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

Child participants will be recruited from databases maintained at each university (see Human Subjects). Initially, parent-report will determine whether the child qualifies as a habitual napper, which will later be confirmed by actigraphy. In each wave, participants will then complete 3 sessions, 3-7 days apart. The first two sessions will consist of nap or wake promotion (counter-balanced, separated by one week) and memory assessments. The third session will consist of a visit to the university for the MRI scan. All of the participants will also have overnight sleep and overnight sleep-dependent memory consolidation assessed on the evenings following the nap/wake sessions. An Actiwatch watch will be given to the participant at the beginning of the first wave. Instructions will be included describing how to put the watch on the child and that the watch should be worn as often as possible. Nap and wake sessions will begin 30 mins prior to the child's typical nap time (if habitual nappers) or approximately 12:30 pm (non-habitual nappers). First, children will complete the encoding and immediate retrieval phases of the mnemonic similarity task. Subsequently, children will either be nap- or wake-promoted. Polysomnography (PSG) will be applied in the nap condition, but timing will be equated in the wake condition. Thirty-minutes following wake (or approximately 3:30 pm for those in the wake group), children will complete the delayed retrieval phase of the mnemonic similarity task.

Participants at both testing sites will also have overnight sleep and memory assessed after the nap and wake sessions. Experimenters will arrive at the participant's house about 30 mins prior the child's typical bedtime. The child will encode a new set of stimuli and complete the immediate retrieval phase of the mnemonic discrimination task. Subsequently, the child will be fitted with PSG. Parents are given instructions on placement of the PSG amplifier during the night and how to remove in the electrodes in the morning. Experimenters will return in the morning to retrieve the equipment and obtain the delayed retrieval phase of the mnemonic similarity task. Approximately 3-7 days after the second visit, children will complete the MRI session. The researchers will attempt to obtain MRI scans in the morning to avoid confounds with nap status. The Actiwatch will also be collected in this session. Children will then be fitted with the Fitbit to wear as often as possible between testing waves. These procedures will be repeated every 6 months for 2-3 waves.

Inclusion criteria

• 36-60 months at the time of enrollment
• must be a habitual napper (defined as napping 5 or more days/week on average for the past month)
• must sleep independently (not bedsharing; in order to maintain consistent sleep not interrupted by others)

Exclusion criteria

• diagnosis of any sleep disorder (other than mild parasomnia which is routine at this age) past or present (Child's Sleep Habit Questionnaire)
• current use of psychotropic or sleep-altering medications (Developmental, Health, and Environment Questionnaire)
• traveling beyond 1 time zone within 1 month prior to testing (phone screening)
• fever or symptoms of respiratory illness at the time of testing (phone screening)
• physical handicap which interferes with assessments (vision, hearing impairment; phone screening)
• diagnosed developmental disability (Developmental, Health, and Environment Questionnaire)
• history of neurological injury such as history of seizures, brain tumor, or stroke (phone screening)
• 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).
• 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).