Hi Isarma,
I've noticed that I get more tired reading for some people than for others, I'm not really sure why.
In my case it has nothing to do with dehydration or anything like that, as I very rarely do more than a couple of readings at a time (I don't do it professionally).
I rarely get tired if I'm reading for myself. But after I've read especially for a specific friend of mine (and a specific subject), I always feel like I've been hit by a truck! I guess I ought to stop!
But I always get at least a little tired when I'm reading for others. Also, I've noticed that after, like, the second reading in a row, the cards seem not to speak to me at all anymore. They seem to be nothing more than pretty pictures at that point, and in order to make any sense of them, I have to use a tarot book that gives set meanings.
But I am still a beginner, so I'm not worried yet.