I don't think anyone actually knows the answer to that one. We can only guess.
Studies have shown that the whole purpose of sleeping is to dream - to the point that if you miss "dreaming time" on one night, you'll actually dream more the following nights to make up for it.
So since dreaming is a type of astral projection, does projecting into the astral planes cut down on the amount of sleep you'll need at night?
eg if you normally spend three hours a night dreaming, and you spend an hour projecting, will you only need two hours of dreaming that night?