The 4-month sleep regression: what happens and what helps

Short answer: between the third and fifth month your baby's sleep changes for good. From now on they sleep in cycles like an adult and wake more often in between. It is a developmental step, not a step back. The hard phase usually lasts two to six weeks.

By Felix Plaschka, developer of HeyFini and dad of a daughter. Published 4 September 2026, last reviewed 4 September 2026.

We are parents, not doctors. This guide does not replace a visit to your midwife, health visitor or pediatrician. Facts and figures come from the sources listed. More about us.

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What changes in your baby's sleep

Newborns drop straight into deep sleep and have only two sleep stages. Around the fourth month the sleep pattern matures: there is now light sleep, deep sleep and dream sleep, and a brief waking between cycles. That stays that way, for life.

So what is new is not the waking, but that your baby notices it. And because they now take in the world much more consciously, they need the same situation when they wake as when they fell asleep, otherwise they call for you.

How you recognize the phase

A baby who used to sleep five hours straight suddenly wakes every one to two hours. Naps get shorter, often exactly 30 to 40 minutes. Settling takes longer, and the baby is fussier during the day than usual. Some feed more often at night again.

A lot is happening at the same time: rolling, grasping, laughing, more interest in everything. The brain is working flat out, at night too.

What helps now

Falling asleep like waking up. If your baby falls asleep at the breast or in your arms, they will look for the breast or the arms when they wake. That is not a mistake. If you want to change it, put them down drowsy but awake sometimes over the coming weeks. If you do not want to change it, that is just as fine.

Adjust the wake window. At four months it is 1.25 to 2 hours. An overtired baby wakes more often at night, not less. The table helps.

Keep daytime naps. Even if naps are short, every one counts. A day without sleep makes the night worse.

Same bedtime routine. Bath, pajamas, feed, song, bed. Always in the same order. The baby learns what comes next.

Stay calm at night. Lights off, little talking, tend to them, put them back down. There is nothing to experience at night.

What you do not need

Starting solids early to make the night longer is not recommended by pediatric societies. Solids begin at the earliest around five months and follow the readiness signs, not sleep. You do not need sleep training either. Whether and when you want to change anything about settling is your decision, not a guidebook's.

When to ask: with fever, poor feeding, unusual breathing during sleep, or if the phase has not improved a step after six weeks. Your midwife, health visitor and pediatrician know your baby.

How HeyFini walks you through the phase

In this time you mainly want to know one thing: is it getting better? What has changed compares this week with the week before and this month with the month before, using the median for each figure. You see in black and white whether nights are getting longer again, even if last night was rough. The Sleep Coach adjusts the wake window with you, and the prediction tells you when the next sleep is likely.

This article does not replace medical advice. If you are worried about your baby's sleep or health, ask your midwife, health visitor or pediatrician.

Frequently asked questions

How long does the 4-month sleep regression last?

Usually two to six weeks. The new sleep pattern stays afterwards, but your baby gets used to it and sleeps more settled again. If there is no improvement at all after six weeks, ask your midwife, health visitor or pediatrician.

Is there also a sleep regression at 8, 12 and 18 months?

Unsettled phases happen at every age, often around crawling, walking and talking. A fixed regression at particular months is not scientifically established. The change at four months is the only one based on a measurable rebuild of sleep.

Do I have to do sleep training now?

No. The phase passes without it too. Whether you want to change anything about settling is your decision. If you would like support, talk to your midwife or health visitor, they know gentle ways that suit your baby.

Sources: Maturation of sleep stages: Grigg-Damberger, Journal of Clinical Sleep Medicine 2016; sleeping through the night in the first year: Henderson et al., Pediatrics 2010; starting solids: ESPGHAN, Fewtrell et al. 2017; NHS Start for Life.