Sometimes I've attributed the feeling to my sleeping waking life body during the dream. Although in those cases, during the dream, I've also felt a tingly feeling in my dream body. When we notice our waking life body during REM sleep, we tend to notice a tingly feeling in our body, which is a sign of REM atonia, the process which immobilizes our body during dreams. Other than that, I haven't directly tied the heavy drained feeling during dreams to anything in the waking world, although I'm prone to mentally exhausting myself by working long and hard on a project and deciding to give up entirely and throw the project away before I'm finished. Sometimes, tingly feeling or not, I have been able to overcome the drained and heavy feeling during the dream by just trying to ignore it and focusing on something else.