How is it that during sleep, consciousness fades away, whilst the brain remains very much active? We tested the hypothesis that during sleep, the brain is less able to integrate information, due to a persistent reorganization of functional brain connectivity. Using fMRI data collected during wakefulness and non rapid eye movement (NREM) sleep in normal human volunteers, we used information theoretic methods to investigate how information is exchanged both within brain networks identified by independent component analysis, as well as between these networks. We show that both at the whole brain level and within each network, NREM sleep is characterized by decrease in functional integration. In addition, in most networks, we observe a hierarchical reorganization of integration towards an increased level of clustering into smaller functionally more independent modules. These 2 types of reorganization of functional connectivity potentially hinder the ability of the brain to integrate information and accounts for the reduction of conscious mental representations during NREM sleep.