Sleep regressions are developmental, not disciplinary
Why they happen when they happen
The well-known disruptions cluster at around four months, eight to ten months, and eighteen months. Those windows line up with major developmental change: sleep architecture maturing, crawling and standing arriving, and language switching on.
A brain that is busy consolidating a new skill wakes more easily and rehearses that skill at unhelpful hours. Parents who have changed nothing at all still get hit, which is the clearest evidence that this is development rather than discipline.
The four month change is not really a regression
At around four months, infant sleep permanently reorganises into cycles closer to the adult pattern, with lighter phases between deeper ones. A baby who used to sink into a long unbroken block now surfaces between cycles.
That is a one-way change. The skill worth building here is not sleeping through, it is being able to resettle at the surfacing point. Everything else follows from that.
What actually helps
A short, boring, identical wind-down sequence does more work than any single technique. Children read sequence better than clocks, and the predictability itself is the signal.
Whatever approach you choose, apply it consistently for a couple of weeks before judging it. Switching methods every third night teaches inconsistency, which is the one thing that reliably makes sleep worse.
How long to expect
Most regressions settle within two to six weeks. That is genuinely a long time when you are inside it at 3am, and it is worth saying plainly rather than pretending otherwise.
Share the nights if there are two of you. Exhaustion makes everything harder to respond to calmly, and a rested adult handles a difficult night far better than two half-broken ones.
When it is not a regression
Some things masquerade as sleep problems. Reflux, a cow's milk protein allergy, eczema that flares at night, obstructive sleep apnoea, and iron deficiency can all wreck sleep and none of them respond to a sleep plan.
Bring your child in if they snore most nights, if you see pauses in their breathing, if they are unusually sweaty asleep, or if the disruption has run well past six weeks with no sign of shifting.
This is general information, not advice about your own child. If something here raises a question, ask us at your next visit or call the office. That conversation is what we are for.