Animals evolved circadian neural circuits to encode time of day. Unlike the mammalian circadian neural circuit, roughly half of the Drosophila circadian neurons express a cell autonomous light sensor called Cryptochrome (CRY). We find that CRY is responsive to blue and near visible ultraviolet light and that light containing these short wavelengths can reset the timing of the circadian neural circuit. Clock neurons may use multiple short wavelength light spectral cues to determine morning time more precisely. Using a newly developed imaging system, we can now visualize the whole circuit level pattern of clock neuron cycling at single cell resolution. A single light exposure at 2 hr before the circuit encoded beginning of the day internal time leads to profound desynchrony of the entire circuit while many light exposures during circuit encoded day internal times strengthen the phase coherence of the entire circadian network. Light protocols simulating social jet lag (an early-to-bed and early-to-rise light/dark schedule during the work week followed by staying up late and late-to-rise light/dark schedule on the weekend, returning to early schedule on Monday morning) chronically destabilizes the circadian circuit, suggesting potential health consequences.