I felt like you after after a session. Not BDSM session but exam session, when I used to cram 20 hours a day, it was intense and after I passed all exams I felt empty.

Usually after a BDSM session I feel exhilarated, rejuvenated and in a very good mood. I might have trouble sleeping before but not after. It is possible that mental activities drain you while physical activities invigorate you. I wonder whether this happens because my sessions usually end in some very active ways.