4 Month Baby Sleep Explained (sometimes 3 months too)

Countless parents come for help with their baby’s sleep around the time he turns 4 months old. Why? This article will explain why “good” sleepers sometimes turn “bad” around the 3 or 4 month old mark.

How baby sleeps in the early weeks

When a newborn falls asleep, she immediately goes into deep sleep (also known as REM sleep, but I will avoid too much technical talk because that’s one reason I made this website, so you won’t have to learn the whole history of sleep if you don’t want to (or can’t stay awake to!)). If you are interested in knowing more about our biological rhythms and how we do sleep in more detail, I highly recommend the book, Solve Your Child’s Sleep Problems). Newborns spend a lot of their time in deep sleep, which is why it is hard to wake them up (even to eat!) in those early days.

4 month old baby sleeps (and sometimes as early as 3 months)…

We all go in and out of light and deep sleep. As adults, we might change positions, look at the clock, or reposition our pillow. Up until now, you may have rocked your baby or simply given him a pacifier and he slept for hours without waking up. Well, at 3 or 4 months old, your baby is now sleeping more like an adult. Now when she falls asleep, instead of entering deep sleep, she enters lighter sleep, first. That’s why if you help her fall asleep, then put her down, she is likely to wake up because she isn’t in her deep sleep, yet.

4 month babies enter deep sleep…

Initially in the night, your 4 month old will enter deep sleep relatively quickly, within 30 minutes (this changes as we get older). However, as I said, we all cycle in and out of light and deep sleep. A child’s sleep cycle is about 45-50 minutes. So, your baby will briefly awake 45-50 minutes after she has been asleep. To put that in perspective, if you are holding your baby to sleep, you would need to hold her for at least 30 minutes to make sure she’s in deep sleep and then she might wake up 15 minutes later. Sound familiar?

Overall, though, your baby’s deepest sleep is in the early part of the night, so after that first sleep cycle, she might sleep just fine for a few hours. So, you’re golden right? Just hold her for an hour? Nope!

4 month old sleeps the rest of the night…

The technical definition of “sleeping through the night” is 5 hours of continuous sleep (i.e. no feedings) and many babies can/will do this by 2 or 3 months old. The beginning of the night is your baby’s deepest sleep and after the first 5 hours (if not sooner, depending on just how challenging he is), he will cycle between light and deep sleep, but not as deep as the beginning of the night. This is where the problem of sleep associations really come into play. If your baby needs your help to go to sleep in the beginning of the night, sometime after midnight or so, he will continue to need your help every 1 or 2 sleep cycles (that means every 45 to 90 minutes or as I often hear, every 1-2 hours).

Between 4-6 a.m., approximately, is the lightest sleep of the whole night (parents’ complaints alone make this true, in my experience, aside from my reading). In the very early morning hours (about 30 minutes to an hour before waking up), he will again go into the very deep sleep.

Although babies commonly wake up early, be sure it is truly their waking up time and not just this lighter sleep and that they are having trouble sleeping. You might notice they want a “nap” just 30 or so minutes after “waking up”. What you experienced was a night waking, not starting the day.

4 month olds sleeping through the night…

How can you help your 4 month old sleeping through the night? Read this website (free article updates via e-mail is a good choice), be sure to check out my free guide, 5 Ways to Help Your Child Sleep Through the Night and/or get a detailed step-by-step baby sleep guide, which includes 2 of my e-mail baby sleep consultations, to answer all those “what if?” questions.

Did your baby stop sleeping well at 3 or 4 months old?

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• Baby Temperament and Sleep Series: Perceptiveness
• Baby Sleep and Temperature
• Baby Temperament and Sleep Series: Part 4