Early light environment has been shown to alter locomotor activity behaviour in adult rats and mice when exposed to constant light (LL), constant darkness (DD) or 12:12 hour light-dark cycles (LD). In particular, exposure to LL during lactation results in an increased ability to cope with exposure to LL as adults, implying that the ability to interpret light information is altered depending on early light environment. Therefore, the aim of this project was to explore how early light environment affects the photic response of the circadian system. The retina forms the first component of the photic response of the circadian system. We wanted to know whether being raised in DD, LD or LL would alter retinal function or structure in adult CD1 (albino) and C57BL/6J (pigmented) mice. We found that in CD1 mice, being raised in LL caused significant retinal damage and a significant reduction in retinal function. In C57BL/6J mice, we saw no such changes, implying that any changes that we see in behaviour would be due to alterations further downstream, such as the suprachiasmatic nucleus (SCN), site of the master circadian clock in the mammalian brain. We next exposed C57BL/6J mice to LL and found that mice raised in DD had significantly longer taus than mice raised in LL, implying that mice raised in DD are more sensitive to light than mice raised in LL.Previous work has shown that early light environment alters neuropeptide and astrocyte expression in the SCN of C57BL/6J mice. Furthermore, early light environment produces opposite behavioural responses in CD1 and C57BL/6J mice when exposed to LD as adults. We therefore examined how neuropeptide and astrocyte expression would be affected by early light environment in CD1 mice and whether this would reflect the differential behavioural response. We found that neuropeptide and astrocyte expression in the SCN seemed to be affected by the level of retinal damage and/or the type and intensity of the light source used. This sensitivity to lighting environment makes CD1 mice unsuitable for further studies on the photic response of the circadian system. Pigmented mice were used for the remainder of this project. The photic response of the circadian system can be quantified using a phase response curve (PRC) which measures behavioural responses to light pulses administered at different times of day. We measured the effect of a light pulse on the delay and advance portion of the PRC and found no differences due to early light environment, implying that the phasic effect of light is not altered by early light environment. Light pulses administered during the subjective night result in the upregulation of SCN intracellular photic signalling pathways. After a light pulse given during the early subjective night, we found no differences in the upregulation of different components of the photic signalling pathway due to early light environment indicating early light environment does not seem to affect the initial photic signalling pathway in the SCN.Finally, recent advances in molecular biology allow for real-time monitoring of clock gene expression in vitro. Using mPer2::luc mice, we monitored in vitro PER2::LUC expression to determine the effects of early light environment on clock gene expression. In the SCN, we found that the amplitude of PER2::LUC expression was significantly reduced in mice raised in DD compared to mice raised in LD and LL. These results suggest that early light environment affects the coupling strength between SCN neurons and this may be the mechanism mediating the changes in behaviour we have measured. In peripheral tissue, we found altered PER2::LUC expression due to early light environment in the heart, lung and spleen, implying that early light environment not only alters behaviour but may also affect heart and lung function and the immune system.