Mechanisms of circadian disruption by the modern light environment

Lead Research Organisation: University of Oxford
Department Name: Clinical Neurosciences


Life on Earth has evolved under a rhythmically changing cycle of day and night. As a result, virtually all organisms have evolved internal biological clocks with a period of ~24h. These circadian clocks (from the Latin 'circa diem', or around a day) enable organisms to anticipate and adapt to predictable changes in their environment. In mammals, the master circadian clock is located in the suprachiasmatic nuclei (SCN) in the brain. Rhythms in the SCN are generated by a genetic feedback mechanism which regulates processes throughout our bodies.

A clock is of no use unless it can be set to the correct time. The SCN receives light information from the eye, which synchronises circadian rhythms to the external light/dark (LD) cycle - a process termed entrainment. This led researchers to investigate the light sensitive cells (photoreceptors) mediating these effects. The retinal contains two classes of photoreceptor - the rods (which mediate night-time vision) and cones (which give us our day-time colour vision). Remarkably, mice lacking both rods and cones still retain circadian responses to light. This led to the discovery of a novel retinal photoreceptor system, consisting of a subset of photosensitive retinal ganglion cells (pRGCs) expressing the blue-light sensitive pigment melanopsin.

The discovery of the melanopsin system has led to a remarkable public awareness of the circadian effects of evening blue light, including a particular concern about light from mobile devices. This has resulted in an increasing interest from the lighting and electronics industry, who are keen to develop lighting to avoid these circadian effects. However, simply reducing blue light overlooks the basic biology of this system. For example, melanopsin pRGCs do not work in isolation, and receive light input from rods and cones. As such, loss of melanopsin does not abolish circadian entrainment. Indeed, increasing data indicate that rods and cones also play important roles, which suggest that reducing blue light alone may be ineffective.

This project will investigate the mechanisms mediating the effects of evening light exposure on circadian rhythms. We have shown that exposure to dim light on an evening over the course of a week produces a misalignment of circadian rhythms in mice - replicating human studies. This provides a model for us to study the role of retinal photoreceptors in circadian disruption to evening light. By studying this response to specific colours and intensities of light at this time, we can define which photoreceptors contribute. We can then develop lighting conditions based upon these photoreceptors, enabling us to minimise these circadian effects. We can also confirm our findings using mouse models which lack the key photoreceptors. These studies will also investigate the role of daytime light levels to determine if brighter light during the day (and specifically the morning) can reduce the disruptive effects of evening light. Based on the findings of this first set of experiments, we will then compare our non-disruptive lighting conditions with disruptive conditions to study how circadian clocks throughout the body are affected by long-term exposure to evening light. We will also use these conditions to study how light activates the brain to enable us to understand the key brain regions involved in these responses. Finally, we will use these lighting conditions to investigate how sleep and performance are influenced by light.

We are exposed to artificial lighting throughout our lives with little appreciation of its biological effects. This proposal will provide critical information about the consequences of the modern light environment and the biological mechanisms underlying these responses. Critically, this work will also provide new data to help design lighting to avoid these detrimental effects.

Technical Summary

This project addresses the specific neural mechanisms by which the modern light environment disrupts circadian rhythms. The rise of artificial lighting and light emitting mobile devices means that we are increasingly exposed to light during our biological night. Accumulating evidence suggests that this produces circadian misalignment, with consequences for long-term health. However, our understanding of the mechanisms underlying these responses is limited. Despite increasing public and industry interest in reducing evening blue-light to reduce melanopsin activation, empirical data suggest that this approach alone may be ineffective.

This proposal will characterise the photoreceptors mediating the effects of chronic evening light exposure on circadian rhythms. Mice exposed to 4h of dim light on an evening over multiple days show a delayed phase of entrainment. This effect is also observed in mice lacking melanopsin. As such, we hypothesise that reducing activation of both melanopsin AND rods will be necessary to minimise the circadian effects of evening light. To test this, we will conduct irradiance response curves to monochromatic evening light to enable us to construct an action spectrum for this response and define the photoreceptors involved. We will then determine the effects of polychromatic light, optimising the activation of specific photoreceptor channels to minimise these disruptive effects.

We will then use disruptive and non-disruptive lighting conditions to study the effects of light on molecular clocks in central and peripheral oscillators. Using whole brain Fos mapping based upon clearing, we will also study the effects of these lighting conditions on the activation of different brain regions to define the neural pathways mediating these responses. Finally, we will compare the effects of these lighting conditions on sleep and cognitive performance, using electrophysiological and behavioural methods, respectively.

Planned Impact

The wider impact of the proposed research will be built on extending the existing networks that the lead researchers have already established within the scientific community, industrial sector, health care sectors and public policy advisors, as well as the promotion of research to the general public.

Beneficiaries and users of the research
Beneficiaries of this research include the scientific and clinical research communities, healthcare professionals and the wider public in general. This research will also benefit the commercial and private sector - particularly with regard to the lighting industry for disrupted circadian rhythms and retinal neuroprotection.

Data from this proposal will enable researchers working with human subjects (both within the SCNi and the wider research community) to develop improved study designs. The work in this proposal is also expected to result in industrial collaborations with lighting and electronic manufacturers interested in optimising lighting in environmental and architectural settings as well as from mobile devices

Communications and Engagement
SCIENTIFIC COMMUNITY: As well as the publication of internationally competitive science in prominent peer-reviewed journals, the applicants regularly present data at both the national and international level. The applicants are also regularly invited to speak at national and international meetings ensuring the widest possible scientific audience.

HEALTH CARE: Ongoing collaborations between the SCNi and Oxford Eye Hospital form an ideal opportunity to communicate general implications and specific details of this work to health care professionals.

PUBLIC: SNP and RGF are regular contributors to national newspapers and television. SNP was one of three researchers invited to take part in the Biosense exhibition at the Natural History Museum in Oxford. This BBSRC Sparking Impact-funded exhibition on contemporary science at Oxford University ran for 3 months, with an estimated 200,000 visitors over this time. He also worked with Oxford Impacts to prepare a short animated film, providing an ideal format for communication of this research to the public (>72,000 views). RGF is a well-known contributor to television and radio programmes relating to circadian rhythms, sleep and light. His TED talk on 'Why do we sleep?' from 2013 has received over 7 million views. The PIs will continue to seek similar opportunities throughout the course of this proposal to maximise the public impact of this work.

PUBLIC POLICY: SNP has close links with the Public Health England (PHE). He also contributed to an expert working group on the measurement of light for non-image forming responses (Lucas et al., 2014, >350 citations to date). This work has formed the basis for proposals under consideration by the International Lighting Commission (CIE).

THIRD SECTOR: As chair of the University of Oxford's 3Rs sub-committee, SNP works closely with the NC3Rs, and is academic lead for the NC3Rs regional programme manager in Oxford.

Exploitation and application
The increasing public and industry interest in the circadian effects of light make this proposal extremely timely and of direct application in a number of different areas. SNP and RGF are regularly approached by the lighting industry to provide advice regarding the non-visual effects of light. SNP and RGF were recently involved in private consultancy via OU Innovation for a major UK electronics manufacturer.

Capacity and involvement
The impact activities will primarily be undertaken by SNP and RGF, with contributions from PDRA, and named collaborators. The SCNi and NDCN also have dedicated staff to facilitate public engagement activities, providing additional added value.


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