In the last decades a vast empirical and theoretical knowledge about sleep mechanisms has been accumulated. Surprisingly, the function of sleep still remains elusive. Moreover, in place of the long-standing question “why do we sleep?” now comes a more fundamental one: “what is sleep?”
Among the key features of sleep are: a) the occurrence of slow waves in NREM sleep and theta waves in REM sleep on the EEG, b) the regular alternation between NREM and REM sleep episodes; and c) the slow homeostatic process that tracks sleep-wake history. The neurophysiologic substrates underlying these phenomena differ considerably and have been investigated in great detail. For example, much is known about cellular and network mechanisms underlying the slow oscillation (Crunelli and Hughes, 2009; Harris and Thiele, 2011) and about the neuroanatomy and neurochemistry of subcortical circuits regulating NREM and REM sleep (Brown et al., 2012; Jones, 2005; Saper et al., 2010). Furthermore, mathematical modelling has been essential for providing formal framework to investigate the global sleep-wake dynamics (Borbély and Achermann, 2005). While much has been learned about each of these phenomena in isolation, little has been done to reconcile the processes occurring at these “micro” and “macro” scales in a unified coherent framework.
Our aim is to understand the mechanisms governing the spatio-temporal dynamics of brain activity during sleep. This will help us to understand not only what sleep is, but also why it is necessary.