Circadian rhythms are only beneficial to animals if they can be synchronized by changes of ambient conditions. Light and temperature are two dominant environmental parameters that synchronize animal circadian clocks. In Drosophila circadian photo-responses are mediated by a solely blue light photoreceptor named CRYPTOCHROME (CRY). Upon photoreception, CRY changes its conformation and initiates the proteosomal dependent degradation of TIMELESS (TIM). TIM is an important pacemaker protein, thus degradation of TIM will reset the circadian clock. Under constant light conditions (LL), wild type flies quickly become arrhythmic because of the constant degradation of TIM, while flies bearing defects with circadian photo-responses will still be rhythmic. Thus LL triggered arrhythmicity has been used for screening of components in circadian light input pathways. A brief short light pulse in the night can also dramatically shift phases of circadian rhythms. As expected, this phase shift response is reduced in flied with defects in circadian photoresponse. Thus analyzing locomotion behavior rhythmicity under LL or phase changes after short light pulse in constant darkness (DD) are two major methods to study circadian photoresponse. Here we describe how to design and analyze LL and phase response experiments. LL arrhythmicity is suitable for screening light input pathways mutants, whereas phase response validates the results and provide further information for light sensitivity.