Dim Nocturnal Illumination and Plasticity of Circadian Entrainment

University Of California-San Diego, La Jolla CA

Investigators

Abstract

In humans and other mammals, marked daily variations in behavioral and physiological function are generated by endogenous neural clocks, which are synchronized with the light environment through both direct and indirect neural projections from the retina to the brain. Previous work has shown that bright light is necessary to reset the phase of circadian clocks, and cells with matching sensitivity to light have been identified in the retina. The present project explores novel actions of very dim light (less intense than dim moonlight) on the circadian system. Counter to expectations, preliminary studies show that dim nocturnal illumination results in marked changes in circadian function: in its presence, animals more readily adjust to seasonal changes in daylength; they entrain their rhythms more successfully to non 24-h days and they reorganize their circadian rhythms into multiple components under bimodal 24-h light:dark:light:dark cycles. In a series of behavioral studies, this project explores the hypothesis that dim light exerts its effects by altering the coupling, or interactions, between component circadian oscillators. Further, the investigators examine the wavelength and intensity requirements of dim light to assess the retinal photoreceptors that mediate its effects. Finally, these effects are examined in animals with lesions of the intergeniculate leaflet of the thalamus through which midbrain visual projections ascend to the circadian pacemaker. These studies may suggest new avenues for rapid manipulation of human circadian rhythms. These findings may be of value in optimizing human performance during shift-work, in trans-meridian and space travel, and in other contexts.