What is the state-of-the-art on lucid dreaming? Recent advances and questions for future research1 Background2 Quantification of dream lucidity as subjective experience3 Lucid vs. non-lucid dreams3.1 Non-lucid dreams3.2 Lucid dreams3.3 Natural frequency of lucid dreams: The brain maturation hypothesis (1)3.4 The hybrid state hypothesis (2) of lucid dreaming3.5 Space of Consciousness Model (3)3.6 EEG changes associated with lucid dreaming3.7 The gamma band hypothesis (4)3.8 Induction of lucidity via electrical stimulation 3.9 Brain Correlates of Lucidity and a Neuropsychological Model.4 Summary and outlook