Why Rocking Puts a Baby To Sleep --and Why Putting Her Down Often Wakes Her Up

Anybody who's ever tried to put a baby to sleep knows it can be hard for a baby to fall asleep on her own, but it's much easier if you just hold her and rock her. Why? What's the mysterious link between vibrations (rocking) and sleep?

A look at our early history as a species can shed some light. Humans spent most of their evolutionary history as nomads. During this period, a baby left sleeping on the ground was in danger not only of predation, but of being left behind when the group got going, perhaps in the rush of fleeing a predator. Crying would ensure the baby did not get left behind. On the contrary, a baby in her mother's arms was ensured not to be left behind, and crying under those circumstances would only endanger the baby by alerting predators while on the move. Thus, babies evolved to be silent on the move and loud when put down.

All you need is an artificial 'mommy on the move' simulator and your baby will sleep, well, like a baby.

Alex Bäcker, Dec. 31, 2006