Daily fluctuations in light and temperature act as environmental cues for synchronising circadian clocks, with light being the dominant synchronising factor. Properly entraining behaviour and physiology in-line with environmental Light: Dark (LD) cycles will contribute to the fitness and well-being of an organism. The circadian clocks of Drosophila melanogaster have been extensively studied with light-induced plasticity assayed with either brief light-pulses, varied photocycles or re-entrainment to a shifted LD regime. In this thesis, extreme equinox photocycles were used to stretch central and peripheral molecular oscillators to the limits of light-induced entrainment. At these limits, the clockwork has to constantly re-set its phase away from its inherent 24 h periodicity and therefore provides a more sensitive measure of plasticity. In doing so we reveal how the molecular oscillators in the brain and peripheral tissues adapt to maintain entrainment to extremely long and short photocycles. We also show that both the CRY/JET pathway and the visual system are required to facilitate central clock entrainment to extreme LD cycles, with CRY/JET expression in the PDF-expressing M-cells playing a significant role. The visual system plays no role in peripheral light entrainment, but both CRY and JET are essential. Our work thus furthers our understanding of; how the underlying circadian oscillator adapts to facilitate light-dependent plasticity at the behavioural and molecular level; the relative contributions of each light input pathway to central and peripheral clock entrainment; and in the case of central clocks, where in the circadian circuitry CRY/JET are required to allow behavioural entrainment to extreme equinox photocycles. Furthermore, we have shown that entrainment to extreme photocycles has a direct impact on fly physiology. As such, it may be possible to utilise such photocycles to induce internal desynchrony in Drosophila and model other physiological aspects of jet-lag and circadian disruption to further investigate the impact of circadian dysfunction on physiology and well-being of other invertebrate species as well as mammals.