I have noticed that my state of mind affects my focus. If my mind is tired and exhausted cause of staying up late, then it is harder for me to focus my thoughts. That's part of why it's better to try to have an OBE when your not tired. With that said, I believe it's pretty normal to slip back into sleep when your mind is tired. I wouldn't stop trying though whenever the opportunity arises. I have had some pretty interesting OBE's while my mind is sluggish from being tired. But yeah, the more awake your mind is, the more focused your experience should be. This, of course, is just my personal experience.... Everyone is a little different. Good luck on your future attempts 
-Phil.

-Phil.