Sleep Slow Wave-Bistability and the Connection Between the Sleeping Brain and the Environment-Neurobiological Considerations.

The aim of this paper is to review the phenomenon of bistability, a basic working mode of NREM sleep. The perpetual oscillations between two opposite states in sleep slow waves, K-complexes as well as in the Cyclic Alternating Pattern (CAP) on a different time scale; appear spontaneously and can also be elicited by sensory stimulation, representing a response of the sleeping brain. Although there is extensive literature on bistability, its significance remains obscure. In this paper, spontaneous and elicited bistable patterns are reviewed and the global presence of bistable oscillations in NREM sleep is highlighted. We discuss the relationship of bistability with sleep homeostasis and the overlap with the CAP, with emphasis on A1 phase. We collect data along the hypothesis that bistability would provide a perpetual and flexible interface between the sleeping brain and the environment across the night, protecting the continuity of sleep and, in parallel, preserving the brain's adaptive connection with the environment.