well i top when the lights are on, and I think it actually does matter.
Here is why (taken from my GF's little sister's "Biggest Ever Book of Questions & Answers, awesome toilet read)
tips of plants such as leaves can detect the direction of light, and chemicals are released to make them grow more on the lower or darker side, thus turning the branch/ leaf upward.
So I can see that if you did this at night, it would just lay there limp and maybe rot off.