Most babies start sleeping through the night between 3 and 6 months of age, though “sleeping through the night” typically means a 6-hour stretch, not the 10 or 12 hours parents often imagine. Some babies reach this milestone earlier, and a surprising number still wake at night well past their first birthday. Understanding what’s happening biologically helps explain why the timeline varies so much from one baby to the next.
What “Sleeping Through the Night” Actually Means
Pediatric sleep researchers generally define sleeping through the night as either 6 or 8 consecutive hours without needing a parent’s help. That’s an important distinction. A baby who falls asleep at 7 p.m. and sleeps until 1 a.m. technically qualifies for the 6-hour benchmark, even though you’re wide awake for a feeding in the middle of the night. Some studies use stricter criteria, like sleeping uninterrupted from 10 p.m. to 6 a.m., but most clinical research sticks with the 6- or 8-hour window.
This definition matters because many parents assume their baby is behind when, by clinical standards, the baby is already there. A 5-month-old who sleeps from 8 p.m. to 2 a.m. and then wakes for a feeding is sleeping through the night by the 6-hour measure.
The Typical Timeline by Age
Newborns are biologically incapable of long sleep stretches. A newborn’s stomach holds roughly 20 milliliters (less than an ounce), and a small breast milk feeding empties from the stomach in about an hour. Their sleep cycles also run about an hour long, synced to their feeding needs. This is why newborns wake so frequently: they genuinely need to eat that often.
By about 3 months, most babies can physically sustain longer stretches without a feeding, particularly once they reach 12 to 13 pounds. Their stomachs are larger, they take in more calories per feeding, and their sleep cycles start consolidating. Between 3 and 6 months, the biological machinery for longer sleep comes online. Babies begin producing their own melatonin (something they can’t do at birth), and their internal clock starts distinguishing day from night. By 6 to 9 months, most infants can manage at least a 6-hour consolidated sleep episode.
But “most” is doing a lot of work in that sentence. Research tracking large groups of babies found that at 6 months old, 37.6% were still not sleeping 6 consecutive hours, and 57% were not sleeping 8 consecutive hours. At 12 months, 27.9% still weren’t hitting the 6-hour mark, and 43.4% weren’t reaching 8 hours. Nearly half of all one-year-olds are still waking at night. If your baby is one of them, that’s common, not a sign of a problem.
Why Some Babies Take Longer
The single biggest factor is brain development. Babies don’t choose to wake up at night. Their circadian system, the internal process that separates daytime alertness from nighttime sleep, builds itself gradually over the first several months. A fetus and newborn don’t produce melatonin at all. In one case study, a breastfed infant exposed only to natural light developed a measurable sleep-wake rhythm by about 45 days and aligned nighttime sleep onset with sunset by day 60. For most babies in modern homes with artificial lighting, this process takes longer.
Temperament also plays a role. Some babies are naturally lighter sleepers or have more difficulty settling themselves back to sleep after a normal nighttime arousal (all humans briefly wake between sleep cycles, but adults fall back asleep without remembering it).
Breastfeeding and Formula
There’s a common belief that formula-fed babies sleep longer because formula is harder to digest and keeps them full. The reality is more nuanced. Breastfed babies do wake more often between 6 and 12 months. But research adjusting for other variables found that fully breastfed infants actually logged longer total night sleep and longer total sleep overall compared to formula-fed infants. They woke more frequently, yes, but they also fell back asleep and accumulated more sleep. Partially breastfed babies performed similarly to exclusively breastfed ones. So switching to formula specifically to improve sleep may not deliver what parents expect.
Sleep Regressions That Reset Progress
Even babies who have been sleeping well for weeks can suddenly start waking again. These regressions aren’t random. They’re linked to developmental changes, and most babies experience at least one during the first year.
Around 4 months, sleep architecture shifts. Babies move from the simpler newborn sleep pattern to a more adult-like cycle with distinct stages, and this transition often disrupts their established routine. Around 9 months, separation anxiety peaks, and babies who previously settled easily may cry when a parent leaves the room. Growth spurts can trigger extra nighttime feedings. Teething causes pain that wakes babies. Learning a new physical skill, like rolling over or pulling to stand, can keep a baby’s brain too activated to settle. Even travel or a new daycare arrangement can throw things off.
These regressions typically last one to three weeks. They feel permanent in the moment, but they pass as the baby adjusts to whatever developmental leap triggered them.
When Sleep Training Makes Sense
Sleep training, which teaches babies to fall asleep independently, is generally considered appropriate starting around 4 months. Some babies do better waiting until 6 months. Before 4 months, babies lack the self-soothing ability and have sleep cycles too short and feeding needs too frequent for training to be effective or appropriate.
Sleep training doesn’t mean a baby won’t wake at night. It means that when they do wake between sleep cycles, they can resettle without a parent picking them up or feeding them. For many families, this is the practical difference between “sleeping through the night” and not. The baby may still briefly rouse but goes back to sleep on their own, so from the parent’s perspective, everyone sleeps.
Medical Conditions Rarely Explain Night Waking
Parents sometimes wonder whether reflux, ear infections, or other health issues are causing their baby’s night waking. While illness and teething can certainly disrupt sleep in the short term, research looking at infants with chronic medical problems or histories of hospitalization found no meaningful difference in night waking compared to healthy babies. About 35% of babies with health issues woke regularly at night, compared to 34% of those without. The number of disrupted nights per week was essentially the same in both groups.
This suggests that for the vast majority of babies, night waking is a normal developmental pattern rather than a medical symptom. Short-term disruptions from an ear infection or a cold are real but temporary, and they don’t typically cause lasting changes to sleep patterns.
Safe Sleep While You Wait It Out
However long your baby takes to consolidate nighttime sleep, the sleep environment matters. Current guidelines recommend placing babies on their back for every sleep, using a firm, flat mattress in a safety-approved crib with only a fitted sheet. No blankets, pillows, bumper pads, or stuffed animals. Room-sharing (but not bed-sharing) is recommended for at least the first 6 months. Offering a pacifier at nap and bedtime is also associated with reduced risk of sleep-related infant death. If you’re breastfeeding, waiting until nursing is well-established before introducing a pacifier is reasonable.
These guidelines apply to naps and nighttime sleep alike, and they remain important through the entire first year, including during regressions when exhausted parents may be tempted to bring a baby into an adult bed or prop them in a lounger.