Babies’ sleep patterns can feel as mysterious as the Bermuda Triangle, and as hard to get through. But like the Bermuda Triangle, in real life, most people do come out the other side just fine. When it comes to those early childhood sleep patterns, as unpredictable as they can start out, sooner or later, babies develop the same rhythms that tell the rest of us when it’s time to get some sleep, and, usually, when we need to wake up again. This is called the circadian rhythm, from the Latin meaning "around the day," and can be thought of as the body's internal clock.
Some of the earliest building blocks of this more regular sleep schedule - REM and non-REM sleep - develop when your baby is still in the womb, in the last few weeks of gestation. Others, though, like the production of the hormone melatonin during the night to help your baby sleep, won’t start kicking into gear until somewhere near her third month.
Babies don’t develop this part of their sleep rhythms in the womb because they’re still piggybacking off of mom’s, and breastfed babies can continue to do so for a while longer. Breast milk produced at night has melatonin in it, and can help make babies sleepier, which is why breastfed babies can start developing circadian rhythms a little earlier than other babies, especially if they don’t drink expressed milk from bottles at night.
Circadian rhythms tend to show up and start helping your baby fall into more regular sleep patterns sometime between 2 and 4 months, but there are some things you can do to help her get there faster. The fact that circadian rhythms develop at around the same amount of time after birth in preemies as they do in babies who were carried to full-term suggests that developing them has more to do with how your baby experiences the world after birth than it does with her development. You can encourage her to develop them by making sure she gets some sun and natural light during the day, and that her room stays fairly dark at night, even when she wakes up for feedings. Having a general routine for your baby’s day, including bedtime, could help, too. Some researchers believe social cues are one of the most important factors in establishing circadian rhythms.