Longitudinal Investigation of Sleep, Memory, and Brain Development Across the Nap Transition

Inclusion Criteria: 1. 36-60 months at the time of enrollment 2. must be a habitual napper (defined as napping 5 or more days/week on average for the past month) 3. must sleep independently (not bedsharing; in order to maintain consistent sleep not interrupted by others) Exclusion Criteria: 1. diagnosis of any sleep disorder (other than mild parasomnia which is routine at this age) past or present (Child's Sleep Habit Questionnaire) 2. current use of psychotropic or sleep-altering medications (Developmental, Health, and Environment Questionnaire) 3. traveling beyond 1 time zone within 1 month prior to testing (phone screening) 4. fever or symptoms of respiratory illness at the time of testing (phone screening) 5. physical handicap which interferes with assessments (vision, hearing impairment; phone screening) 6. diagnosed developmental disability (Developmental, Health, and Environment Questionnaire) 7. history of neurological injury such as history of seizures, brain tumor, or stroke (phone screening) 8. presence of metal in the body (e.g., implant of any form) or other contraindication for MRI (e.g., claustrophobia, which is rare at this age). 9. external influences on nap habits (e.g., inability to nap due to school or caregiver schedule or interfering activities during a typical naptime) including if the child will enroll in full-day kindergarten by the end of the study. Caregivers will also be queried for the presence of interfering activities throughout enrollment (e.g., ecological momentary assessment (or EMA), sleep diaries at each wave).