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