During the perinatal period in mammals when active sleep predominates, skeletal muscles twitch throughout the body. We have hypothesized that myoclonic twitches provide unique insight into the functional status of the human infant's nervous system. However, assessments of the rate and patterning of twitching have largely been restricted to infant rodents. Thus, here we analyze twitching in human infants over the first seven postnatal months. Using videography and behavioral measures of twitching during bouts of daytime sleep, we find at all ages that twitching across the body occurs predominantly in bursts at intervals of 10 s or less. We also find that twitching is expressed differentially across the body and with age. For example, twitching of the face and head is most prevalent shortly after birth and decreases over the first several months. In addition, twitching of the hands and feet occurs at a consistently higher rate than does twitching elsewhere in the body. Finally, the patterning of twitching becomes more structured with age, with twitches of the left and right hands and feet exhibiting the strongest coupling. Altogether, these findings support the notion that twitches can provide a unique source of information about typical and atypical sensorimotor development.

Highlights d In human infants, myoclonic twitches are a key component of active sleep d By 3 months of age, sleep spindles occur reliably during quiet sleep d Also around 3 months of age, twitches unexpectedly emerge during quiet sleep d When twitches occur during quiet sleep, they are synchronized with sleep spindles

Sleep, the predominant state of early infancy, is divided into two sub-states: active sleep (AS; or REM sleep) and quiet sleep (QS; or non-REM sleep). Behaviorally, AS is distinguished from QS by the presence of rapid eye movements (REMs) and abundant twitches across the body. Here, in the early postnatal period in human infants, we report the unexpected developmental emergence of twitches during QS. These twitches are unaccompanied by REMs and occur synchronously with sleep spindles, a hallmark of QS. As QS-related twitching increases with age, sleep spindle rate also increases along the sensorimotor strip. The emerging synchrony between subcortically generated twitches and cortical oscillations suggests the development of functional connectivity among distant sensorimotor structures, with potential implications for more sensitive identification of atypical developmental trajectories.