Fatigue is a grey area. I've been thinking about this myself, lately. We've all had instances where we are just tired, and others where you were so tired it is akin to being "drunk", and furthermore, you can move between these states quite quickly and unconsciously. I know research has been done in the trucking industry- just wondering if there was any way to quantify fatigue in a physical sense eg. hormone levels/blood sugar levels/reaction time response etc.?
V. difficult to check/test for; as soon as you are aware you are being checked, you 'wake up' a bit!
Maybe a super-sophisticated "dead-mans switch"?