Baby Sleep Cycles
When it comes to baby sleep, one thing all parents can agree on is that it’s a wild ride🎢. But getting a handle on how infant sleep cycles work—and how they change over time—can make those 2 a.m. wake-ups a bit easier to manage.
Plus, with so many myths floating around about infant sleep cycles and their development (and honestly, baby sleep in general), it’s good to separate fact from fiction.
Here’s what you need to know about how infant sleep functions, how it changes as your baby grows, and why frequent waking is not only normal but healthy. (Don’t shoot the messenger!).
Newborn Sleep - The Fourth Trimester
Newborns don’t sleep like older babies, toddlers, or adults, and a big reason for this is that they’re not born with a circadian rhythm. The circadian rhythm is our 24-hour internal clock, generated by the brain, that makes us feel alert or sleepy at roughly the same times each day.
While this rhythm is endogenous (originating from within), it’s strongly influenced by external cues known as "zeitgebers," with light being the most impactful. Newborns, however, don’t yet produce melatonin—the hormone that helps regulate sleep—and their circadian rhythm doesn’t develop until they’re around 12 to 14 weeks old, give or take.
This means their sleep is distributed across the entire 24-hour day, often without any clear distinction between daytime and nighttime. Combine that with their short sleep cycles (lasting only 30–50 minutes) and immature arousal systems, and you get the fragmented, unpredictable sleep patterns so typical of the newborn stage.
Understanding this biological reality can help set realistic expectations and reduce stress about getting your baby on a "schedule."
The 4-Month Sleep Progression - What’s Happening?
Around 3 to 5 months, your baby’s sleep undergoes a massive shift. This is often referred to as the 4-month sleep ‘regression’, but it’s really a progression - your baby’s sleep is maturing.
Here’s what’s happening during this sometimes very tricky stage:
Maturing sleep architecture - Your baby’s sleep begins to resemble adult patterns, cycling through multiple stages like REM (lighter sleep) and non-REM (deeper sleep).
Shorter sleep cycles - While adults’ sleep cycles are 90–120 minutes, your baby’s are still 30–60 minutes. This means more opportunities for waking, especially between cycles.
Increased coregulation - Babies often need more help resettling during this time, and that’s completely normal.
This period can feel intense if your baby’s sleep becomes more interrupted. The good news? The “worst” of it typically lasts a few weeks, and not every baby’s sleep will be disrupted.
How to Support Your Baby (and Yourself)
Baby sleep can be hard, especially during big changes. But, there are ways to make it more manageable for everyone. Here are 3 tips:
Rethink sleeping arrangements - Many parents choose to adjust sleeping arrangements during this time, like moving to a safe bedsharing setup or keeping the baby closer for easier night feedings.
Support each other - If you’re parenting with a partner, work as a team. Trade off shifts, naps, or responsibilities to ensure you both get rest when possible.
Remember the positives - This progression is a sign that your baby’s brain is developing healthily. Frequent waking isn’t something you’ve caused, and continuing to respond to your baby is never a bad habit.
Big Picture - Sleep Is a Developmental Process
If this stage feels hard, know that it’s temporary. Your baby’s sleep will continue to evolve, and so will your ability to support them (and yourself!) through it.
-Dr. Jessica Guy, Founder Infant Sleep Scientist
Hey there! I hope this helps!
I strive to provide as much free and high-quality evidence-based content as possible! I also offer products and services for more in-depth education and support to optimise your and your baby’s sleep and well-being in a responsive and attuned way.
Baby Sleep Without The BS: A Science-Based and Relationship Focused Program for Better Sleep (0-18 Months) Course HERE
Struggle Free Toddler Sleep Course HERE
Gentle Night Weaning Guide HERE
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