Is histamine excitatory or inhibitory?

Excitatory: Histamine is actually classified as an excitatory neurotransmitter in the brain responsible for the sleep wake cycle. Too much of it can keep you awake. That is why it is used in many commercial sleep aides seen with the letters "pm" attached to some kind of pain killer. A common side-effect of a histamine blocker like Diphenhydramine (benadryl) is drowsiness.