here is my experience:

I keep having lucid dreams via either DILD or MILD but they are always fuzzy. The last three times I thought - I am lucid now, let me ground myself in the dream. I would try studying my hands, looking at small textures, etc. But it never works for me, It's like lately I have full mental control but they are always so foggy i can barely see anything more than 10 feet in front of me. I also have noticed that they are always very dark. This last time, after studying my hands and the tree bark in front of me I looked up EXPECTING to see a detailed scene - but again foggy. I was pretty dissapointed. What happened next surprised me. i started again focusing on the tree in front of me. I had my arms around the other side of it. I was leaning back and letting my head and back arch away from it - almost like a child does when they are playing on a flagpole. suddenly all my will faded and I actually FELT drained and tired. my hands started slipping down until I was sitting on the ground. I actually felt the lucidity start to fade and for the very first time i didn't even try to fight it. It was like I just sat there saying "fuck it, i'm to exhausted to try"

Does this make sense? I thought that once you were alseep physical exhaustion did not matter since it is all with the mind. Could I have realy been too tired for a lucid or is it something else like stress?