Sleep, like waking, is a complex phenomenon comprising fluctuations in many neural and physiological systems. When we fall asleep, our skeletal muscle loses tone, the electrical activity in our cerebral cortex (i.e., the EEG) changes, and, during active sleep, our eyes dart around and our limbs twitch. The challenge of studying infant sleep is that these various components, which have been studied extensively in adults, do not always present themselves clearly in infants. For example, the EEG of infant rats before eleven days of age does not exhibit the clearly differentiable activity upon which researchers rely so heaviliy when judging adult sleep. These and other factors mean that we must assess infant sleep on its own terms rather than judge it against an adult standard. This is a central tenet of infant research, and one that we forget at our peril.
One of the goals of our research is to identify the role that sleep plays in the development of the nervous system. We view sleep as essential to the process by which sensory and motor systems establish the topographic relations (or somatotopic maps) that make normal function possible. This process is particularly critical during early development but also continues throughout life. We believe that sleep, especially active sleep, is critical to this process because it provides a period of relative quiescence when discrete signals can be sent and received by the nervous system.
Perhaps the most interesting developmental changes in sleep and wakefulness relate to the temporal organization of these states. For example, we have documented seminal developmental changes in the temporal organization of sleep-wake bouts and are seeking to identify the neural mechanisms that underlie these developmental changes in Norway rats.
Developmental analyses can also be helpful for exploring evolutionary issues pertaining to sleep-wake organization. We are currently adopting a developmental comparative approach to understand circadian rhythmicity using nocturnal (i.e., night-active) Norway rats and diurnal (i.e., day-active) Nile grass rats. By tracking the development of sleep and wakefulness across early development and exploring their neural control, we are identifying the key components that have been evolutionarily altered to produce the phenotypes associated with these different species. Over the coming years we hope to expand this approach to different questions and species.