To work as proper timer, the circadian clock needs to be synchronised (entrained) to the
environmental rhythms by appropriate Zeitgebers attaining a functional phase relationship between internal physiology and the world outside allowing flies to perform functions with a selective advantage at the right time of the day. Nevertheless, recent studies show that certain timing is also possible without a functional clock. We will elucidate in which aspects the activity pattern of wild-type differs from clock mutants under natural outdoor and natural-like laboratory conditions. We will especially investigate the interactions between the circadian clock, light, temperature, humidity and nutrition. Finally, we will test whether possessing a circadian clock will provide the flies with an advantage in reproductive fitness and will prolong life-span, especially under not optimal conditions that prevail in nature.