Aging, light sensitivity and circadian health

Abstract

The world is rapidly aging: In 2006, ~11% of the population worldwide were above 60 years, and it is predicted to double by 2050, resulting in more individuals above 60 years than those aged 0-14 [1]. This trend raises important questions, including what are the ramifications of aging for health and well-being. Aging is often accompanied by sleep and circadian disruptions that may adversely affect quality of life. Compared to young individuals, those above 60 years typically experience increased wakefulness during sleep, early sleep and wake times and more sleep fragmentation [2]. Moreover, there is an association between healthy aging and disruption of endogenous circadian rhythms, as indexed by early timing and/or reduced amplitude of e.g., melatonin and core body temperature rhythms in older as compared to young individuals [2]. A key factor underlying these sleep and circadian changes in aging is reduced light sensitivity. Light elicits non-image-forming (NIF) effects primarily driven by intrinsically photosensitive retinal ganglion cells (ipRGCs) [3], by activating the photopigment melanopsin that is sensitive to light in the short-wavelength range (~490nm). Melanopsin-containing ipRGCs project to the suprachiasmatic nucleus (SCN, the circadian central pacemaker) through the retino-hypothalamic tract, and thereof to brain areas implicated in sleep-wake regulation [4]