Elsevier

Sleep Medicine Reviews

Volume 14, Issue 4, August 2010, Pages 269-280
Sleep Medicine Reviews

Clinical Review
The role of environmental light in sleep and health: Effects of ocular aging and cataract surgery

https://doi.org/10.1016/j.smrv.2009.11.002Get rights and content

Summary

Environmental illumination profoundly influences human health and well-being. Recently discovered photoreceptive retinal ganglion cells (pRGCs) are primary mediators of numerous circadian, neuroendocrine and neurobehavioral responses. pRGCs provide lighting information to diverse nonvisual (non-image-forming) brain centers including the suprachiasmatic nuclei (SCN) which serve as the body's master biological clock. The SCN exert functional control over circadian aspects of physiology. The timing and strength (amplitude) of SCN rhythmic signals are affected by light exposure. Light deficiency may attenuate SCN function and its control of physiological and hormonal rhythms which in turn can result in a cascade of adverse events. Inadequate pRGC photoreception cannot be perceived consciously, but may aggravate many common age-associated problems including insomnia, depression and impaired cognition. In this review we (1) summarize circadian physiology, emphasizing light's critical role as the most important geophysical timing cue in humans; (2) analyze evidence that typical residential lighting is insufficient for optimal pRGC requirements in youth and even more so with advancing age; (3) show how ocular aging and cataract surgery impact circadian photoreception; and (4) review some of the diverse morbidities associated with chronodisruption in general and those which may be caused by light deficiency in particular.

Introduction

Retinal rod and cone photoreceptors send conscious visual information through retinal ganglion cells and the lateral geniculate nuclei to the visual cortex. In 2002, a subset of retinal ganglion cells (<1% in humans) were identified as photoreceptors themselves.1 These photosensitive retinal ganglion cells (pRGCs) transmit information through the retinal–hypothalamic tract synapsing directly on neurons in the suprachiasmatic nuclei (SCN) and other nonvisual brain centers.2 pRGCs express the blue light sensitive photopigment melanopsin.3 Approximately 3000 pRGCs cells per eye form a light sensitive network that spans the retina. Peak absorption of isolated pRGCs and melanopsin is ∼480 nm.1 Human nocturnal melatonin suppression is maximally sensitive at ∼460 nm.4, 5 These maxima lie within the blue portion of the visible spectrum. pRGCs mediate a host of nonvisual effects with a blue-shifted sensitivity quite different from longer (redder) wavelengths optimal for conscious vision, as shown in Fig. 1.6, 7 pRGCs detect ambient illumination allowing optimal physiological and neurobiological responses.1, 6, 8, 9 Additional unique features of pRGCs include resistance to bleaching,3 sustained signals with light thresholds significantly higher than those required for conscious vision,2, 3, 10 bistability,10, 1010a seasonal light adaptability and lack of spatiotemporal resolution.11 Deficient pRGC photoreception cannot be perceived consciously.12

Section snippets

Relevant circadian physiology

In mammals the master circadian clock resides within the paired suprachiasmatic nuclei of the anterior hypothalamus.2 The SCN originate daily patterns of most physiologic and hormonal processes,13, 14 timing events to allow preparation for anticipated metabolic and physical activities.*15, *16 Prior to habitual awakening the SCN initiate actions critical in transitioning from sleep to wakefulness including hepatic and adrenal stimulation which increases serum glucose and produces a morning

Short wavelengths optimally mediate non-image-forming responses

Skylight was probably the evolutionary stimulus for pRGC photoreception. The dominant wavelength of skylight is 477 nm (blue),52 close to pRGC's peak sensitivity. Outdoor illumination can exceed 100,000 lux, as shown in Fig. 2. With the advent of electricity and artificial lighting, industrialized society moved indoors isolated from traditional environmental light–dark extremes. Modern lighting provides at best 5% of natural light intensities*11, 53 and its longer (redder) wavelength spectra are

Cataract surgery

Cataract surgery removes a barrier to short wavelength light optimal for circadian photoreception. The yellowed crystalline lens is surgically replaced with an intraocular lens (IOL). UV-only blocking IOLs are the current standard of care in ophthalmology. They absorb most UV radiation and possibly some additional violet light, but they maximally transmit blue light.7, 179 Asplund and Lindblad demonstrated that cataract surgery with UV-blocking IOLs significantly improves nocturnal sleep and

Conclusions

Environmental illumination plays an important role in human health. The eye mediates this effect because retinal ganglion photoreceptors provide vital information about ambient illumination and light–dark cycles to nonvisual (non-image-forming) brain centers including the SCN. These data are necessary to coordinate metabolic homeostasis thus avoiding physiological stress and assure optimal levels and proper timing of the synthesis of essential CNS hormones and neurotransmitters including

Conflict of interest statement

The authors have no proprietary interests in any manufacturers or products. The authors received no funding for this research. Dr. Turner and Dr. Van Someren have no personal competing interests. Dr. Mainster is a consultant for Abbott Medical Optics, Iridex and Ocular Instruments Corporations.

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      Daily bright light exposure declines markedly in elderly adults, with women averaging half that of men.6 Institutionalized Alzheimer patients may receive fewer than 10 minutes of daily light exceeding 1000 lux.6 Morbidity from environmental light deficiency is reversable, with effective environmental light exposure shown to be beneficial for insomnia, depression, cognition, and degenerative disorders including Alzheimer disease.6,61

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