We all experience various brain states such as Beta, Alpha, Theta, Delta and Gamma throughout our 24 hour day. Astral projection and lucid dreaming deal with brain states in the theta and gamma range of which if you are not careful you could easily fall asleep.
I find that going back to bed about 30 minutes after waking up in the morning is a great time to practice astral projection if you are not using binaural beats or isochronic tones. I am a huge fan of binaural beats and isochronic tones which can give you a lot more freedom on when you choose to practice this art or as some might say, science.
I thought Nick explained the correlation between astral projection and the theta/gamma brain wave states very well to help clear up some confusion for some people on this topic.