My father is getting a nightly rash - around 6PM and lasting all night. He is allergic to shell fish and ate a fish sandwich 4 days ago. His doctor ordered him medication - looks like steroids? Take 6 one day, then 5, then 4 etc. He does not have a rash during the daytime hours and it only comes on every night in the evening. Does this sound like the typical allergic rash from food or maybe something else from the house bothering him in evening hours, i.e. furnace running more etc.? It just started and he says he is doing nothing different in the house. No new bedding etc. He said he did clean out the air filter in the furnace and it was really dirty.