The four-month sleep regression
The word is misleading. Nothing has gone backwards — your baby’s sleep has just changed its nature, and it is not going back.
There is a moment, around four months, when everything falls apart at once. A baby who was stringing together five or six hours starts waking every hour. You do the same things, in the same order, in the same room, and nothing works any more.
That is the period I doubted myself the most. You look for what you changed, you blame the teeth, the heat, the milk, the childcare. You changed nothing.
They changed.
What is happening in their head
Until three months, an infant has only two sleep states: active and quiet, with an in-between. They fall asleep straight into active sleep and move between the two without really surfacing.
Around three or four months, their brain installs the adult architecture. Light, deep and REM stages appear. They stop falling asleep through active sleep and start falling asleep through light sleep — like you.
And above all: between two cycles they now come up much higher. High enough to wake properly. High enough to notice the room has changed since they closed their eyes.
A four-month-old’s cycle, after the shift
55 min au total, à partir de l'endormissement
- Sommeil léger
- Sommeil profond
- Sommeil paradoxal
Why the word “regression” is badly chosen
A regression implies you have lost something and will get it back. So many parents wait for it to “pass”, like a teething episode.
It will not pass. Your baby’s sleep has permanently changed structure, and that is good news: it is a major neurological step forward, exactly like grasping an object or holding their head up.
What will improve, on the other hand, is their ability to settle back to sleep alone when they surface between two cycles. And that can be learned.
The point nobody states plainly
That ability is built at bedtime in the evening. Not at 2 a.m.
Whatever your baby has at the precise moment they tip into sleep, they will look for at every cycle end. They do about twelve cycles a night. Fall asleep at the breast, and they will ask for the breast at 1, at 3 and at 5 a.m. Fall asleep being rocked, and they will ask to be rocked. Not out of stubbornness: because on surfacing, their brain checks whether the conditions they fell asleep in are still there. And it finds they have gone.
That is why everyone suddenly talks about “falling asleep alone” from exactly four months, and not before. Before, they were not capable of it. Now they are.
What I would do, in order
One thing at a time. When nights collapse, the temptation is to change everything at once: new routine, new naps, new bed, new milk. That is the surest way never to understand what worked. One change, one week, observe.
Start with the evening bedtime. That is when your baby is most “ready” to fall asleep alone, because sleep pressure is at its highest. Put them down drowsy but still awake, on the evenings you have the energy. On the ones you do not, get them to sleep in your arms without guilt — every other evening is enough to make progress.
Wait before intervening at night. A micro-waking sometimes lasts two minutes and resolves itself. Rushing in at the first sound turns a partial waking into a complete one. Count to sixty before moving: you will be surprised how often it settles.
Look at the naps. A four-month-old short on daytime sleep has worse nights, not better. Three naps, awake windows of around two hours.
Bring bedtime forward. Counter-intuitive, but often the quickest lever. A baby put down too late falls asleep in sleep debt, and the debt fragments the end of the night.
What not to expect from this period
Do not count on a full night the day after a change. Count in weeks. Two to six, for nights to reorganise — and there will be back-and-forth.
Do not compare either. At four months the gap between two babies of the same age is enormous, and it says nothing about what they will be at one year. Your sister-in-law’s baby who has slept through for eight weeks does not have better parents.
The sign that it is working
Not the number of wakings. The speed at which they settle back.
A baby who wakes four times and goes back to sleep in three minutes is doing better than one who wakes twice and takes forty-five minutes. That is the figure to follow, and it is the one that moves first.
Frequently asked questions
How long does the four-month regression last?
The change in sleep architecture is permanent. The phase of difficult nights that goes with it usually lasts two to six weeks, the time it takes the baby to learn to link their cycles. If nothing improves beyond two months, speak to your GP.
Can the four-month regression happen at 3 or 5 months?
Yes. The maturation happens between three and five months depending on the child, and sometimes so gradually that you never notice it. Some parents never see a "regression": that does not mean it did not happen.
Should you start sleep training during the four-month regression?
That is a personal decision, and there are very different approaches, from gentle to directive. What physiology says is only this: from four months, a baby is able to learn to settle back to sleep alone. How you support that comes down to your values and, if in doubt, a conversation with your GP.
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