4 months: what actually changes
It all fell apart within about a week of turning four months old. Nothing has gone backwards - sleep has been rebuilt into its adult form, and the things that used to work were relying on the old one.
3 min read
For the first few months, babies drop almost immediately into deep sleep and remain there for long stretches. From around four months, sleep reorganises into cycles: lighter stages, deeper stages, and a brief surfacing at the end of each cycle. This is the mature pattern and it does not reverse.
Everything families notice at this age follows from that single change. Naps shorten to a single cycle. Night waking multiplies. Transfers that used to work stop working. A baby who is fed or rocked to sleep now arrives at a cycle boundary, finds the conditions have changed, and wakes properly.
What to hold steady
Keep bedtime early. Overtiredness makes surfacings worse. Fifteen minutes earlier is more often the fix than fifteen later.
Keep the routine identical. Same order, same rooms, same light. Predictability is doing more work now than any technique.
Change one thing at a time. And give it four nights. The temptation to overhaul everything at once is enormous and it destroys your ability to learn anything.
Work on the falling-asleep moment, gently. Moving the point of sleep onset into the cot is the change that addresses the actual mechanism. It does not have to be fast and it does not require leaving anyone to cry.
Wake windows and naps at four months
Windows lengthen to roughly one and a half to two and a half hours, shortest in the morning and longest before bed. Naps often collapse to thirty or forty-five minutes for a while - one cycle - and this is the most common complaint at this age. Bringing bedtime forward on short-nap days is more effective than fighting the naps.
Total sleep is typically twelve to sixteen hours, with three to four hours in the day across three or four naps.
Nothing regressed. The thing that used to work simply stopped being invisible.
How long it lasts
Two to six weeks for most families, and rarely in a straight line - a better stretch, two bad nights, a better week. The change itself is permanent; the disruption around it is not.
Night feeds are still needed at this age and reducing them is not the answer to four-month waking. If anything, hunger genuinely increases around now.
Why this one is different from the others
Every other so-called regression is a developmental phase that passes and leaves sleep roughly where it was. This one does not. The change in sleep architecture is permanent - the cycling and the surfacings are how your baby will sleep from now on, and how you sleep too.
That matters practically. Waiting it out works for an eight-month motor leap; here, waiting returns you to a baby who still has to get through cycle boundaries somehow. The disruption around it settles in a few weeks, but whatever was doing the work before - feeding, rocking, motion - will keep being needed at every boundary until something changes about how sleep begins.
Naps during this month
They usually collapse, and it is the most-reported complaint at this age. Thirty or forty-five minutes - one cycle - where there used to be an hour and a half. This is the same architectural change showing up in the day, and it is largely not worth fighting.
What works better than extending naps is adjusting the day around them: bringing bedtime forward on short-nap days, adding a fourth nap if the day is long, and accepting an uneven schedule for a few weeks. Naps generally lengthen again over the following couple of months as cycle-linking develops, without anyone having done anything to them.
When to stop troubleshooting
Painful feeding, refusing feeds, a lot of arching, poor weight gain, snoring or pauses in breathing, or waking that seems to be about pain rather than sleep, all belong with your health visitor or GP rather than in a schedule adjustment.
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