The reason why it may be useful to avoid blue light at night is that circadian rhythms and melatonin production can be disrupted due to ganglion cell stimulation. They are a class of photoreceptors that have peak sensitivity to blue light. They do not appear to contribute much in the way of spatial vision but are important to entrain day night cycles and for pupillary responses. https://en.wikipedia.org/wiki/Intrinsically_photosensitive_r...