I'm referring to mental stress - state of mental or emotional strain or tension resulting from adverse or demanding circumstances.
It really depends on the situtation but genearlly speaking I think a lightening bolt striking through a brain (or possibily a heart for specificly emotional stress) would work quite well in most situations.
Muller-Brockmann did some excellent designs for a sleeping medicine before. I couldn't find them online, but they are a book that I have at home. Will take some pics and add them later.
Perhaps a face with excess sweat drops, or a person sitting with their elbows on a desk with hands touching temples of the head. I like the above idea too.