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

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PROJECT SUMMARY Sleep stabilizes memories in cortical regions and optimizes synaptic organization, freeing up the hippocampus for ongoing learning. In early childhood, naps serve a critical function, enhancing memory through sleep- dependent memory consolidation. Given this, it remains unknown why children transition out of naps when they are still in a stage rich with learning. We predict that critical brain development occurs at this time, making the mid-day “unloading” of memories no longer essential. Here we propose the first longitudinal study of the cognitive, physiological, and neural changes that accompany the nap transition. The central hypothesis is that maturation of the brain (in particular, the hippocampus) during early childhood results in more information being retained without interference, reducing the need for frequent consolidation that takes place during naps. We posit that these changes contribute to the biphasic (nap and overnight sleep) to monophasic (overnight sleep) sleep transition. To examine this, we will recruit 180 participants (36-54 months) and use a longitudinal design to assess brain development in relation to memory, sleep physiology, and nap status across the nap transition. Nap status will be monitored in real-time with actigraphy and verified via parent-reported ecological momentary assessments. Sleep will be assessed with polysomnography during the nap and overnight sleep. Memory will be assessed with a mnemonic similarity task, designed to tap hippocampal-dependent memory. Assessments will occur before and after a nap (with an equivalent interval awake as a control) and again before and after subsequent overnight sleep. Brain macro/micro structure and anatomical/functional connectivity will be measured with MRI. Aim 1 is to identify neural markers that predict the shift from biphasic to monophasic sleep. Aim 2 is to examine changes in sleep-dependent memory processing across the sleep transition. Aim 3 is to examine changes in diurnal and nocturnal sleep microstructure and topography across the sleep transition. Aim 4 is to examine interrelations among sleep architecture, memory, and the brain across the sleep transition using latent change score modeling. Innovative aspects of the proposed work include: real-time tracking of nap transitions longitudinally within the same subjects; examination of nap and overnight sleep microstructure and topography; comprehensive and targeted measures of brain development; measures of hippocampal-mediated memory; and latent change score modeling. This work will also result in a novel and rich dataset for additional analyses by our group and others to test alternative outcomes, explore the contribution of additional brain regions, and assess additional hypotheses from other prominent sleep theories. This work will have significant implications for human health by enhancing understanding of the mechanisms underlying sleep regulation and the consequences of sleep on cognitive health and performance and identification of biomarkers in sleep which may be useful in identifying risk and health outcomes.