How to "attend to the perception of light"?

In [AN 7.58](http://www.accesstoinsight.org/tipitaka/an/an07/an07.058.than.html) , the Buddha instructed Maha Moggallana on what do to about his nodding: > "But if by doing this you don't shake off your drowsiness, then attend to the perception of light, resolve on the perception of daytime, [dwelling] by night as by day, and by day as by night. By means of an awareness thus open & unhampered, develop a brightened mind. It's possible that by doing this you will shake off your drowsiness. Elsewhere ([AN 4.41](http://www.accesstoinsight.org/tipitaka/an/an04/an04.041.than.html)) , this same practice is described as one that leads to knowledge & vision1. How is this to be understood and practiced in detail? What is meant by "perception of daytime, [dwelling] by night as by day, and by day as by night"? Is it as simple as imagining light? Are there ancient or modern detailed comments on this practice? --- 1In Bodhi translation, he says the *Manorathapūranī* understand this as divine eye, but in other places it can be insight knowledge or full enlightement.