Spectral Sensitivity of Circadian, Neuroendocrine and Neurobehavioral Effects of Light

Abstract

The mammalian eye detects light for a range of behavioral and physiological responses separate and apart from sight. In humans, ocular light exposure resets the endogenous circadian pacemaker, suppresses synthesis of the pineal hormone melatonin, enhances morning cortisol production, increases heart rate and core body temperature, induces pupillary constriction, and improves subjective and objective measures of alertness. Several lines of evidence suggest that these responses are mediated primarily via a novel photoreception system with short-wavelength sensitivity (λmax 460–480 nm) different from that used for sight. In order to compare the relative contribution of the novel photoreceptor system with the three-cone visual photopic system for multiple non-visual effects of light, we compared the responses following exposure to an equal photon density (2.8×1013 photons/cm2/s) either 460 nm (n=8) or 555 nm (n=8) monochromatic light (10 nm half-peak bandwidth) at night on circadian phase resetting, melatonin suppression and subjective and objective correlates of alertness. As compared to 555 nm exposure, 460 nm light caused twice the phase shift in the circadian melatonin rhythm, double the amount of melatonin suppression, and significantly reduced auditory reaction time and lapses of attention. Exposure to 460 nm light also preferentially suppressed delta-theta (0.5–5.5 Hz) activity and elevated high alpha power (9.5–10.5 Hz) in the waking electroencephalogram (EEG) recordings, indicating a more alert state. The short-wavelength sensitivity to the non-visual effects of light indicates that the photopic visual system is not the primary photoreceptor system mediating these responses to light, and is consistent with predominant input from the novel melanopsin-containing photosensitive retinal ganglion cells. These data also suggest that short-wavelength light may be an effective therapy for resetting the circadian system in Circadian Rhythm Sleep Disorders and could be used as a direct fatigue countermeasure in a range of clinical and occupational settings. Translation of these basic findings into real-world applications is beginning to occur but the challenge to architects and lighting designers is to provide lighting that optimizes both the visual and non-visual effects of light simultaneously and safely.