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The sleep of a 4-month-old baby

If there is one month when parents land on a page like this at 3 a.m., it is this one. A baby who was sleeping properly suddenly wakes every hour, and nobody understands what happened.

What happened is that their brain grew. Around three or four months, the architecture of their sleep shifts to an adult one: light, deep and REM stages appear, and they stop falling asleep straight into active sleep. Between two cycles they now wake more fully — fully enough to notice the surroundings have changed since they fell asleep.

So the word "regression" is misleading, and I find it a little cruel: it suggests you have lost something and will get it back. It is the opposite. Your baby’s sleep has just moved forward, permanently, and it is your habits that need to adjust.

Sleep per 24 h

12 – 16 h

Of which at night

10 – 11 h

Naps

3

Awake window

2 h – 2 h 30

One cycle lasts

~ 55 min

Before putting down

~ 15 min

The key point at this age

Why it will not just "pass"

A teething episode passes. A cold passes. The four-month maturation does not: it is acquired. What improves in the following weeks is your baby’s ability to settle back to sleep alone between two cycles. And that ability is built at bedtime in the evening, not at 2 a.m. That is why everyone suddenly starts talking about "falling asleep alone" at exactly this age.

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They fall asleep

Their night at 4 months, minute by minute

    The whole night, at a glance

    To send to your partner, the nanny or the grandparents.

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    The timer: when can I put them down?

    Start it the second they fall asleep in your arms.

    --:--before putting them down

    While you wait, keep them against you and try not to move. The countdown will tell you when their sleep is deep enough for the transfer.

    Keep this tab open. The timer stays accurate even if the screen turns off.

    Long nights become possible

    By around six months, a majority of babies string together six hours at least one night in two. "A majority" does not mean "all": a third are not there yet, and they are perfectly fine.

    • Naps anchor themselves at recognisable times: one in the morning, one in the early afternoon, a short one at the end of the day.
    • The late-afternoon nap is the first to go, usually around six to eight months.
    • The need to feed at night drops noticeably, without disappearing for everyone.
    • Deep sleep concentrates at the start of the night: the first few hours are the most solid.

    The classic trap

    Thirty-minute naps are common here: that is one complete cycle and a waking at the wrong moment. It is not a fault, it is arithmetic.

    Falling asleep alone at this age

    ✓ Possible

    Most babies are physically capable of it. Put down awake, they usually need 10 to 20 minutes to fall asleep: that squirming and grumbling is normal, it is not a call.

    Put down awake, a child of this age usually takes 10 to 20 minutes to fall asleep. That squirming is normal.

    How much sleep they need, age by age

    The shaded band is the normal range over 24 hours. Hover to read an age range.

    0h4h8h12h16hbirth6 mo1 year18 mo2 years30 mo3 years
    Total sleep over 24 hOf which at night

    Hover over the curve to read an age range.

    The questions of this age

    How long does the four-month sleep regression last?

    The change itself is permanent. The phase of difficult nights that comes with it usually lasts two to six weeks, the time it takes your baby to learn to link their cycles. If multiple wakings persist beyond two months with no improvement at all, it is worth raising with your GP.

    My four-month-old wakes every 45 minutes at night — why?

    Because that is roughly the length of one of their cycles. At the end of each cycle they come up into light sleep and wake briefly — that is normal, everyone does it. The question is not how to prevent those wakings, it is whether they can settle back without you. If they fell asleep in your arms at 8 p.m., they will call you at every cycle end.

    Should I change anything during this period?

    One thing, not ten: try putting them down drowsy but still awake at the evening bedtime, on the evenings you have the energy. Everything else — new naps, new timings, new bed — waits until the period has passed. You do not change three variables at once on an exhausted baby.

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