Although the neurophysiological origin of the functional magnetic resonance imaging (fMRI) blood oxygen level-dependent (BOLD) signal is still poorly understood, spontaneous fMRI signal fluctuations show consistent spatial correlations in functionally related networks. Large-scale functional brain networks as derived from fMRI time-series can be examined by graph theoretical analysis; such analysis has revealed a small-world organization of human functional brain networks during wakefulness, with high local clustering and short path length. A hierarchical cluster analysis indeed illustrated that frontoparietal clusters could be detected in wakefulness but not in deeper non-rapid eye movement (NREM) sleep stages. Functional connectivity of phasic events allows further spatial and temporal refinement of vigilance-state dependent connectivity patterns, and may be of special interest for phasic electroencephalography (EEG) events during sleep. Finally, although functional connectivity appears to overlap to a considerable extent with brain metabolism, these measures seem to represent correlated but different dimensions.