Why does an allergy attack make one so sleepy?

It saps energy: Suspect you mean respiratory allergy attacks, but all allergies are our immune systems mounting a defensive response against what it believes is a serious invader, specifically, a parasite. In reality, it's confused; harmless particles touching skin or mucous membranes (pollen up the nose, peanut butter in the mouth, or poison ivy on the skin) can cause dramatic reactions sapping energy over days.