Light and the circadian system in humans

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Light is not necessary only for vision. Light also has many non-visual biological functions, which is why controlled light exposure can be used to treat some mood disorders (like winter depression) and sleep-wake disorders (like abnormal sleep timing). Conversely, inappropriate timing of light exposure can have "toxic" effects and be involved in the development of various disorders. The aim of our NSERC research program is to understand how light is perceived, interpreted, and used for non-visual functions, especially for the regulation of our internal biological clock, the circadian system. Contrary to most research on human subjects in this field, we are studying the effects of light modulated over the 24-h, instead of the effects of light stimuli, and we combine real-life field studies with controlled laboratory testing. Two main research projects are planned for the next 5 years. The first project will focus on the effects of light exposure at the "wrong" time of day in night workers. The aim is to identify the characteristics of light exposure that are related to decreased melatonin production in night workers, because decreased melatonin production has been associated with increased risks of cancer. Night workers will wear a small monitor to measure their light exposure, and they will collect their urine for 48 h to assess their melatonin production. We will assess the effects of different patterns of light exposure due to light intensity during night work, and to seasonal variations in daylight duration. The second project will focus on normal subjects with naturally early or late sleep schedules. The biological clock of these individuals is adjusted differently to the external day-night cycle, which makes them an interesting model to study the role of light in circadian physiology. Since the effects of light depend not only on light exposure but also on light sensitivity, we plan to measure both in these subjects. We will develop new methods to measure light sensitivity, specifically adapted to the assessment of the non-visual system of light perception. We hope that the results of our studies will contribute to the development of new prevention and treatment strategies based on controlled light exposure and manipulations of non-visual light sensitivity.