Why is there a 4 month sleep regression?

What the evidence says

Newborn babies sleep in short, simple cycles that flip between light active sleep and deep quiet sleep, each lasting less than an hour. Around 3 to 4 months, a baby's brain starts changing this pattern into the more complex sleep cycles that adults have, with distinct stages that repeat through the night.

As part of this shift, babies start waking briefly at the end of each sleep cycle, the same way adults do. A newborn might sleep right through these moments, but a slightly older baby is more likely to fully wake up and cry out for help getting back to sleep.

Australian pediatric experts explain that this shift is not really a regression at all. It reflects the brain developing, similar to how a baby's feeding or movement skills change over time, and it gradually leads toward more mature sleep patterns over the first five years of life.

Research tracking infants over their first six months shows total night sleep slowly increases and night wakings slowly decrease during this period, though the pace varies a lot between babies. One study found that even by 6 months, only about half of healthy babies had fully settled into consolidated night sleep, so some bumpiness around 4 months is expected and normal.

The good news is this stage passes. Keeping a consistent bedtime routine and predictable sleep cues, as recommended by the Canadian Paediatric Society, can help your baby learn to settle themselves back to sleep between these newly noticeable wake-ups.