Why babies, puppies and other cute things twitch in their sleep

Pretty much every baby mammal that scientists have watched sleep has twitched while doing so. Most of the time its adorable, but why does it happen?

"The sleep field really started off in many ways as an offshoot of Freudian psychoanalysis and the study of dreams," [Mark Blumberg, a psychology professor at the University of Iowa] says. "People see these movements and they think, 'Oh, Fido is chasing rabbits in his dreams.' But it turns out that that's almost certainly not the case." In an engaging new review in Current Biology , Blumberg argues that these sleep twitches actually have an indispensible purpose: to teach a newborn what all of its limbs and muscles can do, and how to use them in concert to interact with the big, wide world.

Using high-speed cameras, Blumberg records young rats while they sleep and tracks and codes their twitches. He's found that there are actually some reliable patterns to the seemingly random movements.

But Blumberg's experiments have shown that the flailing is actually quite ordered in space and time. For example, when an animal brings its right elbow in toward its shoulder, there's a high probability that the left elbow will immediately follow in the same pattern. Similarly, on the same limb, if the shoulder moves in toward the body, there's a high probability that the elbow would then flex. Blumberg suspects that these predictable couplings are building blocks that help the developing motor system learn more complex behaviors.

"The brain is trying to understand, what are my limbs, how many do I have, and how many joints, and muscles, and how do they all move together?" he says. Once these simple commands are learned, he continues, the brain can use them to learn more complex sequences. "So that later, you can fire off a command somewhere in your mind, and generate a whole series of joint movements that would bring a bottle to your mouth, or make it possible to step." [Only Human]