The Activity of Young Children During Sleep was first published in 1939.This study analyzes the general character of the sleep of young children and the relationship of the quietness of their sleep to such factors as length of sleep, time of going to bed, afternoon naps, room and outdoor temperature, season, age and sex of the child, violent exercise between supper and bedtime, sleeping posture, etc.The subjects of the experiment were 22 children aged from 25 to 58 months, who slept in their own homes on especially designed beds having attached to the spring a Johnson kinetograph, which recorded the child’s motions during the night. The parents kept records of supplementary information. Usable records were obtained for a total of 3,339 nights.The techniques and results of this study will be of interest not only to students of child development but to all who are interested in the phenomena associated with sleep.