A condition of the infant (also referred to as irregular or REM sleep and state II) when the eyes are closed, intermittent rapid eye movements can be observed through the eyelids, respirations are irregular and variable in frequency and amplitude, and motor activity varies from apparently random small limb movements to occasionally trunk movements and periods of inactivity (with a decrease in tonic muscle activity). Vocalizations are not present. In this sleep state, however, half-sided smiles (sometimes referred to ‘angel smiles’) can be apparent in young infants.
See Behavioral state concept, Muscle tone (or power), Newborn, NREM sleep, REM sleep