As others have pointed out, this sounds like a sleep disorder. A healthy person who gets enough sleep should not have trouble staying awake for an 8 hour shift. This can be a serious condition and needs to be checked by a doctor. The good news is sleep disorders - and there are various kinds - can be resolved fairly quickly with treatment. Going straight to stimulants is not the answer. You should start with your primary physician and consider seeing a specialist for a sleep study.