From the moment they're born, and even before, we are bombarded with advice from other parents and even non-parents on how "not" to screw this parenting gig up. From breastfeeding vs. bottle feeding to cloth diapers vs. disposable diapers, to organic purees vs. store bought, it's no wonder this job is so overwhelming. Amidst all of those everlasting debates, however, one aspect of caring for a baby that we all know the dangers of doing, so we're told, is creating sleep associations (rocking being one of them).
If you've never heard the term, sleep associations are objects or actions that help us fall asleep. Even adults sometimes have sleep associations (a fan blowing, the TV on, etc.) that they use to sleep soundly. What a lot of us tend to forget is babies spend the first 9 months of their existence constantly being rocked and swayed to sleep with mom's movements. Once they're born, however, they are to lay flat on their backs in an empty crib and must self soothe themselves to sleep. When that doesn't work, we then find ourselves leaving them to cry until they've given up all hope that we'll return or we continuously tend to them frustrated that they'll only sleep while being held and rocked in the rocking chair. Rocking is most babies' sleep association just from being in the womb.
According to The New York Times, scientists at the University of Geneva in Switzerland did a sleep study on adults where they measured brain activity sleeping in a stationary bed compared to sleeping in a bed that swayed gently all night long. To not much of their surprise, the subjects slept better and more soundly while rocking as compared to sleeping completely still. What they deduced was that "rocking induced a kind of synchrony in brain wave activity that varied in tandem with the external motion. Rocking also increased the number of brain oscillations specific to sleep, which are critical for memory consolidation and learning" (Dr. Friedman, The New York Times). Their hypothesis was that rocking caused neurons in the ear to affect what was happening inside the brain (in this case, causing brain waves to flow smoothly with fewer interruptions).
Looking at a newborn baby, all they know is life inside the womb. They don't know what being alone and still is because they've been held their entire life in a warm, comforting, and moving womb. Rocking, especially during the fourth trimester (if you don't know what the fourth trimester is, you can read more about it here), reminds babies of their home and lets them know that they are still safe and still loved. Yes, it's exhausting tending to a baby's every cry, but in those first few critical months outside the womb, rocking is a gentle reminder that this world isn't as scary as it feels because mom, dad, or whoever is there for them as they adjust to this big change.