You should not be driving if you are experiencing symptoms of drowsiness such a the inability to keep your eyes open, feeling like your eyelids are unusually heavy, frequently blinking or rubbing your eyes or the feeling that they are dryer than normal, if you have trouble remembering or keeping your focus especially of how long you've been driving or how much further you have left to go, yawning, drifting between lanes, and physically slumping your head or body.