The impact of daytime light exposure on diurnal and circadian rhythms in the diurnal rodent Rhabdomys pumillio

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Throughout our evolutionary history, the rotation of the earth upon its axis imposed a highly predictable and large amplitude twenty four hour rhythm in the amount of light to which we were exposed. The advent of artificial lighting has fundamentally disrupted this relationship in two ways. Firstly, it allows us to experience unnaturally high levels of light at night. Secondly, it allows us to spend most of our waking time indoors, exposed to unnaturally low light levels during the day. Significant attention has been paid to the deleterious effects of light at night, with evidence from both animal and human studies that this disrupts the endogenous timing mechanisms (circadian clocks) that define 24hr rhythms in our physiology and behaviour and inhibits sleep, with knock-on consequences for general health and daytime performance. By contrast, the impact of reduced light intensity during the day has been much less studied. This is a shame because, while it is practically inconceivable that we could reverse the progress of the last >100 years and persuade people to avoid unnatural sources of light at night, increasing levels of daytime light could be readily achieved by changes to architectural lighting design or even just by encouraging people to spend more time outdoors. There is evidence from humans that increasing light during the day improves alertness and cognitive performance, and that in the long term it can change the phase, increase the amplitude, and reduce the photosensitivity of circadian rhythms. However, the literature is sparse and lacks a systematic study of the impact of daytime light levels on temporal physiology. Here we set out to address this deficit and ask what impact, if any, changing daytime light exposure has on 24hr rhythms in behaviour and physiology. We then continue to ask whether increases in daytime light intensity can ameliorate some of the disruptive effects of light at night. We will address these questions by studying laboratory rodents. There is a rich history of translating findings from laboratory animals to humans in the field of circadian biology. Working in laboratory animals has the huge advantage of allowing researchers to carefully control environmental conditions over many days/weeks in a way that is difficult to achieve in human studies in the laboratory, let alone under field conditions. Fortunately, general principles established in the laboratory have proven to have good utility for understanding rhythmic physiology of humans in the real world. A substantial challenge in applying this well-worn approach to the questions posed in this proposal is that commonly employed laboratory rodents (mice, rats, hamsters) are nocturnal and would not normally experience bright daytime light. Indeed, in the lab they employ behavioural strategies to reduce their daytime light exposure - build nests, seek shade, shield their eyes during sleep. If we are to understand the impact of changing daytime light levels on circadian organisation and the disruptive effects of light at night we therefore need a species that, like humans, actively seeks daytime light exposure. We have established a colony of such animals in Manchester in preparation for this proposal. Rhabdomys pumillio is a species of mouse from Africa, slightly larger than ordinary lab mice. Importantly though, in both their natural environment and in the lab they are substantially day active. Accordingly, they also have a visual system that, like our own, is optimised for seeing in bright light. In this proposal we will carefully change the intensity of daytime light and monitor the impact on daily rhythms in behaviour and physiology in Rhabdomys. We will then ask whether increasing daytime light exposure ameliorates the disruptive effects of nighttime light exposure, with especial emphasis on the sort of nocturnal lighting patterns experienced by people.