The Surprising Truth about the Baby Sleep Cycle

Have you ever wondered what is happening when your baby sleeps? Why they make so many little noises or funny movements and then seem to be perfectly still (and you are literally holding your breath watching them breathe). Well, whether they are a newborn sleeping in your arms or an infant sleeping in their crib there is a distinct process happening as they snooze””the baby sleep cycle.

The Surprising Truth about the Baby Sleep Cycle

Just like their little brains and bodies, with time, the structure of the baby sleep cycle changes to be the same as an adult sleep cycle and that is what we are going to talk about in this article.


To understand baby sleep cycles, it is useful to see them juxtaposed to your own sleep cycles. As an adult you experience sleep in cycles that are about 90 to 120 minutes long. These cycles contain two different phases, one called rapid eye movement (REM) sleep and the other called non-REM (NREM) sleep. NREM sleep is further broken down into stages which I describe in detail below.

Adults initially cycle through the NREM sleep stages. These stages take them from light to deep sleep, then go to the beginning of deep sleep again and then they move onto REM. Once an adult has gone through a whole sleep cycle they either wake up or come close to waking up and then start the cycle over again until their alarm, dog, toddler, or baby wakes them up in the morning.

The 4 Stages of Sleep

As promised, let’s talk about the different NREM sleep stages. NREM sleep consists of 4 stages which go from very light to the deepest of sleep. Stage 1 is a very light sleep where you can feel yourself nodding off and if you were woken up you would say you hadn’t fallen asleep at all. In stage 2, it is still relatively light sleep but is described as the first “true sleep” stage where upon waking you would realize you were actually asleep. Stage 3 is the beginning of deep sleep and during this stage our body grows and heals.

Stage 4 is the deepest stage of sleep, sometimes called slow-wave sleep. It is very hard to wake someone up who is in deep sleep and if they are woken up during this stage they tend to be very drowsy.

After going through all 4 stages of NREM sleep, your body goes back into Stage 3 for 10 minutes and then into REM. REM sleep is where most dreams occur, the brain consolidates information, forms memory and releases serotonin. A fun fact about REM sleep is that while the mind is very active here, the body is not, in fact adults experience muscle atonia (essentially paralysis) so we can’t act out our dreams. As adults cycle through sleep at night we naturally wake a few times but generally, we aren’t able to recall those wake-ups and if someone asked us about them the following morning we wouldn’t know they occurred.


In contrast to adults, most babies have sleep cycles that last about 30 to 50 minutes and lengthen to about 90 minutes when they reach adolescence. Even before birth, babies are sleeping! Around 25 weeks gestation a baby will be sleeping 16 to 20 hours a day. In utero, REM sleep appears first and then a month later (about 8 months gestation) NREM follows.

After birth, a newborn baby still only experiences those two stages of sleep per cycle. Their NREM sleep is often referred to as “quiet sleep” and their REM sleep is also called “active sleep”. Upon falling asleep, newborns immediately go into REM sleep. Unlike older babies and adults, newborns don’t experience muscle atonia so during REM sleep they twitch, smile, make noises and breathe irregularly (actions which are often perceived by parents as waking up).

After “active sleep” they transition to “quiet sleep” where they breathe deeply and lie still (actually “sleeping like a baby”). Newborns spend about half of their sleep cycle in REM sleep and then the other half in deep sleep. Between three and four months of age babies experience a permanent reorganization in the architecture of their sleep cycle.

4 Month Baby Sleep Cycle

Most parents know this event as the infamous four-month sleep regression. Here they gain two more sleep stages, Stage 1 and Stage 2, both of which are lighter than REM sleep. In order to accommodate two more stages their sleep cycle shifts from being made up of 50% REM sleep to only 25% REM sleep. Furthermore, they now begin their sleep cycle in Stage 1 vice REM sleep. This means they are more prone to waking up as they are learning to adjust to these lighter sleep stages.

Thus, mom and dad be prepared for the four-month change to your baby’s sleep cycle. Understand that your baby is likely to be up more as they adjust to their new cycle. Plan ahead, and make sure to get in those much needed naps when your baby naps in the daytime.

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How did you adjust to your baby’s changing sleep cycle? Let us know in the comments

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