Significance: The potential benefits of infant sleep for memory processing are largely unexplored. Here we show evidence that having an extended nap (≥30 min) within 4 h of learning helps 6- and 12-month-old infants to retain their memories for new behaviors across a 4- and 24-h delay. These results suggest that infants rely on frequent naps for the formation of long-term memories.

Significance: Infancy is regarded as a period of increased vulnerability to stress, but it is currently unknown how stress influences cognitive functioning in infants. Studies with adults have revealed that stress promotes rigid habitual behavior at the cost of flexible behavior. Here we show that stress similarly affects 15-mo-old infants. Half of the infants underwent a stress induction, the other half did not. Then, all infants learned to habitually perform an action that produced an interesting effect. Once the action had become ineffective, only stressed infants continued performing this action, showing rigid habitual behavior. Nonstressed infants explored an alternative action, showing flexible behavior. If stress occurs frequently, infants’ failure to explore alternative behaviors could hamper knowledge acquisition in the long term.