Reb Moshe brings a few reasons why its assur. One is because its mekalkal the guf. ANother is that it makes you a non-bar daas and if one does any mitzvos at that time, he will not be mekayaim them. Another is being over not honoring your parents because they will have tzar from you doing this. He ends by saying that one needs to stay away from this and befrat people who learn in yeshiva.
This is just a bikitur, if one wants to see the whole thing, look it up or if someone else here wants, they can expound of this and also fix any mistakes, I might have made.