Humans and animals exhibit brief awakenings from sleep (arousals), which are traditionally viewed as random disruptions of sleep caused by external stimuli or pathologic perturbations. However, our recent findings show that arousals represent previously unrecognized intrinsic aspects of sleep, and exhibit complex temporal organization and scale-invariant behavior characterized by a power-law probability distribution for their durations. In contrast, sleep-stage durations exhibit exponential behavior. The co-existence of these two very different processes in the sleep regulatory mechanism has not been observed in any other physiological system, and resembles a special class of non-equilibrium physical systems exhibiting self-organized criticality (SOC). Such organization of arousals makes it unlikely that they are merely a response to random stimuli, however, the role arousals play in healthy sleep and in sleep disorders remains unknown.Since our preliminary analyses show that SOC-type dynamics persist throughout the sleep period in humans and across several mammalian species with different sleep architecture, we hypothesize that arousals are an integral part of healthy sleep and relate to basic neuronal interactions. To address this hypothesis we will combine human sleep data and bio-molecular/genetic animal experiments with modern approaches from statistical physics of non-equilibrium systems and the theory of complex networks. Our objective is to uncover basic principles of self-organization in sleep that would elucidate the interrelation between sleep-stage dynamics and arousals, and to derive novel and more sensitive diagnostic markers of sleep disorders.