Why do we sleep? Loosing consciousness makes us completely vulnerable to any dangers of the environment for a third of our life time and must therefore yield a critical evolutionary advantage. A likely core function of sleep is the regulation of neuronal plasticity. The absence of interfering conscious information processing sets the stage for the basic maintenance of synaptic homeostasis to the complex reorganization and consolidation of memories. However, the underlying neuronal mechanisms subserving these processes are still largely unknown. The (...) aim of this thesis is to further elucidate the essential role of sleep-specific neuronal oscillations in brain plasticity. In a multimodal neuroscientific approach, non-invasive imaging methods (EEG and fMRI) and brain stimulation techniques (TMS and tDCS) are combined in a series of empirical studies to investigate how slow oscillations and sleep spindles are influenced by and in turn mediate plastic changes in the sleeping brain. (shrink)