A diffuse network of photosensitive retinal ganglion cells (pRGCs), which also receive input from rods and cones, are maximally sensitive to blue light between 470 and 480 nm (A). These cells have direct connections to the central circadian oscillator in the SCN where depending on the time of day (circadian time, CT) light induces changes in gene expression (B). pRGCs also mediate the synchronisation to LD cycles of locomotor activity, and light-induced phase shifts (C). pRGC connections to the olivary pretectal nucleus mediate light-sensitive pupil constriction (D), and indirect input via the SCN regulates the light-sensitive suppression of melatonin production in the pineal (E). The pRGC network has direct connections to sleep regulatory structures such as the VLPO and thereby modulates sleep and the ECoG during wakefulness (F). Blue light can modify brain responses to an executive task, as measured using fMRI (G) (figure adapted from [33], with permission), and can improve alertness (H) during the morning, lunch time, and early evening (figure based on data published in [32]).