Some babies sleep through the night by three months, while others are still waking multiple times well past their first birthday. The difference comes down to a mix of genetics, brain maturity, temperament, feeding patterns, physical comfort, and the sleep habits that form in those early weeks. No single factor explains it, but understanding each one can help you make sense of your own baby’s sleep patterns.
Genetics Set the Baseline
A large meta-analysis published in JAMA Network Open estimated that about 35% of the variation in infant sleep duration and night waking is heritable. That means genetics account for roughly a third of why one baby sleeps differently from another. But the bigger share, around 45%, comes from the shared environment: the household, routines, and caregiving patterns a baby experiences. The remaining 20% is attributed to individual, non-shared experiences unique to each child.
What this tells you is that your baby’s sleep isn’t purely hardwired. Genetics matter, but they’re not destiny. Two siblings raised with identical routines can still sleep differently because of that 35% genetic component, which influences things like how deeply a baby sleeps, how easily they’re aroused, and how their nervous system processes stimulation. If your baby seems like a lighter sleeper than your friend’s baby despite similar routines, biology is a real part of the explanation.
Their Internal Clock Takes Weeks to Develop
Newborns don’t produce melatonin, the hormone that signals nighttime drowsiness. A rhythm of the stress hormone cortisol begins developing around 8 weeks of age. Melatonin production and sleep efficiency follow at approximately 9 weeks, and body temperature rhythms come online around 11 weeks. Until these systems mature, babies genuinely cannot distinguish day from night in the way adults do.
This timeline varies from baby to baby. Some infants develop a clear day-night pattern by 2 months, while others take closer to 4 months. Light exposure plays a role in helping this process along: babies exposed to natural daylight during the day and dim conditions at night tend to develop their circadian rhythm faster. If your newborn seems to have their days and nights mixed up, it’s not a behavioral problem. Their brain simply hasn’t built the clock yet.
Shorter Sleep Cycles Mean More Waking
Babies cycle through sleep stages much faster than adults. Where an adult’s sleep cycle runs about 90 minutes, an infant’s is considerably shorter. Babies also spend less time in deep sleep and more time in lighter, more easily disrupted stages. At the end of every cycle, there’s a brief moment of partial waking. Adults usually roll over and fall back asleep without remembering it. Babies, especially those under 6 months, often wake fully at these transition points.
This is where individual differences become obvious. Some babies learn to pass through these transitions quietly. Others surface to full wakefulness and need help getting back to sleep. That ability to resettle independently, sometimes called self-soothing, is partly developmental and partly shaped by what happens at bedtime, which leads to one of the biggest factors in infant sleep variation.
How They Fall Asleep Shapes How They Stay Asleep
The conditions present when a baby falls asleep become the conditions they expect when they wake between cycles. If a baby always falls asleep while being rocked, nursed, or held, they often need that same input to fall back asleep at 1 a.m. and 3 a.m. and 5 a.m. Researchers call these “sleep onset associations,” and they’re one of the most consistent predictors of frequent night waking.
Babies who fall asleep independently in their crib tend to resettle more easily between sleep cycles, not because they never wake, but because the environment they wake into matches the one they fell asleep in. This doesn’t mean rocking or nursing your baby to sleep is wrong. It means the pattern explains a significant portion of why some babies seem to “sleep through the night” while others don’t. Bed-sharing, for instance, has been linked to reduced self-soothing ability in infants, likely because the parent’s constant proximity makes independent resettling less necessary.
Behavioral sleep interventions that address these associations have measurable effects. In one controlled study, babies whose parents received sleep training guidance were far less likely to wake twice or more per night compared to a control group: 31% versus 60%. Parent-reported severe sleep problems also dropped significantly, from 14% to 4%.
Feeding Method Plays a Nuanced Role
The relationship between feeding and sleep is more complex than the common advice suggests. A study tracking 654 infants found that fully breastfed babies actually had longer total and nighttime sleep durations than formula-fed babies at multiple time points through 24 months. However, breastfed babies also woke more frequently between 6 and 12 months.
So breastfed babies sleep more overall but wake more often within that sleep. Formula-fed babies may have fewer wakings, but their total sleep isn’t necessarily longer. The frequent advice to switch to formula for “better sleep” doesn’t hold up as straightforwardly as it sounds. The waking pattern in breastfed babies is likely related to breast milk digesting faster than formula, leading to more frequent hunger cues, plus the strong sleep-onset association that nursing creates.
Reflux and Physical Discomfort
Some babies sleep poorly because something physical is bothering them. Gastroesophageal reflux is one of the most common culprits. Research tracking reflux episodes during infant sleep found a strong link between the onset of a reflux episode and disrupted sleep stages. During the moments surrounding a reflux event, sleep stage changes occurred 47 times per hour, compared to 28 times per hour during reflux-free periods. That’s nearly double the disruption.
What makes reflux tricky is that “silent reflux,” where the stomach acid rises but the baby doesn’t visibly spit up, can be hard to identify. A baby with silent reflux may simply seem fussy, arch their back, or wake frequently without an obvious cause. Other physical factors like ear infections, teething pain, food sensitivities, and nasal congestion also fragment sleep in ways that have nothing to do with habits or temperament. If your baby’s sleep suddenly deteriorates or has never been good despite consistent routines, a physical cause is worth investigating.
Sensory Sensitivity Varies Widely
Babies differ in how much environmental input it takes to disturb them. Some infants can sleep through a loud conversation in the next room. Others startle awake at a door closing down the hall. Research on sensory processing in young children has found that babies with heightened sensitivity to stimuli tend to take longer to settle to sleep, while those with higher sensory-seeking behavior tend to sleep less during the day.
These aren’t problems to fix. They’re differences in how each baby’s nervous system is wired. A baby with a low sensory threshold is more easily overstimulated by light, noise, or even the texture of their sleepwear, and that overstimulation makes it harder to wind down. For these babies, the sleep environment matters more than it does for their easygoing peers.
The Environment You Can Control
Room temperature is one of the simplest variables to optimize. Research suggests a range of 68 to 72 degrees Fahrenheit (20 to 22 degrees Celsius) is comfortable for most babies. Temperatures above 72°F may be too warm, which can increase restlessness and, more seriously, is associated with higher risk of sleep-related safety concerns. Babies are poor at regulating their own body temperature, so the room does that job for them.
Beyond temperature, darkness matters more for babies than many parents realize, especially after that 9-week mark when melatonin production begins. Even small amounts of light can suppress melatonin in infants. White noise can help babies with higher sensory sensitivity by masking sudden environmental sounds that would otherwise trigger a wake-up. These aren’t magic fixes, but for a baby who’s already on the edge of waking, they can be the difference between a full sleep cycle and a fragmented one.
Why “Normal” Covers a Huge Range
When you combine genetic predisposition, circadian maturity, sensory wiring, feeding patterns, sleep associations, physical comfort, and environment, the number of possible combinations is enormous. Two healthy, well-cared-for babies of the same age can have wildly different sleep patterns, and both can be completely normal. A baby waking twice a night at 8 months isn’t necessarily experiencing a sleep problem. They may just be on a different developmental timeline, have a more sensitive temperament, or have built stronger associations with parental help at bedtime.
The factors you can influence, like sleep environment, bedtime routines, and how your baby falls asleep, are genuinely powerful. But they operate within the boundaries set by your baby’s unique biology. Understanding that interplay is more useful than chasing an idealized sleep schedule that may not match your particular child.