Quick answer: Most babies don’t consistently sleep through the night (defined as 5–6 continuous hours) until 4–6 months — and many don’t until 9–12 months or later. There’s no single ‘right’ age, and the variation is enormous.
What ‘sleeping through the night’ actually means
The medical definition of ‘sleeping through the night’ is a continuous stretch of 5 hours — not the 12-hour block parents imagine. In research, a baby who sleeps 11pm–4am has ‘slept through.’ The 8–12 hour stretches that parents are hoping for are developmentally normal from around 6–9 months for many babies, but not all. Many perfectly healthy, typically developing babies continue waking at night well into toddlerhood — particularly in non-sleep-trained families.
Why babies wake at night: it’s biological, not a problem
Newborns cannot sleep through the night for multiple physiological reasons: their stomach holds only 1–2 tablespoons at birth and genuinely needs refilling every 2–3 hours; their blood sugar drops quickly; their circadian rhythm (the biological day/night clock) doesn’t mature until approximately 3–4 months; and they spend approximately 50% of their sleep in active, light sleep (protective against SIDS). Night waking isn’t a failure of parenting — it’s infant biology working correctly.
The developmental timeline
Week 1–6: feeds every 1.5–3 hours around the clock. Week 6–12: some babies begin producing one longer stretch of 3–5 hours. Month 3–4: circadian rhythm begins to establish; many babies consolidate to one longer nighttime stretch with 1–3 wakings. Month 4–6: neurological changes can paradoxically worsen sleep (the 4-month sleep regression). Month 6–9: with or without sleep training, many babies begin sleeping longer. Month 9–12: most babies capable of sleeping 8+ hours — whether they do depends on temperament, sleep associations, and parental approach. The 12-month range: studies consistently find that roughly 1 in 4 12-month-olds still wake 1–3 times per night in families that did not sleep train.
What you can actually do
From 4–6 months onward, sleep training has strong evidence for improving infant sleep consolidation without long-term harm. The evidence base covers graduated extinction (Ferber), full extinction, and fading approaches — all show comparable outcomes. Before 4 months, the focus is on establishing day/night differentiation (bright mornings, quiet dark nights), beginning a simple bedtime routine (bath, feed, song, sleep), and accepting that biology is largely in charge. Dream feeds (a feed given while baby is drowsy at parents’ bedtime), strategic light management, and consistent settling approaches all contribute.
Frequently Asked Questions
Is it normal for a 9-month-old to still wake twice a night?
Yes — developmentally normal. ‘Normal’ and ‘common’ are different from ‘unavoidable.’ If waking twice is working for your family, there’s nothing to fix. If it’s unsustainable, sleep training from 6+ months is evidence-based and effective.
Can formula help babies sleep longer?
Formula takes slightly longer to digest than breast milk (3–4 hours vs 2–3 hours), so formula-fed babies sometimes go slightly longer between night feeds. However, multiple studies have found no significant difference in total nighttime sleep between breastfed and formula-fed babies. Switching to formula to improve sleep is not evidence-based.
Why did my baby suddenly start waking more after sleeping well?
Sleep regressions — periods of disrupted sleep following a period of consolidation — are normal and occur around 4 months, 8–10 months, 12 months, and 18 months. They typically coincide with developmental leaps (motor skills, language, object permanence). They last 2–6 weeks on average.
Related Reading
- 3 month old baby: sleep regression or just a growth spurt?
- Baby sleep schedule generator by age
- Best baby sleep products 2025: swaddles, white noise and more
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