Regulation of Sleep by Environmental Light

We spend around a third of our lives asleep and sleep disruption is a major contributing factor to a health problems ranging from poor vigilance and memory, reduced reaction times, reduced motivation, depression, metabolic abnormalities, obestity, reduced immunity and elevated risks of cancer and coronary heart disease. Sleep is a complex process, involving multiple areas of the brain and numerous neurotransmitters. It is regulated by the familiar process whereby there is a increased requirement for sleep with prolonged waking (the sleep homeostat) as well as a drive for wakefulness produced by the body's internal 24h clock (circadian clock). In addition, environmental light plays a critical role in regulating the timing of the sleep/wake cycle as well as acute changes in sleep and arousal states. Changes in the light environment are detected by eye via a number of different photoreceptive cells. These include the rods and cones which mediate image-forming vision. However, the last decade has witnessed the remarkable discovery of a new class of light sensitive cell in the eye. A small subset of the retinal ganglion cells that form the optic nerve that projects to the brain have been found to be directly photosensitive (pRGCs) due to the presence of the light-sensing protein melanopsin. Rather than enabling us to create precise images of the external world, these cells simply detect the brightness of environmental light (irradiance), playing a critical role in many non-image forming responses such as setting the body clock to the day-night cycle and regulating sleep. As such, the eye performs two very different sensory tasks - firstly, to generate images of the world around us and secondly, to sample the light environment to regulate a range of non-image forming responses, including sleep. Recent studies have shown that mice lacking melanopsin show reduced sleep when exposed to light during the night. However, the sleep timing with respect to the light/dark cycle is normal in these animals, and exposure to light at other times can still produce normal sleep responses. Our recent unpublished data show that whilst melanopsin contributes to sleep regulation in response to light, other photoreceptors also contribute. The light environment provides a complex stimulus, and as well as the irradiance, the wavelength and rate of change of the light are expected to determine which photoreceptors are involved. Little is known about the contribution of these photoreceptors under different lighting conditions, and as a result we have only a poor understanding of the optimum light environment for housing laboratory mice. This proposal aims to determine the contribution of rods, cones and melanopsin pRGCs to the regulation of sleep under different lighting conditions. In mice, sleep is normally measured by electroencephalography (EEG) which involves implanting electrodes to measure electrical activity of the brain. In mice, this is invasive as well as time-consuming and expensive. We have recently developed a method of measuring sleep in mice using miniature night-vision cameras placed above the animal's cage. Using computer software to track the animal's movement, and defining sleep as a period of extended immobility we are able to measure sleep with remarkable precision compared to simultaneous EEG measures. We will use this non-invasive, high-throughput approach to study responses to light in a range of mouse models in which rods, cones and melanopsin are genetically altered. We will also screen sleep responses in mutant mice produced by MRC Harwell to identify novel genes which are involved in the regulation of sleep in response to light. This work will lead to a greater understanding of the role of rods, cones and melanopsin pRGCs in the regulation of sleep and is expected to improve our understanding of how retinal disease may give rise to sleep disruption.

Sleep is the product of multiple brain structures and neurotransmitter systems. This coordinated neural activity in turn drives alternating patterns of behaviour characterised by changes in rest/activity, body posture and responsiveness to stimuli. In addition to homeostatic and circadian oscillators, environmental light plays a critical role in both the entrainment of the sleep/wake cycle and the acute modulation of sleep and arousal states. The intensity, duration, rate of change and spectral composition of the light environment all contribute to sleep regulation and this complex stimulus is decoded by multiple retinal photoreceptors. These photoreceptors include the rods and cones which mediate image-forming vision, as well as the recently identified photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin. Melanopsin pRGCs mediate many non-image forming (NIF) responses to light, and recent studies have shown that the regulation of sleep is impaired in melanopsin knockout mice. However, this data does not preclude a contribution from rods and cones and our preliminary data using twilight simulation suggest that different photoreceptors may contribute, depending upon the nature of the stimulus. How the various photoreceptor classes interact in the intact retina and what features of the light environment are important remain unknown. This proposal will use both forward and reverse genetic approaches to define the contribution of different classes of photoreceptors to both the timing and direct effects of light on sleep regulation. Using high-throughput sleep scoring based upon video tracking we will study sleep in a range of photoreceptor transgenics in response to different environmental light stimuli. Behavioural data will be supported by the use of molecular and electrophysiological correlates of sleep. The results are expected to provide major advances in our understanding of how sleep is regulated by environmental light.

The impact of this proposal is expected to extend well beyond the research community, building upon the existing communication networks and expertise of the applicants. Both Dr Peirson (SNP) and Prof Foster (RGF) have a track record in securing industrial collaborations, and given interest in high-throughput compound screening, this is expected to lead to further industrial partnerships. Strong research interaction between the Nuffield Laboratory of Ophthalmology (NLO) and the Oxford Eye Hospital (OEH) also enables engagement and involvement of those working in front-line health care, and presentations on the OEH lecture series and the NLO's annual 'Updates in Ophthalmology' meeting will enable both specific details and health care implications of this work to be communicated to health care professionals. Both SNP and RGF have been involved in communicating their research to third sector organisations such as the RNIB, and in addition SNP has been involved with presentations as part of workshops for both the NC3Rs as well as the Health Protection Agency. The applicants have a strong track record of innovation and developing novel research tools and applications. This background provides a working knowledge of the processes required to protect and exploit research findings, which may have applications far beyond the scope of the immediate project. Finally, the applicants have previously been involved in a range of public engagement events to aid the communication of research to people of all ages and backgrounds, and with the development of a new NLO website have a further tool to ensure the current proposal achieves the greatest possible impact. Dr Nolan (PMN) has been involved in numerous collaborations with industrial partners, clinicians and basic scientists. Projects have been specifically associated with screening for mutants in a number of phenotypic domains and with developing and refining novel phenotyping methodologies and technologies to characterise mouse mutant lines.