Light is a potent stimulus for regulating circadian, neuroendocrine and neurobehavioural responses in humans. Further, light therapy is effective for treating select affective, circadian and sleep disorders. Over the past decade, discoveries show that these biological and behavioural effects of light are mediated by a newly discovered photoreceptor in the eye that is distinct from the classical rods and cones for vision. These findings are providing a basis for major changes in lighting of advanced habitats for space exploration, as well as everyday buildings on Earth.