Sleep Deprived

Does it ever happen (say towards the end of a third consecutive 12-hour shift) that you feel so overtired that you can't think straight? If you ever do feel in that state, what do you do? I know it's your responsibility to be well-rested for work and keep yourself in top form, but let's say despite your best efforts you feel that you're not able to focus enough to do your job optimally. What do you do then? do you ask to leave work early? Try to change to a different schedule that will work better for you?