RecruitingBehavioural interventionThe Function of Biphasic Sleep in InfantsThis study is designed to investigate developmental changes in naps and nap function on memory from 9 to 15 months of age. Memory is measured by a task in which the experimenter interacts with a toy and the measure of memory is whether the child imitates that action when given the toy. Sleep is assessed with a watch that detects motion which provides an estimate of sleep and a set of electrodes placed on the head that measures brain activity during sleep. Infants are recruited at 9 months and sleep and memory are measured again 3- and 6-months later. At each visit, memory is tested before and after a nap (either the morning or afternoon nap) and naps are recorded with the sleep electrodes.