Babies move their mouths during sleep primarily because they spend enormous amounts of time in REM (active) sleep, a phase that triggers small, involuntary movements throughout the face and body. Newborns sleep roughly 16 hours a day, and about half of that time is spent in REM sleep. That’s far more than older children or adults experience, which is why these little mouth movements are so visible and frequent in young babies.
These movements are almost always normal. They range from sucking motions and lip smacking to jaw twitches and brief grimaces, and they serve real purposes in your baby’s developing brain and body.
REM Sleep Drives Most Mouth Movements
During REM sleep, motor areas in the brain generate brief, seemingly random movements in the face and limbs. In infants, this active sleep phase produces a wide repertoire of mouth-related behaviors: sucking, facial grimaces, jaw jerks, chin tremors, and stretching. These are so characteristic that clinicians actually use mouthing and sucking as markers to identify REM sleep in babies under two months old.
Behavioral signs of REM sleep appear remarkably early, visible in babies as young as 25 to 30 weeks gestational age. Babies also fall asleep directly into REM sleep (rather than cycling through deeper stages first) until around two to three months of age, which means you’re likely to see mouth movements almost immediately after your baby drifts off.
Twitches Help Build the Brain
Those tiny mouth and face twitches aren’t just random misfires. Research shows that sleep twitches act as a form of self-stimulation, sending sensory information back to the brain during a period when the baby is disconnected from the outside world. Each twitch triggers activity in the spinal cord, sensory and motor areas, and the hippocampus, a brain region involved in memory.
Jan Born, a professor of medical psychology at the University of Tübingen in Germany, has described this process as a way for the brain to “tune and advance the functional connectivity of the sensory and motor systems” without the distraction of external stimuli. In other words, your baby’s brain is using sleep as a quiet workshop to wire up the connections it will need for coordinated movement later on. Newborns can even learn during sleep: studies have demonstrated that babies in their first days of life form new associations and show measurable changes in brain activity while sleeping, particularly over the frontal areas of the brain associated with memory.
The Sucking Reflex Doesn’t Turn Off at Night
Babies are born with a sucking reflex that operates automatically, without conscious effort. This reflex is present in neurologically healthy infants born at 32 weeks gestational age or older, and it doesn’t simply switch off when your baby falls asleep. Non-nutritive sucking (the kind that happens without actual feeding) persists during sleep and gradually fades by about four to five months of age if it isn’t reinforced through breastfeeding or pacifier use. The rooting reflex, where a baby turns toward a touch on the cheek with an open mouth, disappears around six months.
So when you see your sleeping newborn making sucking or chewing motions, you’re often watching a hardwired reflex cycling on its own. It’s the same reflex that helps your baby locate and latch onto a food source while awake, just running in the background during sleep.
Sleep Movements vs. Hunger Cues
One of the trickiest parts of new parenthood is figuring out whether those mouth movements mean your baby is hungry or just cycling through active sleep. The Mayo Clinic lists lip smacking, rooting, and hand-to-mouth activity as early hunger signs, but these same behaviors occur during normal REM sleep.
The key difference is escalation. A baby in active sleep may suck, grimace, or root briefly, then settle back down without fully waking. A hungry baby will progress: the movements become more persistent, the baby stirs more, eyes may open, and fussing or crying follows. If your baby makes mouth movements but stays asleep or quickly returns to stillness, they’re almost certainly just in a normal active sleep cycle. Picking them up at this point can actually wake a baby who wasn’t ready to eat.
When Mouth Movements Signal Something Else
There’s a condition called benign sleep myoclonus of infancy that causes clusters of jerky movements during quiet (non-REM) sleep. Despite its intimidating name, it’s harmless. Importantly, it involves the limbs and trunk but not the face, and it’s not associated with oral movements, eye deviation, or color changes. It typically starts within the first two weeks of life and resolves on its own.
The movements parents should watch for are ones that look rhythmic, forceful, or cluster in a pattern that builds and then fades (sometimes described as “crescendo-decrescendo”). These can resemble normal sleep twitches but may indicate seizure activity such as infantile spasms. The distinguishing features: spasms tend to involve the whole body stiffening or folding, they repeat in clusters, and they may occur during wake periods as well. Normal REM mouth movements, by contrast, are brief, gentle, and mixed in with other small facial twitches and limb movements.
Mouth Breathing During Sleep
If your baby’s mouth movements look less like sucking and more like sustained open-mouth breathing, that’s a different situation. Babies are preferential nose breathers, so consistent mouth breathing during sleep often means something is blocking the nasal airway. Common causes include nasal congestion from a cold or allergies, and in some children, enlarged adenoids or tonsils. Occasional open-mouth moments during a stuffy nose are expected, but if your baby routinely sleeps with their mouth open and seems to struggle to breathe through their nose, it’s worth having their airway evaluated.