Why doesn't my infant sleep through the night?

In the first few weeks of life, babies don’t have a circadian rhythm going yet which helps them to know the difference between daylight and nighttime. This rhythm, or body clock, starts ticking at around six weeks of age. But what’s more critical to the sleep rhythms of infants is their need for nourishment. In other words, it’s not so much that babies are missing the “on” and “off” switch that adults have when it comes to day-and-night cycles. It’s that babies cannot hold enough food (i.e., breast milk or formula) in their bellies to get through the night without needing more calories for growth.

It takes a baby a few months to grow a belly big enough to accommodate enough calories so they may rest for longer periods without needing to fill up again. And because breast milk in particular gets digested more quickly than formula, moms who exclusively breastfeed typically find themselves tending to their tots more frequently than those who are formula-fed.