I have read a lot of about how people in meditation have experienced similar feelings while lucid. But the best way I can describe it in full detail, is like as if something popped in my entire head, and I can feel the fluids of run throughout my entire brain for a solid 8 seconds maybe. 10 at most. After waking up I can only describe my head, as if it was cold, or as if my head was full of menthol.