Nap for Memory (HSR Trial)

To examine the relations between sleep (nap transitions, sleep physiology), memory, and brain development longitudinally, the researchers will assess n=180 children (in order to acquire n=152 usable data sets) who are 36-54 months of age and habitual nappers at enrollment. In each wave, the researchers will assess memory, memory change over a nap and equivalent waking interval, sleep physiology of the nap, and brain structure and function (using Magnetic Resonance Imagining or MRI). Additionally, overnight sleep physiology will be assessed in all participants. Waves will take place approximately every 6 months. For all children, three waves will be collected. With these data, the researchers will address the following aims: * Examine neural markers that predict the sleep transition (Aim 1); * Examine changes in sleep-dependent memory processing (mnemonic discrimination) over both nap and overnight sleep intervals, across the sleep transition (Aim 2); * Examine changes in sleep microstructure in both nap and overnight sleep across the sleep transition (Aim 3) * Examine interrelations among brain, memory and sleep microstructure across the sleep transition (Aim 4)