The Science Behind 4 Month Sleep Regression, Night Wakes and Cat Naps

If you’re here in the middle of the night searching “why won’t my 4 month old sleep,” you’re in good company. The 4 month sleep regression is one of the most common challenges parents face and one of the most misunderstood. It feels like your baby has gone backwards, but in reality their brain is making a permanent developmental shift!!

Newborn sleep vs 4 month old sleep

Newborns sleep very differently from older babies and adults. In the early weeks, they drift between two main stages: active sleep (similar to REM) and quiet sleep (similar to deep sleep). These cycles are short and not well organised. That’s why newborns often fall asleep anywhere and can be transferred without noticing.

By around 4 months, your baby’s sleep architecture changes permanently. Instead of just two stages, they now experience adult-like sleep cycles with multiple phases:

1. light sleep (stage 1 and 2 non-REM) – the drowsy, easy-to-wake stage

2. deep sleep (stage 3 non-REM) – restorative, growth and immune function

3. REM sleep – brain activity, memory, emotional regulation

Each cycle lasts about 45–50 minutes. At the end of a cycle, babies now come into a light sleep or brief waking. For adults, this is when we roll over and fall back asleep without noticing. For babies, it often means fully waking, especially if they’ve learned to rely on rocking, feeding or contact to get back to sleep.

This is why the 4 month sleep regression feels so intense: your baby is no longer a newborn, and they can’t just drift back to sleep the way they did before. They need to learn how to link those cycles on their own.

Why sleep feels so disrupted at 4 months

• more frequent waking: your baby surfaces every 45–50 minutes instead of “sleeping through” long stretches of newborn sleep

• short naps: cycles end with a light wake, so naps get stuck at 30–40 minutes

• lighter sleep overall: adult-like architecture means more time in light non-REM sleep, making them easier to wake

• circadian rhythm development: melatonin and cortisol are starting to follow a day-night pattern, which means your baby is more sensitive to light, dark and routine (Kennaway, 2010)

How long does the regression last

The short answer: the disruption can last 2 to 6 weeks. The long answer: this isn’t really a regression at all. This is how your baby will now sleep for the rest of their life. The challenge is not that sleep cycles exist, but that your baby hasn’t yet learned how to move between them independently.

How to support your baby through the 4 month sleep regression

1. set up the environment

   • pitch-black room with blackout blinds

   • continuous white noise

   • safe sleep space with a firm mattress and sleep sack

2. focus on routine a short, predictable bedtime routine helps cue melatonin release (Mindell et al., 2015). Do the same order every night: feed, bath, sleep sack, story, bed.

3. watch the clock most 4 month olds manage 90–120 minutes awake before overtiredness. Longer wake windows often lead to harder nights.

4. teach baby to fall asleep in the cot this is the key. Babies who fall asleep in arms expect to wake in arms. Babies who fall asleep in the cot learn it is safe and familiar, and can resettle there. Gentle approaches like pick up put down, hands-on settling or spaced soothing can all work, as long as you’re consistent.

5. feed fully in the day ensure regular, full feeds in daylight to avoid reverse cycling where baby makes up calories at night.

When to ask for help

If your baby is waking every cycle all night long, naps never extend beyond 20–30 minutes, or you feel like you’re at breaking point, it’s time to get support. Sleep deprivation is not a rite of passage ...... it’s a recognised health risk linked to maternal mental health, relationship stress and safety concerns (Tikotzky, 2016).

The takeaway

The 4 month sleep regression is not your baby “forgetting” how to sleep. It’s the moment their sleep matures to mirror adult sleep architecture. From now on, sleep will always run in cycles. The challenge is teaching your baby how to connect those cycles without constant help. With the right environment, routine and consistent settling, your baby can learn this skill .... and you can finally get some sleep.

Written by: Sophia Masur who is an internationally certified Infant & toddler sleep consultant.