Babies smile in their sleep because of involuntary muscle twitches triggered by brain activity during REM (rapid eye movement) sleep, not because they’re dreaming or passing gas. These reflexive smiles are a normal part of early neurological development and appear from birth, well before a baby is capable of smiling on purpose.
What Happens in the Brain During Sleep Smiles
Newborns spend a remarkable amount of time in REM sleep. Full-term infants average 16 to 18 hours of sleep per day, and roughly 50% of that time is spent in REM. Premature infants sleep even more, with up to 80% of their sleep in the REM phase. During REM sleep, the brainstem sends signals that cause small, involuntary contractions of skeletal muscles throughout the body. These are called myoclonic twitches, and they can show up as finger movements, leg kicks, or facial expressions, including smiles.
The facial muscles involved in smiling are just one of many muscle groups firing during this active sleep state. Researchers have documented eye movements, body jerks, grimaces, and other fleeting expressions alongside the smiles. This is why early infant sleep is often called “active sleep” rather than the still, quiet sleep adults are used to.
These twitches aren’t random glitches. Research led by Mark Blumberg found that myoclonic twitches during early-life REM sleep send sensory feedback to the developing brain, helping to build and calibrate the sensorimotor system. In other words, each tiny facial contraction is part of how a baby’s brain learns to map and control its own muscles. The brainstem structures responsible for generating REM sleep and its associated twitches are functional even before birth, which is why premature babies display these same sleep smiles.
The Gas Myth
A persistent piece of folk wisdom holds that babies smile in their sleep because of gas or digestive discomfort. Research has directly tested this idea and found no connection. A study published in Developmental Science examined neonatal smiles and found they were not related to burps, regurgitation, or flatulence. The “gas smile” explanation, while widespread, has no scientific support.
Reflex Smiles vs. Social Smiles
Sleep smiles in the first weeks of life are purely reflexive. They don’t mean a baby is happy, amused, or responding to anything in their environment. They’re generated by brainstem activity without any involvement from the higher brain regions responsible for emotion and social connection. Those cortical areas simply aren’t mature enough yet.
The shift to real, intentional smiling happens gradually. Most babies begin producing genuine social smiles between 6 and 12 weeks of age, with the CDC placing the typical milestone at around 2 months. You’ll know the difference: a social smile happens when your baby is awake, making eye contact, and responding to your face or voice. It looks more sustained and involves the eyes, not just the mouth. If a baby hasn’t started imitating a caregiver’s smile by 12 weeks, it’s worth mentioning to a pediatrician.
Reflex smiles don’t disappear once social smiling begins. You may continue to see fleeting sleep smiles for months, but they’ll gradually become less frequent as the balance between REM and deeper sleep stages shifts with brain maturation.
Why Sleep Smiles May Still Matter
Even though sleep smiles aren’t intentional, they appear to serve a purpose beyond brain wiring. Research on infant facial expressions suggests that baby smiles, whether reflexive or social, trigger powerful physiological responses in caregivers. A study from the University of Miami found that infant smiling evokes maternal closeness and interactive behavior, and that seeing a baby smile actually helps moderate the physiological stress caused by infant crying. The researchers found that a baby’s smile activates complex cognitive processes in the mother’s brain that calm her stress response, essentially restoring a kind of emotional equilibrium.
From an evolutionary standpoint, this makes sense. A newborn who smiles, even accidentally, may strengthen the bond with a caregiver who is sleep-deprived, stressed, and biologically primed to respond to that expression. The smile doesn’t need to be “real” to have a real effect on the parent.
When Smiling Could Signal Something Else
In extremely rare cases, repetitive episodes of laughing or smile-like movements can be a sign of gelastic seizures, a type of epileptic event characterized by sudden, unprovoked bursts of laughter. These differ from normal sleep smiles in several important ways. Gelastic seizures are stereotypical, meaning they look the same every time and recur in a pattern. The laughter or smiling appears forced or unnatural, happens without any social or environmental trigger, and may be accompanied by a loss of awareness or contact with the surroundings.
Normal sleep smiles are brief, varied in timing and intensity, and occur during otherwise peaceful sleep. If your baby’s smile-like movements seem mechanical, repetitive, and identical in appearance each time, or are accompanied by stiffening, eye-rolling, or unresponsiveness, that’s a different picture entirely and warrants medical evaluation. For the vast majority of babies, sleep smiles are exactly what they appear to be: a developing brain doing its job.