I have honestly found practice is a bit more important than concentration... yes, it is needed when you are first learning, but as you progress, it will become easier, to the point that it is almost second nature to simply sit down and slip into the needed mindset for Astral Projection. Just remember though, practice makes permanent.
As to waking up in the middle of the night, I'm not going to down the idea... I just don't suggest it for the fact that breaking a sleep cycle can have adverse affects, not only physically and mentally but also doing so can inadvertantly lead to problems on a sub-concious level. It can leave you reseptive to outside influence and even very unwanted nightmares... trust me, it's a very tried interrogation technique to break sleep cycles at opportune times...
Anyways, I didn't want to put that out there to offend people who use this technique, I just felt you should know both sides of the coin before attempting it. Good luck.