Babies don’t just have REM sleep, they spend far more time in it than adults do. A full-term newborn sleeps 16 to 18 hours a day, and roughly half of that time is spent in REM sleep. That’s about eight hours of REM daily, compared to the roughly two hours most adults get. Premature infants spend even more of their sleep in REM, up to 80%.
REM Sleep Begins Before Birth
REM-like sleep doesn’t start at birth. It develops in the womb. Fetal eye movements can be detected as early as 14 weeks of gestation, and by 28 to 30 weeks, those eye movements begin to synchronize with body movements during sleep in a pattern researchers call “active sleep,” the precursor to true REM. This is the point at which the brain centers responsible for REM sleep start functioning. A second surge in eye movement activity occurs around 36 to 37 weeks of gestation, suggesting the system continues maturing right up until delivery.
What REM Looks Like in a Baby
In adults, REM sleep is internally active but externally still (aside from the eyes). In babies, it’s much noisier. Infant REM sleep is called “active sleep” because it comes with visible twitching of the arms, legs, and face, along with rapid eye movements, irregular breathing, and occasional sounds. At one month of age, these twitches and eye movements are so frequent and obvious that they’re one of the main ways clinicians identify active sleep in infants.
This can confuse new parents. A baby who is twitching, grimacing, or making small noises may look restless or uncomfortable, but they’re often in the deepest part of their REM cycle. Picking them up at this point can actually wake a sleeping baby unnecessarily.
Why Babies Need So Much REM
The enormous amount of REM sleep in early life isn’t accidental. It serves a specific developmental purpose: building the brain’s sensory and motor wiring. During active sleep, a baby’s brain generates spontaneous neural activity that helps establish and refine the maps connecting body parts to the brain regions that will eventually control them. The twitches you see during a baby’s REM sleep aren’t random. They activate sensorimotor pathways with surprising precision, helping the brain learn which signals correspond to which body parts.
Research in developing mammals has shown that active sleep provides a unique context for this work. It activates the primary motor cortex and builds a body map long before the infant has any deliberate motor control. Sleep, more so than waking, appears to be the brain state in which sensory input gets processed and functional connections between brain regions get expressed. REM sleep also contributes to broader neural processes: the birth of new brain cells, cell migration, the pruning of unnecessary connections, circuit formation, and the brain’s ability to reorganize itself.
In short, a baby’s brain is doing some of its most important construction work while the baby appears to be doing nothing at all.
How Infant Sleep Cycles Differ From Adults
A newborn’s sleep cycle lasts about 45 to 60 minutes, roughly half the length of an adult cycle. Each cycle moves between active sleep (REM) and quiet sleep (non-REM), but the proportion of REM within each cycle is much higher than in adults. This is one reason babies wake so frequently: they pass through more complete cycles per hour, and the transitions between cycles are natural arousal points.
Babies also enter sleep differently. Adults typically fall into non-REM sleep first and don’t reach REM for about 90 minutes. Newborns often drop directly into REM sleep, which is another reason they may seem restless shortly after falling asleep.
Research on infant arousal thresholds shows that babies are easier to wake during active sleep than during quiet sleep, and that quiet sleep becomes progressively harder to rouse from as time passes. This means the lightest, most easily disrupted sleep tends to come at the beginning of a nap, when the baby may still be in REM.
How REM Sleep Changes With Age
The 50% REM proportion at birth doesn’t last. As the brain matures and the most intensive period of neural wiring tapers off, the share of sleep spent in REM gradually declines. By six months, babies are spending a smaller fraction of their sleep in REM, though still considerably more than adults. By early childhood, the proportion approaches adult levels of roughly 20 to 25% of total sleep time. Total sleep hours also decrease, so the absolute amount of REM drops on two fronts: less sleep overall and a smaller percentage of it in REM.
Do Babies Dream During REM?
This is one of the most common follow-up questions, and the honest answer is that no one knows for certain. In adults, REM sleep is strongly associated with dreaming. Some scientists have argued that because newborns spend so much time in REM, they must also be dreaming. But infant REM sleep differs from adult REM sleep in both brain wave patterns and behavioral characteristics, and having REM sleep doesn’t automatically mean dreams are occurring.
The research on when dreaming begins is, as one review put it, “strikingly controversial,” with no consensus in the field. The challenge is fundamental: you can’t ask a newborn what they experienced. What is clear is that the primary purpose of all that infant REM sleep is brain development, not the kind of narrative dream experience adults have. Whether some simpler form of sensory experience accompanies that neural activity remains an open question.