Active sleep is the infant version of REM sleep, and it makes up over half of a newborn’s total sleep time. During active sleep, your baby’s brain is highly active even though they’re asleep, which is why you’ll see twitching, facial expressions, fluttering eyelids, and hear small sounds. It looks restless, but it’s one of the most productive things happening in your baby’s developing brain.
What Active Sleep Looks Like
Active sleep can be startling to watch if you don’t know what’s happening. Your baby’s eyes move beneath closed lids, sometimes visibly. Their arms and legs may jerk, kick, or punch in seemingly random directions. You might notice fine muscle twitches, facial grimaces, sucking motions, stretching, or tremors in the chin and limbs. These movements can look mildly rhythmic or like a vibration.
The sounds are just as varied. Grunting, cooing, soft crying, and little peeping noises are all normal during active sleep. Your baby’s breathing will be irregular, speeding up and slowing down, and their heart rate fluctuates too. This is the opposite of quiet sleep, where breathing is deep and steady, the body is still, and the baby lies motionless.
Why It Matters for Brain Development
Active sleep isn’t just filler between deeper sleep stages. It serves a specific developmental purpose. During active sleep, the brain generates its own internal stimulation, which helps build and refine neural connections through a process of forming new synapses and pruning unnecessary ones. This is essentially the brain wiring itself, and it happens most intensely in the earliest months of life when brain growth is fastest.
Sleep also plays a direct role in memory. After your baby takes in new information while awake, sleep consolidates those experiences into lasting memory. There’s even evidence that infants process sensory input and learn about patterns in their environment while they’re asleep. Active sleep, with its high brain activity, appears to set the stage for later learning by preparing the brain to process and explore the world in increasingly complex ways.
The fact that newborns spend so much time in active sleep lines up with how rapidly their brains are developing. As the brain matures and needs less of this internal stimulation, the proportion of active sleep naturally decreases.
How Much Time Babies Spend in Active Sleep
Newborns spend more than 50% of their total sleep time in active sleep. That’s a dramatic contrast to adults, who spend only about 20% of sleep in REM. Young babies cycle through sleep in short loops of roughly 50 to 60 minutes, alternating between active sleep and quiet sleep within each cycle. Because these cycles are short, your baby transitions between sleep states frequently, which is one reason they wake so often.
Over the first year, the balance gradually shifts. Active sleep decreases as quiet sleep (the precursor to deep, restorative NREM sleep) takes up a larger share. By early childhood, sleep architecture starts resembling an adult pattern, with distinct stages of light sleep, deep sleep, and dream sleep. REM sleep continues declining through childhood and adolescence until it settles at the adult level of around 20%.
Active Sleep vs. Quiet Sleep
Infant sleep has two main states, and they look nothing alike. During quiet sleep, your baby is essentially motionless. Breathing is regular, heart rate is steady, and there are no eye movements. This is the deep, restorative phase where your baby is hardest to wake.
Active sleep is the opposite on nearly every measure. Breathing is irregular, heart rate fluctuates, eyes dart under closed lids, and the body produces a wide range of movements, from tiny finger twitches to full limb stretches. Babies are much easier to wake during active sleep, which is worth knowing if you’re trying to transfer a sleeping baby to a crib. Waiting until they’ve settled into quiet sleep, when they’re completely still and breathing evenly, gives you a better chance of putting them down without waking them.
How to Tell Active Sleep From Being Awake
This is the practical question most parents are really asking. Your baby is squirming, making noises, maybe even letting out a brief cry. Are they waking up, or are they still asleep?
The key is to pause before responding. During active sleep, newborns move around and make sounds that can easily be mistaken for waking. But if you watch for a moment, a baby in active sleep will usually settle back into a quieter phase without intervention. Their eyes stay closed (or only flutter briefly), and the movements are disorganized rather than purposeful. A truly awake baby will open their eyes, and their movements will become more sustained and directed. Picking up a baby during active sleep can actually wake them from a sleep cycle they would have continued on their own.
Active Sleep and Easier Arousal
Babies wake more easily during active sleep than quiet sleep, and that gap widens as they get older. Research measuring arousal thresholds in infants found that the stimulus needed to wake a baby during quiet sleep was significantly higher than during active sleep, at both two to three weeks and two to three months of age. By two to three months, the arousal threshold during quiet sleep nearly doubled from the first quiet sleep period to the second, meaning babies became progressively harder to rouse the deeper into quiet sleep they went.
This easier arousal during active sleep is actually considered protective. The ability to wake up in response to something like a change in breathing or temperature is an important safety mechanism in young infants. Babies who arouse more readily from active sleep can respond to potential threats during a period when their autonomic nervous system is still maturing.