Sleeping at work is generally viewed as a slovenly, completely inappropriate thing to do. Accepted social mores associate employment with productivity and productivity with compensation, thus making the idea of earning compensation while sleeping a morally indefensible one. In most workplaces, sleeping on the job is probably on the short list of things to do to get yourself fired. Particularly if it is a flagrant "I'm gonna catch some zzzz's" display and the sleeping position appears to be arranged, like say, under a desk or reclining back in a chair. Dozing off unintentionally is certainly less rude and probably a more excusable offense, however you're still likely to lose some respect from your co-workers. They'll probably think you were out getting wasted the night before or something.
Have you ever fallen asleep at your workplace? Did you get caught?