Don’t worry. If you drift off during your Reiki session it doesn’t mean that you “wasted” the energy or anything like that. It’s actually a good sign. While there are definite advantages to consciously calming your mind and looking within, falling asleep is an indication that you were indeed extremely relaxed. And when you’re in such a relaxed state, or asleep, you are even more open to the Reiki energy.
I’ve had quite a few people tell me that when they try meditating, they usually fall asleep, which discourages them. I don’t take this to be a bad sign when you are first starting. I used to fall asleep all the time. But falling asleep while meditating is the same as falling asleep during your Reiki session. The brain-wave pattern when you’re super relaxed or in deep meditation is close to the pattern that we experience during sleep (Delta brain-wave pattern), so it’s easy for your brain to just slip the rest of the way into that state. But if you keep practicing you’ll eventually be able to keep your balance, so to speak, and stay in that meditative or relaxed state.
So don’t be too hard on yourself if you doze off. Being so relaxed that you fall asleep is certainly better than spending your entire session going over your todo list in your head.