Frequently Asked Questions
Why do newborns smile in their sleep?
Newborns smile in their sleep mostly because of spontaneous reflex smiles that occur during active sleep, the infant version of REM sleep. These smiles are linked to brain activity and development rather than to a response to the outside world.
Do babies dream when they smile in their sleep?
Scientists do not know for sure. Newborns spend about half of their sleep in active sleep, which is associated with dreaming in adults, but researchers believe this phase mainly supports brain development in infants.
When do babies start smiling for real?
The first social smile usually appears around six to eight weeks old, and most babies smile at the people around them by about two months.
Is a baby's sleep smile caused by gas?
Internal sensations like digestion can sometimes trigger reflexive facial movements, but most sleep smiles are simply part of the natural activity of active sleep.
Do animals smile in their sleep?
Yes. Researchers at Kyoto University observed spontaneous sleep smiles in newborn Japanese macaques, and newborn chimpanzees show them too, suggesting the smile has very ancient evolutionary roots.
Conclusion: A Tiny Smile With a Long History
A baby's sleep smile lasts only a second or two, but it carries a surprising amount of science and history. It reflects the intense activity of a developing brain, it may be a rehearsal for the social smiles to come, and it connects us to primate ancestors who lived tens of millions of years ago.
So the next time you see a newborn smiling in their sleep, you can tell yourself that it is probably not "just gas". It is a little glimpse of a brain building itself, of a face practicing its first expressions, and of one of the oldest gestures of tenderness in the living world.