Asymmetry in sleep-stage transitions and critical self-organization in sleep micro-architecture

We study sleep-stage transitions and dynamical aspects of sleep micro-architecture. 
We find that sleep-stage transitions exhibit 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 further demonstrate an intriguing temporal organization in sleep micro-architecture at short time scales that is typical for physical systems exhibiting self-organized criticality, and indicates non-equilibrium critical dynamics in brain activity during sleep.