Determining The Role of Photic and Non-Photic Time Cues in Resetting Lipid Circadian Rhythms in Humans

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Project Summary/Abstract Circadian rhythm disruption is experienced by patients with Circadian Rhythm Sleep-Wake Disorders and millions of shiftworkers worldwide, which may increase their risk of developing chronic health disorders including cardiovascular disease. Treatment of circadian rhythm disruption requires appropriately-timed intervention to either shift the circadian system earlier (advance) or later (delay). A Phase Response Curve (PRC) informs when to administer the intervention, without which the disruption may either be prolonged due to inadequate phase resetting or worsened due to shifting the system in the wrong direction. Currently, the field relies on the PRC for resetting the melatonin rhythm as guide to reset the entire circadian system, despite the fact that circadian rhythms are present in many other physiological features besides melatonin. Our preliminary data show that there are robust circadian rhythms in circulating levels of total cholesterol and triglyceride in healthy young individuals and that these rhythms can also be shifted. Our pilot studies further indicate that the timing of these lipid rhythms may be more responsive to shifts in the timing of meals rather than light exposure. We have constructed preliminary PRCs of these lipid rhythms in response to a combined stimulus of light exposure and meals distributed across the 24-h day and detected robust phase advances and delays. Moreover, the shifts in these lipid rhythms are larger than those for melatonin. We do not know, however, whether light exposure or meal timing is the primary time cue for resetting these lipid rhythms. Without this knowledge, developing a comprehensive treatment for circadian rhythm disruption of lipid rhythms that likely underlies the cardiometabolic consequences of shiftwork, will remain difficult. The objective of this proposal is to construct three PRCs that systematically examine the contribution of light and meal timing on resetting lipid circadian rhythms. Young healthy adults will be randomized to three conditions: (1) bright light + 12-h meal window, (2) dim light + 12-h meal window, and (3) dim light + 6.5-h meal window (time redistricted eating), each distributed across the 24-h day. The primary outcomes include phase resetting of lipid and melatonin circadian rhythms measured under each of the three conditions, and the area-under-the curve of the lipids during the 6.5-h time restricted eating. The aims of the study are to: (1) determine if light is the primary time cue for resetting melatonin but not lipid circadian rhythms, (2) determine if meal timing is the primary time cue for resetting lipid but not melatonin circadian rhythms, and (3) evaluate the acute effects of eating across the 24-h day on circulating lipid levels. Our work will be a comprehensive evaluation of how two daily events – light exposure and meals – synchronize lipid circadian rhythms in humans. We expect our analytic paradigm to be a foundational resource that can be extended to future studies of other peripheral systems under circadian regulation in humans, and have a positive public health impact by guiding therapeutic strategies for patients with circadian disruption and the population at large, many of whom experience recurrent circadian disruption due to irregular sleep-wake schedules.