What happens if you mix seroquel- a sleeping aid, with allergy medication?

Depends. On what allergy med ur taking: traditional antihistamines like benadryl/tavist will add drowsiness; newer "non-sedating" ones like Allegra (fexofenadine) should be ok; zyrtec may add drowsiness in some people. Ask ur doc or pharmacist.
Drowsiness. It can make you very drowsy and possible affect your heart rate and rhythm. Do no mix these on your own.