Understanding Baby Sleep Cycles

Before your baby hits 4 months old, their sleep is relatively simple - they are either awake, in ‘active sleep’, or in ‘quiet sleep’. Newborn sleep is made up of these light sleep cycles that are instinctively designed for survival. It’s why newborns sleep so much, yet seem so easily disturbed. You may also notice that your newborn is particularly noisy and snuffly in their ‘active sleep’ which fools you into thinking they are awake, when in fact they are still snoozing.

But there is a major change that happens around 4-months. This is when your baby’s sleep cycles go through a permanent shift in their brain architecture, and it's the only time we can truly call it a “regression.”

The Real 4-Month Sleep Regression

At around 4 months old, the development of baby sleep cycles take a huge leap forward. They begin to mature and resemble that of an adult, meaning they now cycle through both light and deep stages of sleep. This is a good thing, as each of these sleep stages are essential for memory building, emotional regulation, brain development, and physical restoration.

Understanding Baby Sleep Cycles

After this regression, your baby will cycle through each sleep stage approximately every 90 minutes. Here’s a simplified breakdown:

The first 20–30 minutes of each sleep cycle are light sleep. During this time, babies are much more prone to waking, especially if there’s any noise, light, or movement.
Then comes around 20 minutes of deep sleep
The last part of the cycle is REM sleep, which is where the brain performs it's magic, such as memory formation and emotional regulation

And just like adults, your baby will briefly wake between cycles. As adults, we’re so good at falling back to sleep that we hardly notice it. But babies, not so much, especially if they’ve never learned how to fall asleep on their own.

In the first few hours after falling asleep, your baby will take the majority of their deep sleep for the night - you may notice they take about 3 hours of decent sleep after you put them to bed. As the night goes on, this deep sleep becomes less and less, meaning they are more likely to wake up - especially during the early hours of the morning, and towards their morning wake up time.

Why Is My Baby Waking Every 90 Minutes?

If your baby relies on an external source to fall asleep—rocking, feeding, patting, or being held, they’ll likely wake up between each sleep cycle and wonder where that extra support has gone.

That’s when the cycle begins all over again - you help them back to sleep, only for them to wake again 90 minutes later.

How to Break the 90-Minute Wake-Up Cycle

The most effective way to get longer stretches of sleep (and protect your own sanity!) is to teach your baby how to self-settle - in other words, falling asleep on their own without needing external help. Once your baby knows how to do this, they’re more likely to transition between cycles independently and get longer, more restful sleep.

And this doesn't have to involve leaving your baby to cry alone. There are many responsive, gentle sleep support methods that work with your baby’s temperament and your parenting style. With patience and consistency, your baby can learn this essential skill in a way that feels secure and nurturing.

The Bottom Line

The 4-month sleep regression is not just a bump in the road, it’s a permanent change in your baby’s brain development. While it can feel like a big setback, it's actually a sign that your baby is growing and maturing.

By understanding what's happening behind the scenes, and helping your little one learn to self-settle, you can turn this challenge into a stepping stone toward better, more restful sleep for the whole family.