If nuclear receptors are key components of the mammalian molecular clock, their role in the Drosophila circadian rhythms remained poorly understood. In this thesis, I first focused on the nuclear receptor E75, which appeared to be a crucial modulator for the fly molecular clock. I then focused on a non-circadian neuronal structure of the Drosophila's brain called the Mushroom Body. Despite not expressing the molecular clock itself, this center modulates numerous complex circadian behaviors. Circadian RNA-seq analysis of this structure revealed key genes involved in sleep regulation.