Communication within the clock network is essential for normal rhythmic activity patterns. Globally interfering with neuronal excitability or neuropeptide signalling strongly affects the rhythms; but so far a timed manipulation of specific selected neurons was not possible. We aim to use current optogenetic methods to manipulate the neuronal activity and cAMP-levels of selected clock neurons and to investigate the consequences on oscillations in clock protein levels in the clock network and on rhythmic behaviour. Furthermore, we aim to develop new light sensitive genetic tools that are less sensitive to the current hurdles: 1) Rhodopsins which are excitable by longer wavelengths, outside the spectral sensitivity of the fly`s photoreceptors. 2) Rhodopsins with higher light sensitivity and more resistance to degradation of the opsin. 3) Better supply of retinal to the target cells of the optogenetic tools.