Research over the past decade has shown that the rods and cones are not the onlyphotoreceptors of the eye. There also exists a population of directly light sensitive ganglion cells that act as brightness detectors and regulate a wide variety of different photosensory tasks, including the regulation of 24-h circadian clocks, synthesis of the hormone melatonin, pupil size and behaviour. These receptors may even help modulate our mood and sense of well-being.