Virtually all organisms possess a biological representation of the 24 hour day. These circadian rhythms (often termed the body clock) optimise an organism's physiology and behaviour in anticipation of the varying demands of the day/night cycle. Light provides the primary input to the circadian system, setting the clock to the correct time in a process termed 'photoentrainment'.

In mammals, photoentrainment is mediated by retinal photoreceptors which project to the master circadian pacemaker in the suprachiasmatic nuclei (SCN) of the hypothalamus. These photoreceptors include the rods and cones as well as the recently identified photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin. Research in the Nuffield Laboratory of Ophthalmology covers the basic mechanisms of photoentrainment, the molecular mechanisms producing 24h rhythms, light regulated physiology and behaviour, as well as the impact of circadian rhythm disturbances on human illness.