Asymmetry and Basic Pathways in Sleep-Stage Transitions.

We study dynamical aspects of sleep micro-architecture. We find that sleep dynamics exhibits a high degree of asymmetry, and that the entire class of sleep-stage transition pathways underlying the complexity of sleep dynamics throughout the night can be characterized by two independent asymmetric transition paths. These basic pathways remain stable under sleep disorders, even though the degree of asymmetry is significantly reduced. Our findings indicate an intriguing temporal organization in sleep dynamics at short time scales that is typical for physical systems exhibiting self-organized criticality (SOC).