How long does the 4-month sleep regression last?

Usually 2 to 6 weeks. The underlying change in sleep architecture is permanent, but the restlessness settles once your baby is used to the new sleep cycles.

Do I need to sleep train?

Not necessarily. What almost always helps: a fixed routine, respecting wake windows and putting your baby down drowsy but awake where possible. Choices beyond that are personal — your health visitor can think along without judgement.

Will there be more regressions?

Restless sleep around 8–10 months is also common — separation anxiety and big motor leaps (crawling, standing) play a part then. That too is temporary.

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The 4-month sleep regression

Was your baby just starting to sleep longer stretches, and suddenly they're waking every hour again? Around 3 to 5 months your baby's sleep architecture matures for good: they start sleeping in cycles of about 45 minutes with lighter phases in between — the way adults sleep too. Not a setback, but a developmental step that temporarily looks restless.

What's actually happening?

A newborn drops straight into deep sleep. Around 4 months the sleep system matures: your baby now moves through full sleep cycles with light and deep phases. Between cycles everyone briefly half-wakes — adults roll over and never notice, but a baby who can't yet resettle independently calls for help. Hence: more night waking, shorter naps and harder settling, while nothing is wrong.

What helps

A fixed wind-down routine — a short, predictable sequence (feed, change, song) teaches the body "sleep is coming".
Respect wake windows. Staying up too long causes overtiredness, which makes settling harder. At this age 1.5–2.5 hours awake between naps is typical.
Put your baby down drowsy but awake where possible — that practises self-settling, exactly the skill the new sleep cycles require.
Pause briefly before responding to stirring between cycles — sometimes your baby resettles by themselves.
Enough feeds during the day help calm the night.

How long does it last?

It usually settles within 2 to 6 weeks, once your baby is used to the new way of sleeping. The change in sleep architecture itself is permanent — the restlessness on top of it is temporary.

Worried, or is it dragging on much longer? Your health visitor is happy to think along — nothing needs to be "wrong" to ask.

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Also handy: how much sleep does a baby need? (chart by age)

Guidance by age: 0–6 weeks · 6–12 weeks · 3–4 months · 4–6 months · 6–9 months · 9–12 months · 12–18 months · 18–24 months · 2–3 years · 3–4 years · 4–5 years