Infant sleep is a challenge among parents and a problem seen frequently in pediatric practice. Actigraphy, an activity-based sleep monitoring system, uses a wristwatch-like device with an accelerometer sensitive to body movements. Activity counts derived from actigraphy serve as the basis for sleep-wake determination. Although it has been concluded that actigraphy is a valid measure of sleep during infancy,1,2 external motion, a common experience for infants (eg, being carried or put in an infant swing), is usually overlooked in studies that use actigraphy to assess infant sleep. In some studies, actigraphy records containing external motion were removed prior to analysis2,3; in others, data involving external motion were included,4,5 with most studies not specifying how such data were analyzed. The objective of this study was to assess the activity count and sleep-wake identification accuracy by actigraphy when an infant doll was exposed to external motion.