Hi Nick,
I am not in the same type of therapy as you, mine is a combo of vestubular/physical therapy, but i have noticed I feel completely wiped out and even spaced out somedays (especially when we work on visual exercises).
I also tend to have a much more aggresive headache the next day. Must be from really trying to concentrate, as well as focusing eyes, movements etc.
But, I do feel the therapy is beneficial.
Take Care