Changes in the daily light-dark cycle can influence mood, as in seasonal affective disorder. Ambient light is detected by intrinsically photosensitive retinal ganglion cells (ipRGCs) that project to brain regions such as the suprachiasmatic nucleus (where it entrains circadian rhythms) and limbic areas (which regulate mood). LeGates et al. now provide evidence that exposure of ipRGCs to light can directly influence behaviour, learning and synaptic plasticity without altering circadian rhythmicity or sleep.
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