Like Miramel noted, talk with the managers/supervisors about your impending burnout. Stress that you love the job and that you want to keep with it, but the lack of availble workers means you and your co-workers are getting burnt out too fast and they might have a crisis on their hands in the future when people can't work from stress-related maladies.

Has your workplace advertised for people recently? I know as a university student looking for work right now that you could make a killing if you could harness the local version of my situation...

BEB

gotta say, i was dealing w/ the same thing... i don't know what kind of job your talking about, but:
i work at an ice cream shop: its spring/summer, i.e. very busy; anyway, people started quitting (first the store manager, then some shift managers, then some cashiers), and for a while we had something like 4 managers and 6 cashiers for the 84ish hours a week we're open; it got to where i had shifts as the manager, even though i was in NO way qualified.
ok, so i now realize that i actually have very little advise; i guess i would say, talk to whoever's in charge, let them know exactly how many hours you *can* work, and stick with it; also, keep in mind that since you're so short on workers, they would much rather that you work less then not at all.