As FearlessLeader just mentioned, saying that you don't FEEl like you'll fall asleep doesn't mean you're not fatigued. It really is like saying you don't feel drunk.
I too have seen plenty of tests which back that up... looking at how people respond given certain amounts of sleep, or disturbed sleep, etc. You may FEEL fine, you may feel as though you're reasonably well rested, but that in itself doesn't mean that you're not fatigued and therefore a danger. It's about how your brain responds, and how long it takes to respond, given a lack of sleep. Yes, everyone will vary, but on the whole our brains require a full night's sleep to function at their full capacity.