Do guppies Prefer light or Darkness?

Keep her in a normal day/night lightcycle. They normally prefer to birth at night. Lights go off at night.

You should keep guppies in low wattage blue tinted light ( such as T-5 Deep Blue), although it doesn't really matter for one that is way along in it's pregnancy. It matters much to get get good guppy matings. Female guppies repond the brightness of the color orange in male's tails. (Which is why the original non-hybrid guppies have a lot of orange). As it happens in low light orange stands out in the tail, and male's body basically looks gray but the females look at the tails. Dim blue light makes the orange stand out most, A number of scientific papers have been written about this